Trap Dispatching
Interrupts and exceptions are operating system conditions that divert the processor to code outside the normal flow of control. Either hardware or software can detect them. The term trap refers to a processor's mechanism for capturing an executing thread when an exception or an interrupt occurs and transferring control to a fixed location in the operating system. In Windows 2000, the processor transfers control to a trap handler, a function specific to a particular interrupt or exception. Figure 3-1 illustrates some of the conditions that activate trap handlers.
Figure 3-1 Trap dispatching
The kernel distinguishes between interrupts and exceptions in the following way. An interrupt is an asynchronous event (one that can occur at any time) that is unrelated to what the processor is executing. Interrupts are generated primarily by I/O devices, processor clocks, or timers, and they can be enabled (turned on) or disabled (turned off). An exception, in contrast, is a synchronous condition that results from the execution of a particular instruction. Running a program a second time with the same data under the same conditions can reproduce exceptions. Examples of exceptions include memory access violations, certain debugger instructions, and divide-by-zero errors. The kernel also regards system service calls as exceptions (although technically they're system traps).
Either hardware or software can generate exceptions and interrupts. For example, a bus error exception is caused by a hardware problem, whereas a divide-by-zero exception is the result of a software bug. Likewise, an I/O device can generate an interrupt, or the kernel itself can issue a software interrupt (such as an APC or DPC, described later in this chapter).
When a hardware exception or interrupt is generated, the processor records enough machine state so that it can return to that point in the control flow and continue execution as if nothing had happened. To do this, the processor creates a trap frame on the kernel stack of the interrupted thread into which it stores the execution state of the thread. The trap frame is usually a subset of a thread's complete context. (Thread context is described in Chapter 6.) The kernel handles software interrupts either as part of hardware interrupt handling or synchronously when a thread invokes kernel functions related to the software interrupt.
In most cases, the kernel installs front-end trap handling functions that perform general trap handling tasks before and after transferring control to other functions that field the trap. For example, if the condition was a device interrupt, a kernel hardware interrupt trap handler transfers control to the interrupt service routine (ISR) that the device driver provided for the interrupting device. If the condition was caused by a call to a system service, the general system service trap handler transfers control to the specified system service function in the executive. The kernel also installs trap handlers for traps that it doesn't expect to see or doesn't handle. These trap handlers typically execute the system function KeBugCheckEx, which halts the computer when the kernel detects problematic or incorrect behavior that, if left unchecked, could result in data corruption. (For more information on bug checks, see the section "System Crashes" beginning in Chapter 4.) The following sections describe interrupt, exception, and system service dispatching in greater detail.
Interrupt Dispatching
Hardware-generated interrupts typically originate from I/O devices that must notify the processor when they need service. Interrupt-driven devices allow the operating system to get the maximum use out of the processor by overlapping central processing with I/O operations. A thread starts an I/O transfer to or from a device and then can execute other useful work while the device completes the transfer. When the device is finished, it interrupts the processor for service. Pointing devices, printers, keyboards, disk drives, and network cards are generally interrupt driven.
System software can also generate interrupts. For example, the kernel can issue a software interrupt to initiate thread dispatching and to asynchronously break into the execution of a thread. The kernel can also disable interrupts so that the processor isn't interrupted, but it does so only infrequently—at critical moments while it's processing an interrupt or dispatching an exception, for example.
The kernel installs interrupt trap handlers to respond to device interrupts. Interrupt trap handlers transfer control either to an external routine (the ISR) that handles the interrupt or to an internal kernel routine that responds to the interrupt. Device drivers supply ISRs to service device interrupts, and the kernel provides interrupt handling routines for other types of interrupts.
In the following subsections, you'll find out how the hardware notifies the processor of device interrupts, the types of interrupts the kernel supports, the way device drivers interact with the kernel (as a part of interrupt processing), and the software interrupts the kernel recognizes (plus the kernel objects that are used to implement them).
Hardware Interrupt Processing
On x86 systems, external I/O interrupts come into one of the lines on an interrupt controller. The controller in turn interrupts the processor on a single line. Once the processor is interrupted, it queries the controller to get the interrupt request (IRQ). The interrupt controller translates the IRQ to an interrupt number, uses this number as an index into a structure called the interrupt dispatch table (IDT), and transfers control to the appropriate interrupt dispatch routine. At system boot time, Windows 2000 fills in the IDT with pointers to the kernel routines that handle each interrupt and exception.
EXPERIMENT
Viewing the IDTYou can view the contents of the IDT, including information on what trap handlers Windows 2000 has assigned to interrupts (including exceptions and IRQs), using the !idt command implemented in the Kdex2x86.dll debugger extension library. Passing the !idt command a 0 flag shows device driver ISRs that are registered for hardware device interrupts.
The following example shows how you load the Kdex2x86.dll debugger extension library and what the output of the !idt command looks like:
29: 00000000 2a: 804628fe (ntkrnlmp!KiGetTickCount) 2b: 804629f0 (ntkrnlmp!KiCallbackReturn) 2c: 80462b10 (ntkrnlmp!KiSetLowWaitHighThread) 2d: 8046375c (ntkrnlmp!KiDebugService) 2e: 80462420 (ntkrnlmp!KiSystemService) 2f: 80465aac (ntkrnlmp!KiTrap0F) 30: 80461a50 (ntkrnlmp!KiStartUnexpectedRange)
Some of the interrupt numbers of interest in the example are in the range 0x0 to 0x10, which includes x86 exception interrupts (for example, a page fault, which is exception 0xe, is handled by KiTrap0E) and the range 0x2a to 0x2e, which includes the system service dispatcher and other software interrupts that the kernel uses internally as fast entry points into the kernel from environment subsystems. On the system used to provide the output for this experiment, the clock interrupt handler is at interrupt number 0xd1 and the keyboard device driver's (I8042prt.sys) keyboard ISR is at interrupt number 0x52.
Windows 2000 maps hardware IRQs to interrupt numbers into the IDT, and the system also uses the IDT to configure trap handlers for exceptions. For example, the x86 exception number for a page fault (an exception that occurs when a thread attempts to access a page of virtual memory that isn't defined or present) is 0xe. Thus, entry 0xe in the IDT points to the system's page fault handler. Although the x86 architecture can support up to 256 IDT entries, the number of IRQs a particular machine can support is determined by the design of the interrupt controller the machine uses.
