IRQL备忘

IRQL是Interrupt ReQuest Level，中断请求级别。一个由windows虚拟出来的概念，划分在windows下中断的优先级，这里中断包括了硬中断和软中断，硬中断是由硬件产生，而软中断则是完全虚拟出来的。

　　处理器在一个IRQL上执行线程代码。IRQL是帮助决定线程如何被中断的。在同一处理器上，线程只能被更高级别IRQL的线程能中断。每个处理器都有自己的中断IRQL。

　　我们在调用NDIS API时，在DDK帮助文档中都有该API函数的所在级别。

PASSIVE_LEVEL

　　IRQL最低级别,没有被屏蔽的中断，在这个级别上，线程执行用户模式，可以访问分页内存。

APC_LEVEL

　　在这个级别上,只有APC级别的中断被屏蔽，可以访问分页内存。当有APC发生时，处理器提升到APC级别，这样，就屏蔽掉其它APC，为了和APC执行一些同步，驱动程序可以手动提升到这个级别。比如，如果提升到这个级别，APC就不能调用。在这个级别，APC被禁止了，导致禁止一些I/O完成APC，所以有一些API不能调用。

DISPATCH_LEVEL

　　这个级别，DPC 和更低的中断被屏蔽，不能访问分页内存，所有的被访问的内存不能分页。因为只能处理非分页内存，所以在这个级别，能够访问的Api大大减少。

DIRQL (Device IRQL)

　　一般的，更高级的驱动在这个级别上不处理IRQL，但是几乎所有的中断被屏蔽，这实际上是IRQL的一个范围，这是一个决定某个驱动有更高的优先级的方法。

The IRQL at which a driver routine executes determines which kernel-mode driver support routines it can call. For example, some driver support routines require that the caller be running at IRQL = DISPATCH_LEVEL. Others cannot be called safely if the caller is running at any IRQL higher than PASSIVE_LEVEL.

Following is a list of IRQLs at which the most commonly implemented standard driver routines are called. The IRQLs are listed from lowest to highest priority.

PASSIVE_LEVEL

Interrupts Masked Off — None.

Driver Routines Called at PASSIVE_LEVEL — DriverEntry, AddDevice, Reinitialize, Unload routines, most dispatch routines, driver-created threads, worker-thread callbacks.

APC_LEVEL

Interrupts Masked Off — APC_LEVEL interrupts are masked off.

Driver Routines Called at APC_LEVEL — Some dispatch routines (see Dispatch Routines and IRQLs).

DISPATCH_LEVEL

Interrupts Masked Off — DISPATCH_LEVEL and APC_LEVEL interrupts are masked off. Device, clock, and power failure interrupts can occur.

Driver Routines Called at DISPATCH_LEVEL — StartIo, AdapterControl, AdapterListControl, ControllerControl, IoTimer, Cancel(while holding the cancel spin lock), DpcForIsr, CustomTimerDpc, CustomDpc routines.

DIRQL

Interrupts Masked Off — All interrupts at IRQL<= DIRQL of driver's interrupt object. Device interrupts with a higher DIRQL value can occur, along with clock and power failure interrupts.

Driver Routines Called at DIRQL — InterruptService, SynchCritSection routines.

The only difference between APC_LEVEL and PASSIVE_LEVEL is that a process executing at APC_LEVEL cannot get APC interrupts. But both IRQLs imply a thread context and both imply that the code can be paged out.

Lowest-level drivers process IRPs while running at one of three IRQLs:

• PASSIVE_LEVEL, with no interrupts masked off on the processor, in the driver's Dispatch routine(s)

  DriverEntry, AddDevice, Reinitialize, and Unload routines also are run at PASSIVE_LEVEL, as are any driver-created system threads.

• DISPATCH_LEVEL, with DISPATCH_LEVEL and APC_LEVEL interrupts masked off on the processor, in the StartIoroutine

  AdapterControl, AdapterListControl, ControllerControl, IoTimer, Cancel (while it holds the cancel spin lock), andCustomTimerDpc routines also are run at DISPATCH_LEVEL, as are DpcForIsr and CustomDpc routines.