Most x86 systems rely on either the i8259A Programmable Interrupt Controller (PIC) or a variant of the i82489 Advanced Programmable Interrupt Controller (APIC); the majority of new computers include an APIC. The PIC standard originates with the original IBM PC. PICs work only with uniprocessor systems and have 15 interrupt lines. APICs work with multiprocessor systems and have 256 interrupt lines. Intel and other companies have defined the Multiprocessor Specification (MP Specification), a design standard for x86 multiprocessor systems that centers on the use of APIC. To provide compatibility with uniprocessor operating systems and boot code that starts a multiprocessor system in uniprocessor mode, APICs support a PIC compatibility mode with 15 interrupts and delivery of interrupts to only the primary processor. Figure 3-2 depicts the APIC architecture. The APIC actually consists of several components: an I/O APIC that receives interrupts from devices, local APICs that receive interrupts from the I/O APIC and that interrupt the CPU they are associated with, and an i8259A-compatible interrupt controller that translates APIC input into PIC-equivalent signals. The I/O APIC is responsible for implementing interrupt routing algorithms—which are software-selectable (the HAL makes the selection on Windows 2000)—that both balance the interrupt load across processors and attempt to take advantage of locality, delivering interrupts to the same processor that has just fielded a previous interrupt of the same type.
Figure 3-2 x86 APIC architecture
Each processor has a separate IDT so that different processors can run different ISRs, if appropriate. For example, in a multiprocessor system, each processor receives the clock interrupt, but only one processor updates the system clock in response to this interrupt. All the processors, however, use the interrupt to measure thread quantum and to initiate rescheduling when a thread's quantum ends. Similarly, some system configurations might require that a particular processor handle certain device interrupts.
EXPERIMENT
Viewing the PIC and APICYou can view the configuration of the PIC on a uniprocessor and the APIC on a multiprocessor by using the !pic and !apic I386kd (or Kd) commands, respectively. (You can't use LiveKd for this experiment because LiveKd can't access hardware.) Here's the output of the !pic command on a uniprocessor. (Note that the !pic command doesn't work if your system is using an APIC HAL.)
Here's the output of the !apic command on a system running with the MPS HAL. The "0:" prefix for the I386kd prompt indicates that commands are running on processor 0, so this is the I/O APIC for processor 0:
The following output is for the !ioapic command, which displays the configuration of the I/O APIC, the interrupt controller component connected to devices.
Most of the routines that handle interrupts reside in the kernel. The kernel updates the clock time, for example. However, external devices such as keyboards, pointing devices, and disk drives also generate many interrupts, and device drivers need a way to tell the kernel which routine to call when a device interrupt occurs.
Software Interrupt Request Levels (IRQLs)
Although interrupt controllers perform a level of interrupt prioritization, Windows 2000 imposes its own interrupt priority scheme known as interrupt request levels (IRQLs). The kernel represents IRQLs internally as a number from 0 through 31, with higher numbers representing higher-priority interrupts. Although the kernel defines the standard set of IRQLs for software interrupts, the HAL maps hardware-interrupt numbers to the IRQLs. Figure 3-3 shows IRQLs defined for the x86 architecture.
Figure 3-3 Interrupt request levels (IRQLs)
Interrupts are serviced in priority order, and a higher-priority interrupt preempts the servicing of a lower-priority interrupt. When a high-priority interrupt occurs, the processor saves the interrupted thread's state and invokes the trap dispatchers associated with the interrupt. The trap dispatcher raises the IRQL and calls the interrupt's service routine. After the service routine executes, the interrupt dispatcher lowers the processor's IRQL to where it was before the interrupt occurred and then loads the saved machine state. The interrupted thread resumes executing where it left off. When the kernel lowers the IRQL, lower-priority interrupts that were masked might materialize. If this happens, the kernel repeats the process to handle the new interrupts.
IRQL priority levels have a completely different meaning than thread-scheduling priorities (which are described in Chapter 6). A scheduling priority is an attribute of a thread, whereas an IRQL is an attribute of an interrupt source, such as a keyboard or a mouse. In addition, each processor has an IRQL setting that changes as operating system code executes.
EXPERIMENT
Viewing the IRQLA data structure called the processor control region (PCR) and its extension the processor control block (PRCB) contain information about the state of each processor in the system, such as the current IRQL, a pointer to the hardware IDT, the currently running thread, and the next thread selected to run. The kernel and the HAL use this information to perform architecture-specific and machine-specific actions. Portions of the PCR and PRCB structures are defined publicly in the Windows 2000 Device Driver Kit (DDK) header file Ntddk.h, so examine that file if you want a complete definition of these structures.
You can view the contents of the PCR with the kernel debugger by using the !pcr command:
Each processor's IRQL setting determines which interrupts that processor can receive. IRQLs are also used to synchronize access to kernel-mode data structures. (You'll find out more about synchronization later in this chapter.) As a kernel-mode thread runs, it raises or lowers the processor's IRQL either directly by calling KeRaiseIrql and KeLowerIrql or, more commonly, indirectly via calls to functions that acquire kernel synchronization objects. As Figure 3-4 illustrates, interrupts from a source with an IRQL above the current level interrupt the processor, whereas interrupts from sources with IRQLs equal to or below the current level are masked until an executing thread lowers the IRQL.
Figure 3-4 Masking interrupts
Because accessing a PIC is a relatively slow operation, HALs that use a PIC implement a performance optimization, called lazy IRQL, that avoids PIC accesses. When the IRQL is raised, the HAL notes the new IRQL internally instead of changing the interrupt mask. If a lower-priority interrupt subsequently occurs, the HAL sets the interrupt mask to the settings appropriate for the first interrupt and postpones the lower-priority interrupt until the IRQL is lowered. Thus, if no lower-priority interrupts occur while the IRQL is raised, the HAL doesn't need to modify the PIC.
A kernel-mode thread raises and lowers the IRQL of the processor on which it's running, depending on what it's trying to do. For example, when an interrupt occurs, the trap handler (or perhaps the processor) raises the processor's IRQL to the assigned IRQL of the interrupt source. This elevation masks all interrupts at and below that IRQL (on that processor only), which ensures that the processor servicing the interrupt isn't waylaid by an interrupt at the same or a lower level. The masked interrupts are either handled by another processor or held back until the IRQL drops. Therefore, all components of the system, including the kernel and device drivers, attempt to keep the IRQL at passive level (sometimes called low level). They do this because device drivers can respond to hardware interrupts in a timelier manner if the IRQL isn't kept unnecessarily elevated for long periods.
Because changing a processor's IRQL has such a significant effect on system operation, the change can be made only in kernel mode—user-mode threads can't change the processor's IRQL. This means that a processor's IRQL is always at passive level when it's executing user-mode code. Only when the processor is executing kernel-mode code can the IRQL be higher.
Each interrupt level has a specific purpose. For example, the kernel issues an inter-processor interrupt (IPI) to request that another processor perform an action, such as dispatching a particular thread for execution or updating its translation look-aside buffer cache. The system clock generates an interrupt at regular intervals, and the kernel responds by updating the clock and measuring thread execution time. If a hardware platform supports two clocks, the kernel adds another clock interrupt level to measure performance. The HAL provides a number of interrupt levels for use by interrupt-driven devices; the exact number varies with the processor and system configuration. The kernel uses software interrupts (described later in this chapter) to initiate thread scheduling and to asynchronously break into a thread's execution.
Mapping interrupts to IRQLs These IRQL levels aren't the same as the interrupt requests (IRQs) of the x86 system—the x86 architecture doesn't implement the concept of IRQLs in hardware. So how does Windows 2000 determine what IRQL to assign to an interrupt? The answer lies in the HAL. In Windows 2000, a type of device driver called a bus driver determines the presence of devices on its bus (PCI, USB, and so on) and what interrupts can be assigned to a device. The bus driver reports this information to the Plug and Play manager, which decides, after taking into account the acceptable interrupt assignments for all other devices, which interrupt will be assigned to each device. Then it calls the HAL function HalpGetSystemInterruptVector, which maps interrupts to IRQLs.
The algorithm for assignment differs for the uniprocessor and multiprocessor HALs that Windows 2000 includes. On a uniprocessor system, the HAL performs a straightforward translation: the IRQL of a given interrupt vector is calculated by subtracting the interrupt vector from 27. Thus, if a device uses interrupt level 5, its ISR executes at IRQL 22. On a multiprocessor system, the mapping isn't as simple. APICs support over 100 interrupt vectors, so there aren't enough IRQLs for a one-to-one correspondence. The multiprocessor HAL therefore assigns IRQLs to interrupt vectors in a round-robin manner, cycling through the device IRQL (DIRQL) range. As a result, on a multiprocessor system there's no easy way for you to predict or to know what IRQL Windows 2000 assigns to APIC IRQs. You can use the !idt kernel debugger command (shown earlier in the chapter) to view IRQL assignments for hardware interrupts.
Predefined IRQLs Let's take a closer look at the use of the predefined IRQLs, starting from the highest level shown in Figure 3-4:
- The kernel uses high level only when it's halting the system in KeBugCheckEx and masking out all interrupts.
- Power fail level originated in the original Microsoft Windows NT design documents, which specified the behavior of system power failure code, but this IRQL has never been used.
- Inter-processor interrupt level is used to request another processor to perform an action, such as dispatching a particular thread for execution, updating the processor's translation look-aside buffer (TLB) cache, system shutdown, or system crash.
- Clock level is used for the system's clock, which the kernel uses to track the time of day as well as to measure and allot CPU time to threads.
- The system's real-time clock uses profile level when kernel profiling, a performance measurement mechanism, is enabled. When kernel profiling is active, the kernel's profiling trap handler records the address of the code that was executing when the interrupt occurred. A table of address samples is constructed over time that tools can extract and analyze. The Windows 2000 resource kits include a tool called Kernel Profiler (Kernprof.exe) that you can use to configure and view profiling-generated statistics. See the Kernel Profiler experiment for more information on using Kernprof.
- The device IRQLs are used to prioritize device interrupts. (See the previous section for how hardware interrupt levels are mapped to IRQLs.)
- DPC/dispatch-level and APC-level interrupts are software interrupts that the kernel and device drivers generate. (DPCs and APCs are explained in more detail later in this chapter.)
- The lowest IRQL, passive level, isn't really an interrupt level at all; it's the setting at which normal thread execution takes place and all interrupts are allowed to occur.
EXPERIMENT
Using Kernel Profiler to Profile ExecutionYou can use the Kernel Profiler tool that comes with the Windows 2000 resource kits to enable the system profiling timer, collect samples of the code that is executing when the timer fires, and display a summary showing the frequency distribution across image files and functions. Kernel Profiler is most useful in situations in which performance-critical code is running in a repeatable manner and you want to obtain a breakdown of where the system is spending time when the code executes. For the output to be useful, Kernel Profiler requires that the Windows 2000 symbols be installed on your system. Below is sample output from Kernel Profiler after collecting information for 30 seconds on a system that was relatively idle. (Notice that the majority of the samples were in KiIdleLoop, the idle thread loop, which is explained in Chapter 6.)
One important restriction on code running at DPC/dispatch level or above is that it can't wait on an object if doing so would necessitate the scheduler to select another thread to execute. Another restriction is that only nonpaged memory can be accessed at IRQL DPC/dispatch level or higher. This rule is actually a side effect of the first restriction because attempting to access memory that isn't resident results in a page fault. When a page fault occurs, the memory manager initiates a disk I/O and then needs to wait for the file system driver to read the page in from disk. This wait would in turn require the scheduler to perform a context switch (perhaps to the idle thread if no user thread is waiting to run), thus violating the rule that the scheduler can't be invoked (because the IRQL is still DPC/dispatch level or higher at the time of the disk read). If either of these two restrictions is violated, the system crashes with an IRQL_NOT_LESS_OR_EQUAL crash code. (See Chapter 4 for a thorough discussion of system crashes.) Violating these restrictions is a common bug in device drivers. The Windows 2000 Driver Verifier, explained in the section "Driver Verifier" in Chapter 7 has an option you can set to assist in finding this particular type of bug.