• Device IRQL (DIRQL), with all interrupts at less than or equal to the SynchronizeIrql of the driver's interrupt object(s) masked off on the processor, in the ISR and SynchCritSection routines

Most higher-level drivers process IRPs while running at either of two IRQLs:

• PASSIVE_LEVEL, with no interrupts masked off on the processor, in the driver's dispatch routines

  DriverEntry, Reinitialize, AddDevice, and Unload routines also are run at PASSIVE_LEVEL, as are any driver-created system threads or worker-thread callback routines or file system drivers.

• DISPATCH_LEVEL, with DISPATCH_LEVEL and APC_LEVEL interrupts masked off on the processor, in the driver'sIoCompletion routine(s)

  IoTimer, Cancel, and CustomTimerDpc routines also are run at DISPATCH_LEVEL.

In some circumstances, intermediate and lowest-level drivers of mass-storage devices are called at IRQL APC_LEVEL. In particular, this can occur at a page fault for which a file system driver sends an IRP_MJ_READ request to lower drivers.

Most standard driver routines are run at an IRQL that allows them simply to call the appropriate support routines. For example, a device driver must call AllocateAdapterChannel while running at IRQL DISPATCH_LEVEL. Since most device drivers call these routines from a StartIo routine, usually they are running at DISPATCH_LEVEL already.

Note that a device driver that has no StartIo routine because it sets up and manages its own queues of IRPs is not necessarily running at DISPATCH_LEVEL IRQL when it should call AllocateAdapterChannel. Such a driver must nest its call toAllocateAdapterChannel between calls to KeRaiseIrql and KeLowerIrql so that it runs at the required IRQL when it callsAllocateAdapterChannel and restores the original IRQL when the calling routine regains control.

When calling driver support routines, be aware of the following.

• Calling KeRaiseIrql with an input NewIrql value that is less than the current IRQL causes a fatal error. CallingKeLowerIrql except to restore the original IRQL (that is, after a call to KeRaiseIrql) also causes a fatal error.
• While running at IRQL >= DISPATCH_LEVEL, calling KeWaitForSingleObject or KeWaitForMultipleObjects for kernel-defined dispatcher objects to wait for a nonzero interval causes a fatal error.
• The only driver routines that can safely wait for events, semaphores, mutexes, or timers to be set to the signaled state are those that run in a nonarbitrary thread context at IRQL PASSIVE_LEVEL, such as driver-created threads, the DriverEntry and Reinitialize routines, or dispatch routines for inherently synchronous I/O operations (such as most device I/O control requests).
• Even while running at IRQL PASSIVE_LEVEL, pageable driver code must not call KeSetEvent, KeReleaseSemaphore, or KeReleaseMutex with the input Wait parameter set to TRUE. Such a call can cause a fatal page fault.
• Any routine that is running at greater than IRQL APC_LEVEL can neither allocate memory from paged pool nor access memory in paged pool safely. If a routine running at IRQL greater than APC_LEVEL causes a page fault, it is a fatal error.

• A driver must be running at IRQL DISPATCH_LEVEL when it calls KeAcquireSpinLockAtDpcLevel andKeReleaseSpinLockFromDpcLevel.

  A driver can be running at IRQL <= DISPATCH_LEVEL when it calls KeAcquireSpinLock but it must release that spin lock by calling KeReleaseSpinLock. In other words, it is a programming error to release a spin lock acquired withKeAcquireSpinLock by calling KeReleaseSpinLockFromDpcLevel.

  A driver must not call KeAcquireSpinLockAtDpcLevel, KeReleaseSpinLockFromDpcLevel, KeAcquireSpinLock, orKeReleaseSpinLock while running at IRQL>DISPATCH_LEVEL.

• Calling a support routine that uses a spin lock, such as an ExInterlockedXxx routine, raises IRQL on the current processor either to DISPATCH_LEVEL or to DIRQL if the caller is not already running at a raised IRQL.
• Driver code that runs at IRQL > PASSIVE_LEVEL should execute as quickly as possible. The higher the IRQL at which a routine runs, the more important it is for good overall performance to tune that routine to execute as quickly as possible. For example, any driver that calls KeRaiseIrql should make the reciprocal call to KeLowerIrql as soon as it can.

For more information about determining priorities, see the  Scheduling, Thread Context, and IRQL white paper that is available on the Microsoft Windows Hardware Developer Central (WHDC) Web site.