Interrupt objects The kernel provides a portable mechanism—a kernel control object called an interrupt object—that allows device drivers to register ISRs for their devices. An interrupt object contains all the information the kernel needs to associate a device ISR with a particular level of interrupt, including the address of the ISR, the IRQL at which the device interrupts, and the entry in the kernel's IDT with which the ISR should be associated. When an interrupt object is initialized, a few instructions of assembly language code, called the dispatch code, are copied from an interrupt handling template, KiInterruptTemplate, and stored in the object. When an interrupt occurs, this code is executed.
This interrupt-object resident code calls the real interrupt dispatcher, the kernel's KiInterruptDispatch routine, passing it a pointer to the interrupt object. The interrupt object contains information this second dispatcher routine needs in order to locate and properly call the ISR the device driver provides.
The interrupt object also stores the IRQL associated with the interrupt so that another routine, KiDispatchInterrupt, can raise the IRQL to the correct level before calling the ISR and then lower the IRQL after the ISR has returned. This two-step process is required because there's no way to pass a pointer to the interrupt object (or any other argument for that matter) on the initial dispatch since the initial dispatch is done by hardware. On a multiprocessor system, the kernel allocates and initializes an interrupt object for each CPU, enabling the local APIC on that CPU to accept the particular interrupt.
Windows 2000 and Real-Time Processing
Deadline requirements, either hard or soft, characterize real-time environments. Hard real-time systems (for example, a nuclear power plant control system) have deadlines that the system must meet to avoid catastrophic failures such as loss of equipment or life. Soft real-time systems (for example, a car's fuel-economy optimization system) have deadlines that the system can miss, but timeliness is still a desirable trait. In real-time systems, computers have sensor input devices and control output devices. The designer of a real-time computer system must know worst-case delays between the time an input device generates an interrupt and the time the device's driver can control the output device to respond. This worst-case analysis must take into account the delays the operating system introduces as well as the delays the application and device drivers impose.
Because Windows 2000 doesn't prioritize device IRQs in any controllable way and user-level applications execute only when a processor's IRQL is at passive level, Windows 2000 isn't always suitable as a real-time operating system. The system's devices and device drivers—not Windows 2000—ultimately determine the worst-case delay. This factor becomes a problem when the real-time system's designer uses off-the-shelf hardware. The designer can have difficulty determining how long every off-the-shelf device's ISR or DPC might take in the worst case. Even after testing, the designer can't guarantee that a special case in a live system won't cause the system to miss an important deadline. Furthermore, the sum of all the delays a system's DPCs and ISRs can introduce usually far exceeds the tolerance of a time-sensitive system.
Although many types of embedded systems (for example, printers and automotive computers) have real-time requirements, the special embedded version of Windows NT (currently based on Windows NT 4 but eventually to be updated for a future version of Windows 2000) doesn't have real-time characteristics. It is simply a version of Windows NT that makes it possible, using system designer technology that Microsoft licensed from VenturCom, to produce small-footprint versions of Windows NT suitable for running on devices with limited resources. For example, a device that has no networking capability would omit all the Windows NT components, including network management tools and adapter and protocol stack device drivers, related to networking.
Still, there are third-party vendors that supply real-time kernels for Windows NT 4 and Windows 2000. The approach these vendors take is to embed their real-time kernel in a custom HAL and to have Windows 2000 or Windows NT 4 run as a task in the real-time operating system. The task running Windows 2000 or Windows NT serves as the user interface to the system and has a lower priority than the tasks responsible for managing the device. See VenturCom's Web site, www.venturcom.com, for an example of a third-party real-time kernel extension for Windows NT or Windows 2000.
Associating an ISR with a particular level of interrupt is called connecting an interrupt object, and dissociating an ISR from an IDT entry is called disconnecting an interrupt object. These operations, accomplished by calling the kernel functions IoConnectInterrupt and IoDisconnectInterrupt, allow a device driver to "turn on" an ISR when the driver is loaded into the system and to "turn off" the ISR if the driver is unloaded.
Using the interrupt object to register an ISR prevents device drivers from fiddling directly with interrupt hardware (which differs among processor architectures) and from needing to know any details about the IDT. This kernel feature aids in creating portable device drivers because it eliminates the need to code in assembly language or to reflect processor differences in device drivers.
Interrupt objects provide other benefits as well. By using the interrupt object, the kernel can synchronize the execution of the ISR with other parts of a device driver that might share data with the ISR. (See Chapter 9 for more information about how device drivers respond to interrupts.)
Furthermore, interrupt objects allow the kernel to easily call more than one ISR for any interrupt level. If multiple device drivers create interrupt objects and connect them to the same IDT entry, the interrupt dispatcher calls each routine when an interrupt occurs at the specified interrupt line. This capability allows the kernel to easily support "daisy-chain" configurations, in which several devices share the same interrupt line. The chain breaks when one of the ISRs claims ownership for the interrupt by returning a status to the interrupt dispatcher. If multiple devices sharing the same interrupt require service at the same time, devices not acknowledged by their ISRs will interrupt the system again once the interrupt dispatcher has lowered the IRQL. Chaining is permitted only if all the device drivers wanting to use the same interrupt indicate to the kernel that they can share the interrupt; if they can't, the Plug and Play manager reorganizes their interrupt assignments to ensure that it honors the sharing requirements of each.
Software Interrupts
Although hardware generates most interrupts, the Windows 2000 kernel also generates software interrupts for a variety of tasks, including these:
- Initiating thread dispatching
- Non-time-critical interrupt processing
- Handling timer expiration
- Asynchronously executing a procedure in the context of a particular thread
- Supporting asynchronous I/O operations
These tasks are described in the following subsections.
Dispatch or deferred procedure call (DPC) interrupts When a thread can no longer continue executing, perhaps because it has terminated or because it voluntarily enters a wait state, the kernel calls the dispatcher directly to effect an immediate context switch. Sometimes, however, the kernel detects that rescheduling should occur when it is deep within many layers of code. In this situation, the ideal solution is to request dispatching but defer its occurrence until the kernel completes its current activity. Using a DPC software interrupt is a convenient way to achieve this delay.
The kernel always raises the processor's IRQL to DPC/dispatch level or above when it needs to synchronize access to shared kernel structures. This disables additional software interrupts and thread dispatching. When the kernel detects that dispatching should occur, it requests a DPC/dispatch-level interrupt; but because the IRQL is at or above that level, the processor holds the interrupt in check. When the kernel completes its current activity, it sees that it's going to lower the IRQL below DPC/dispatch level and checks to see whether any dispatch interrupts are pending. If there are, the IRQL drops to DPC/dispatch level and the dispatch interrupts are processed. Activating the thread dispatcher by using a software interrupt is a way to defer dispatching until conditions are right. However, Windows 2000 uses software interrupts to defer other types of processing as well.
In addition to thread dispatching, the kernel also processes deferred procedure calls (DPCs) at this IRQL. A DPC is a function that performs a system task—a task that is less time-critical than the current one. The functions are called deferred because they might not execute immediately.
DPCs provide the operating system with the capability to generate an interrupt and execute a system function in kernel mode. The kernel uses DPCs to process timer expiration (and release threads waiting on the timers) and to reschedule the processor after a thread's quantum expires. Device drivers use DPCs to complete I/O requests. To provide timely service for hardware interrupts, Windows 2000—with the cooperation of device drivers—attempts to keep the IRQL below device IRQL levels. One way that this goal is achieved is for device driver ISRs to perform the minimal work necessary to acknowledge their device, save volatile interrupt state, and defer data transfer or other less time-critical interrupt processing activity for execution in a DPC at DPC/dispatch IRQL. (See Chapter 9 for more information on DPCs and the I/O system.)
A DPC is represented by a DPC object, a kernel control object that is not visible to user-mode programs but is visible to device drivers and other system code. The most important piece of information the DPC object contains is the address of the system function that the kernel will call when it processes the DPC interrupt. DPC routines that are waiting to execute are stored in kernel-managed queues, one per processor, called DPC queues. To request a DPC, system code calls the kernel to initialize a DPC object and then places it in a DPC queue.
By default, the kernel places DPC objects at the end of the DPC queue of the processor on which the DPC was requested (typically the processor on which the ISR executed). A device driver can override this behavior, however, by specifying a DPC priority (low, medium, or high, where medium is the default) and by targeting the DPC at a particular processor. A DPC aimed at a specific CPU is known as a targeted DPC. If the DPC has a low or medium priority, the kernel places the DPC object at the end of the queue; if the DPC has a high priority, the kernel inserts the DPC object at the front of the queue.
When the processor's IRQL is about to drop from an IRQL of DPC/dispatch level or higher to a lower IRQL (APC or passive level), the kernel processes DPCs. Windows 2000 ensures that the IRQL remains at DPC/dispatch level and pulls DPC objects off the current processor's queue until the queue is empty (that is, the kernel "drains" the queue), calling each DPC function in turn. Only when the queue is empty will the kernel let the IRQL drop below DPC/dispatch level and let regular thread execution continue. DPC processing is depicted in Figure 3-5.
Figure 3-5 Delivering a DPC
DPC priorities can affect system behavior another way. The kernel usually initiates DPC queue draining with a DPC/dispatch-level interrupt. The kernel generates such an interrupt only if the DPC is directed at the processor the ISR is requested on and the DPC has a high or medium priority. If the DPC has a low priority, the kernel requests the interrupt only if the number of outstanding DPC requests for the processor rises above a threshold or if the number of DPCs requested on the processor within a time window is low. If a DPC is targeted at a CPU different from the one on which the ISR is running and the DPC's priority is high, the kernel immediately signals the target CPU (by sending it a dispatch IPI) to drain its DPC queue. If the priority is medium or low, the number of DPCs queued on the target processor must exceed a threshold for the kernel to trigger a DPC/dispatch interrupt. The system idle thread also drains the DPC queue for the processor it runs on. Although DPC targeting and priority levels are flexible, device drivers rarely need to change the default behavior of their DPC objects. Table 3-1 summarizes the situations that initiate DPC queue draining.
Table 3-1 DPC Interrupt Generation Rules
| DPC Priority | DPC Targeted at ISR's Processor | DPC Targeted at Another Processor |
|---|---|---|
| Low | DPC queue length exceeds maximum DPC queue length or DPC request rate is less than minimum DPC request rate | DPC queue length exceeds maximum DPC queue length or System is idle |
| Medium | Always | DPC queue length exceeds maximum DPC queue length or System is idle |
| High | Always | Always |
Because user-mode threads execute at low IRQL, the chances are good that a DPC will interrupt the execution of an ordinary user's thread. DPC routines execute without regard to what thread is running, meaning that when a DPC routine runs, it can't assume what process address space is currently mapped. DPC routines can call kernel functions, but they can't call system services, generate page faults, or create or wait on dispatcher objects (explained later in this chapter). They can, however, access nonpaged system memory addresses, since system address space is always mapped regardless of what the current process is.
DPCs are provided primarily for device drivers, but the kernel uses them too. The kernel most frequently uses a DPC to handle quantum expiration. At every tick of the system clock, an interrupt occurs at clock IRQL. The clock interrupt handler (running at clock IRQL) updates the system time and then decrements a counter that tracks how long the current thread has run. When the counter reaches 0, the thread's time quantum has expired and the kernel might need to reschedule the processor, a lower-priority task that should be done at DPC/dispatch IRQL. The clock interrupt handler queues a DPC to initiate thread dispatching and then finishes its work and lowers the processor's IRQL. Because the DPC interrupt has a lower priority than do device interrupts, any pending device interrupts that surface before the clock interrupt completes are handled before the DPC interrupt occurs.
EXPERIMENT
Monitoring Interrupt and DPC ActivityUsing the Windows 2000 Performance tool, you can watch the percentage of time your system spends on handling interrupts and DPCs. The Processor object has % Interrupt Time and % DPC Time counters, including Total and per-processor instances, which means you can monitor the activity on a per-CPU or a systemwide basis. These objects also have counters to measure the number of interrupts and DPCs per second.
One situation in which you might want to look at these counters is if your system is spending an inordinate amount of time in kernel mode and you can't attribute all the kernel-mode CPU time to processes. If total kernel-mode CPU time is greater than the total kernel-mode CPU time of all processes, the remaining time has to be interrupts or DPCs because time spent at interrupt level and DPC level isn't charged to any thread's CPU time performance counter. Be aware that there are inherent inaccuracies in the way Windows 2000 accounts for CPU time, inaccuracies that relate to the granularity of the system timer. (See the section "Quantum Accounting" in Chapter 6 for an explanation of how time accounting works.)
Asynchronous procedure call (APC) interrupts Asynchronous procedure calls (APCs) provide a way for user programs and system code to execute in the context of a particular user thread (and hence a particular process address space). Because APCs are queued to execute in the context of a particular thread and run at an IRQL less than 2, they don't operate under the same restrictions as a DPC. An APC routine can acquire resources (objects), wait on object handles, incur page faults, and call system services.
APCs are described by a kernel control object, called an APC object. APCs waiting to execute reside in a kernel-managed APC queue. Unlike the DPC queue, which is systemwide, the APC queue is thread-specific—each thread has its own APC queue. When asked to queue an APC, the kernel inserts it into the queue belonging to the thread that will execute the APC routine. The kernel, in turn, requests a software interrupt at APC level, and when the thread eventually begins running, it executes the APC.
There are two kinds of APCs: kernel mode and user mode. Kernel-mode APCs don't require "permission" from a target thread to run in that thread's context, while user-mode APCs do. Kernel-mode APCs interrupt a thread and execute a procedure without the thread's intervention or consent.
The executive uses kernel-mode APCs to perform operating system work that must be completed within the address space (in the context) of a particular thread. It can use kernel-mode APCs to direct a thread to stop executing an interruptible system service, for example, or to record the results of an asynchronous I/O operation in a thread's address space. Environment subsystems use kernel-mode APCs to make a thread suspend or terminate itself or to get or set its user-mode execution context. The POSIX subsystem uses kernel-mode APCs to emulate the delivery of POSIX signals to POSIX processes.
Device drivers also use kernel-mode APCs. For example, if an I/O operation is initiated and a thread goes into a wait state, another thread in another process can be scheduled to run. When the device finishes transferring data, the I/O system must somehow get back into the context of the thread that initiated the I/O so that it can copy the results of the I/O operation to the buffer in the address space of the process containing that thread. The I/O system uses a kernel-mode APC to perform this action. (The use of APCs in the I/O system is discussed in more detail in Chapter 9.)
Several Win32 APIs, such as ReadFileEx, WriteFileEx, and QueueUserAPC, use user-mode APCs. For example, the ReadFileEx and WriteFileEx functions allow the caller to specify a completion routine to be called when the I/O operation finishes. The I/O completion is implemented by queueing an APC to the thread that issued the I/O. However, the callback to the completion routine doesn't necessarily take place when the APC is queued because user-mode APCs are delivered to a thread only when it's in an alertable wait state. A thread can enter a wait state either by waiting on an object handle and specifying that its wait is alertable (with the Win32 WaitForMultipleObjectsEx function) or by testing directly whether it has a pending APC (using SleepEx). In both cases, if a user-mode APC is pending, the kernel interrupts (alerts) the thread, transfers control to the APC routine, and resumes the thread's execution when the APC routine completes.
APC delivery can reorder the wait queues—the lists of which threads are waiting on what, and in what order they are waiting. (Wait resolution is described in the section "Executive Synchronization" later in this chapter.) If the thread is in a wait state when an APC is delivered, after the APC routine completes, the wait is reissued or reexecuted. If the wait still isn't resolved, the thread returns to the wait state, but now it will be at the end of the list of objects it's waiting on. For example, because APCs are used to suspend a thread from execution, if the thread is waiting on any objects, its wait will be removed until the thread is resumed, after which that thread will be at the end of the list of threads waiting to access the objects it was waiting on.
Exception Dispatching
In contrast to interrupts, which can occur at any time, exceptions are conditions that result directly from the execution of the program that is running. Win32 introduced a facility known as structured exception handling, which allows applications to gain control when exceptions occur. The application can then fix the condition and return to the place the exception occurred, unwind the stack (thus terminating execution of the subroutine that raised the exception), or declare back to the system that the exception isn't recognized and the system should continue searching for an exception handler that might process the exception. This section assumes you're familiar with the basic concepts behind Win32 structured exception handling—if you're not, you should read the overview in the Win32 API reference documentation on the Platform SDK or chapters 23-25 in Jeffrey Richter's book Programming Applications for Microsoft Windows (fourth edition, Microsoft Press, 2000) before proceeding. Keep in mind that although exception handling is made accessible through language extensions (for example, the __try construct in Microsoft Visual C++), it is a system mechanism and hence isn't language-specific. Other examples of consumers of Windows 2000 exception handling include C++ and Java exceptions.
On the x86, all exceptions have predefined interrupt numbers that directly correspond to the entry in the IDT that points to the trap handler for a particular exception. Table 3-2 shows x86-defined exceptions and their assigned interrupt numbers. Because the first entries of the IDT are used for exceptions, hardware interrupts are assigned entries later in the table, as mentioned earlier.
Table 3-2 x86 Exceptions and Their Interrupt Numbers
| Interrupt Number | Exception |
|---|---|
| 0 | Divide Error |
| 1 | DEBUG TRAP |
| 2 | NMI/NPX Error |
| 3 | Breakpoint |
| 4 | Overflow |
| 5 | BOUND/Print Screen |
| 6 | Invalid Opcode |
| 7 | NPX Not Available |
| 8 | Double Exception |
| 9 | NPX Segment Overrun |
| A | Invalid Task State Segment (TSS) |
| B | Segment Not Present |
| C | Stack Fault |
| D | General Protection |
| E | Page Fault |
| F | Intel Reserved |
| 10 | Floating Point |
| 11 | Alignment Check |
All exceptions, except those simple enough to be resolved by the trap handler, are serviced by a kernel module called the exception dispatcher. The exception dispatcher's job is to find an exception handler that can "dispose of" the exception. Examples of architecture-independent exceptions that the kernel defines include memory access violations, integer divide-by-zero, integer overflow, floating-point exceptions, and debugger breakpoints. For a complete list of architecture-independent exceptions, consult the Win32 API reference documentation.
The kernel traps and handles some of these exceptions transparently to user programs. For example, encountering a breakpoint while executing a program being debugged generates an exception, which the kernel handles by calling the debugger. The kernel handles certain other exceptions by returning an unsuccessful status code to the caller.
A few exceptions are allowed to filter back, untouched, to user mode. For example, a memory access violation or an arithmetic overflow generates an exception that the operating system doesn't handle. An environment subsystem can establish frame-based exception handlers to deal with these exceptions. The term frame-based refers to an exception handler's association with a particular procedure activation. When a procedure is invoked, a stack frame representing that activation of the procedure is pushed onto the stack. A stack frame can have one or more exception handlers associated with it, each of which protects a particular block of code in the source program. When an exception occurs, the kernel searches for an exception handler associated with the current stack frame. If none exists, the kernel searches for an exception handler associated with the previous stack frame, and so on, until it finds a frame-based exception handler. If no exception handler is found, the kernel calls its own default exception handlers.
When an exception occurs, whether it is explicitly raised by software or implicitly raised by hardware, a chain of events begins in the kernel. The CPU hardware transfers control to the kernel trap handler, which creates a trap frame (as it does when an interrupt occurs). The trap frame allows the system to resume where it left off if the exception is resolved. The trap handler also creates an exception record that contains the reason for the exception and other pertinent information.
If the exception occurred in kernel mode, the exception dispatcher simply calls a routine to locate a frame-based exception handler that will handle the exception. Because unhandled kernel-mode exceptions are considered fatal operating system errors, you can assume that the dispatcher always finds an exception handler.
If the exception occurred in user mode, the exception dispatcher does something more elaborate. As you'll see in Chapter 6, the Win32 subsystem has a debugger port and an exception port to receive notification of user-mode exceptions in Win32 processes. The kernel uses these in its default exception handling, as illustrated in Figure 3-6.
Figure 3-6 Dispatching an exception
Debugger breakpoints are common sources of exceptions. Therefore, the first action the exception dispatcher takes is to see whether the process that incurred the exception has an associated debugger process. If so, it sends the first-chance debug message (via an LPC port) to the debugger port associated with the process that incurred the exception. (The message is sent to the session manager process, which then dispatches it to the appropriate debugger process.)
If the process has no debugger process attached, or if the debugger doesn't handle the exception, the exception dispatcher switches into user mode and calls a routine to find a frame-based exception handler. If none is found, or if none handles the exception, the exception dispatcher switches back into kernel mode and calls the debugger again to allow the user to do more debugging. (This is called the second-chance notification.)
All Win32 threads have an exception handler declared at the top of the stack that processes unhandled exceptions. This exception handler is declared in the internal Win32 start-of-process or start-of-thread function. The start-of-process function runs when the first thread in a process begins execution. It calls the main entry point in the image. The start-of-thread function runs when a user creates additional threads. It calls the user-supplied thread start routine specified in the CreateThread call.
EXPERIMENT
Viewing the Real User Start Address for Win32 ThreadsThe fact that each Win32 thread begins execution in a system-supplied function (and not the user-supplied function) explains why the start address for thread 0 is the same for every Win32 process in the system (and why the start addresses for secondary threads are also the same). The start address for thread 0 in Win32 processes is the Win32 start-of-process function; the start address for any other threads would be the Win32 start-of-thread function. To see the user-supplied function address, use the Tlist utility in the Windows 2000 Support Tools. Type tlist process-name or tlist process-id to get the detailed process output that includes this information. For example, compare the thread start addresses for the Windows Explorer process as reported by Pstat (in the Platform SDK) and Tlist:
The start address of thread 0 reported by Pstat is the internal Win32 start-of-process function; the start addresses for threads 1 through 3 are the internal Win32 start-of-thread functions. Tlist, on the other hand, shows the user-supplied Win32 start address (the user function called by the internal Win32 start function).
The generic code for these internal start functions is shown here:
void Win32StartOfProcess( LPTHREAD_START_ROUTINE lpStartAddr, LPVOID lpvThreadParm){ __try { DWORD dwThreadExitCode = lpStartAddr(lpvThreadParm); ExitThread(dwThreadExitCode); } __except(UnhandledExceptionFilter( GetExceptionInformation())) { ExitProcess(GetExceptionCode()); } } |
Notice that the Win32 unhandled exception filter is called if the thread has an exception that it doesn't handle. This function looks in the registry in the HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\AeDebug key to determine whether to run a debugger immediately or to ask the user first.
The default "debugger" on Windows 2000 is \Winnt\System32\Drwtsn32.exe (Dr. Watson), which isn't really a debugger but rather a postmortem tool that captures the state of the application "crash" and records it in a log file (Drwtsn32.log) and a process crash dump file (User.dmp), both found by default in the \Documents And Settings\All Users\Documents\DrWatson folder. To see (or modify) the configuration for Dr. Watson, run it interactively—it displays a window with the current settings, as shown in Figure 3-7.
The log file contains basic information such as the exception code, the name of the image that failed, a list of loaded DLLs, and a stack and instruction trace for the thread that incurred the exception. For a detailed description of the contents of the log file, run Dr. Watson and click the Help button shown in Figure 3-7.
Figure 3-7 Dr. Watson default settings
The crash dump file contains the private pages in the process at the time of the exception. (The file doesn't include code pages from EXEs or DLLs.) This file can be opened by WinDbg, the Windows debugger that comes with the Windows 2000 debugging tools package (which is part of Windows 2000 Customer Support Diagnostics, Platform SDK, and DDK). Because the User.dmp file is overwritten each time a process crashes, unless you rename or copy the file after each process crash, you'll have only the latest one on your system.
If you install one of the Microsoft Visual Studio compilers, the Debugger value of the AeDebug registry key is changed to Msdev.exe (including the path), so that you can debug programs that incur unhandled exceptions. Another product that changes the Debugger value is Lotus Notes—it runs a Notes-specific postmortem tool named Qnc.exe.
If the debugger isn't running and no frame-based handlers are found, the kernel sends a message to the exception port associated with the thread's process. This exception port, if one exists, was registered by the environment subsystem that controls this thread. The exception port gives the environment subsystem, which presumably is listening at the port, the opportunity to translate the exception into an environment-specific signal or exception. For example, when POSIX gets a message from the kernel that one of its threads generated an exception, the POSIX subsystem sends a POSIX-style signal to the thread that caused the exception. However, if the kernel progresses this far in processing the exception and the subsystem doesn't handle the exception, the kernel executes a default exception handler that simply terminates the process whose thread caused the exception.
EXPERIMENT
Unhandled ExceptionsTo see an example Dr. Watson log file, run the program \Tools\Accvio.exe from this book's companion CD. This program generates a memory access violation by attempting to write to address 0, which is always an invalid address in Win32 processes. (See Table 7-6 in Chapter 7.)
- Run the Registry Editor, and locate HKLM\SOFTWARE\ Microsoft\Windows NT\CurrentVersion\AeDebug.
- If the Debugger value is "drwtsn32 -p %ld -e %ld -g," your system is set up to run Dr. Watson as the default debugger. Proceed to step 4.
- If the value of Debugger was not set up to run Drwtsn32.exe, you can still test Dr. Watson by temporarily installing it and then restoring your previous debugger settings:
- Save the current value somewhere (for example, in a Notepad file or in the current paste buffer).
- Select Run from the taskbar Start menu, and then enter drwtsn32 -i. (This initializes the Debugger field to run Dr. Watson.)
- Run the test program \Tools\Accvio.exe.
- The Program Error message box should come up—when the log file and crash dump have been created, the button will change from Cancel to OK. (Note: This error message occurs by default only on Windows 2000 Professional installations of Dr. Watson—Windows 2000 Server by default will not show a message box, but the dump files will still be created. You also might see a different error message if Dr. Watson is not your default debugger.)
- Click OK to dismiss the message box.
- Run Drwtsn32.exe. (Select Run from the Start menu, and then enter drwtsn32.)
- In the list of Application Errors, click on the last entry and then click the View button—the portion of the Dr. Watson log file containing the details of the access violation from Accvio.exe will be displayed. (For details on the log file format, press Help in the Dr. Watson For Windows 2000 dialog box and select Dr. Watson Log File Overview.)
- If the original value of Debugger wasn't the default Dr. Watson settings, restore the saved value from step 1.
As another experiment, try changing the value of Debugger to another program, such as Notepad.exe (Notepad editor) or Sol.exe (Solitaire). Rerun Accvio.exe, and notice that whatever program is specified in the Debugger value is run—that is, there's no validation that the program defined in Debugger is actually a debugger. Make sure you restore your registry settings. (As noted in step 3b, to reset to the system default Dr. Watson settings, type drwtsn32 -i in the Run dialog box or at a command prompt.)
System Service Dispatching
As Figure 3-1 illustrated, the kernel's trap handlers dispatch interrupts, exceptions, and system service calls. In the preceding sections, you've seen how interrupt and exception handling work; in this section, you'll learn about system services. A system service dispatch is triggered as a result of executing an int 0x2e instruction (46 decimal) on x86 processors. Because executing the int instruction results in a trap, Windows 2000 fills in entry 46 in the IDT to point to the system service dispatcher. (Refer to Table 3-1.) The trap causes the executing thread to transition into kernel mode and enter the system service dispatcher. A numeric argument passed in the EAX processor register indicates the system service number being requested. The EBX register points to the list of parameters the caller passes to the system service. The following code illustrates the generic code for a system service request:
NtWriteFile: mov eax, 0x0E ; build 2195 system service number for NtWriteFile mov ebx, esp ; point to parameters int 0x2E ; execute system service trap ret 0x2C ; pop parameters off stack and return to caller |
As Figure 3-8 illustrates, the kernel uses this argument to locate the system service information in the system service dispatch table. This table is similar to the interrupt dispatch table described earlier in the chapter except that each entry contains a pointer to a system service rather than to an interrupt handling routine.
NOTE
System service numbers can change between service packs—Microsoft occasionally adds or removes system services, and the system service numbers are generated automatically as part of a kernel compile.
Figure 3-8 System service exceptions
The system service dispatcher, KiSystemService, verifies the correct minimum number of arguments, copies the caller's arguments from the thread's user-mode stack to its kernel-mode stack (so that the user can't change the arguments as the kernel is accessing them), and then executes the system service. If the arguments passed to a system service point to buffers in user space, these buffers must be probed for accessibility before kernel-mode code can copy data to or from them.
As you'll see in Chapter 6, each thread has a pointer to its system service table. Windows 2000 has two built-in system service tables, but up to four are supported. The system service dispatcher determines which table contains the requested service by interpreting a 2-bit field in the 32-bit system service number as a table index. The low 12 bits of the system service number serve as the index into the table specified by the table index. The fields are shown in Figure 3-9.
Figure 3-9 System service number to system service translation
A primary default array table, KeServiceDescriptorTable, defines the core executive system services implemented in Ntosrknl.exe. The other table array, KeServiceDescriptorTableShadow, includes the Win32 USER and GDI services implemented in the kernel-mode part of the Win32 subsystem, Win32k.sys. The first time a Win32 thread calls a Win32 USER or GDI service, the address of the thread's system service table is changed to point to a table that includes the Win32 USER and GDI services. The KeAddSystemServiceTable function allows Win32k.sys and other device drivers to add system service tables. If you install Internet Information Services (IIS) on Windows 2000, its support driver (Spud.sys) upon loading defines an additional service table, leaving only one left for definition by third parties. With the exception of the Win32k.sys service table, a service table added with KeAddSystemServiceTable is copied into both the KeServiceDescriptorTable array and the KeServiceDescriptorTableShadow array.
The system service dispatch instructions for Windows 2000 executive services exist in the system library Ntdll.dll. Subsystem DLLs call functions in Ntdll to implement their documented functions. The exception is Win32 USER and GDI functions, in which the system service dispatch instructions are implemented directly in User32.dll and Gdi32.dll—there is no Ntdll.dll. These two cases are shown in Figure 3-10.
Figure 3-10 System service dispatching
As shown in Figure 3-10, the Win32 WriteFile function in Kernel32.dll calls the NtWriteFile function in Ntdll.dll, which in turn executes the appropriate instruction to cause a system service trap, passing the system service number representing NtWriteFile. The system service dispatcher (function KiSystemService in Ntoskrnl.exe) then calls the real NtWriteFile to process the I/O request. For Win32 USER and GDI functions, the system service dispatch calls functions in the loadable kernel-mode part of the Win32 subsystem, Win32k.sys.
EXPERIMENT
Viewing System Service ActivityYou can monitor system service activity by watching the System Calls/Sec performance counter in the System object. Run the Performance tool, and in chart view, click the Add button to add a counter to the chart; select the System object, select the System Calls/Sec counter, and then click the Add button to add the counter to the chart.
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Pages: 121
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