KERNEL-SIDE DEBUG in Visual Studio .NET

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KERNEL-SIDE DEBUG
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Example code for testing the capturing of hardware exceptions and panic events can be found here:
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\base\e32utils\d_exc\d_exc.mmp
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14.3.1.4 Context switch event
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To complete the set of noti cations available to a debug agent, the kernel provides a context switch event. This event is triggered on every change to the currently scheduled thread. This noti cation is particularly useful for the implementation of software pro lers for example, to record the length of time spent in each thread. In this API, we wanted to allow a device driver or extension to provide a callback function that would be called by the scheduler after every context switch. During the implementation, we were conscious that this was impacting a critical part of the scheduler and we were not prepared to compromise its performance. The cost for a typical implementation of the callback mechanism on an ARM processor would be three instructions: 1. 2. 3. Load the function pointer for the callback Compare it to NULL Execute the callback if non-NULL.
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This three-instruction cost is paid at every call to the scheduler, even if the callback function is not provided. To work around this performance impact, we devised an alternative mechanism for installing the callback function: the kernel provides two implementations of the scheduler code segment affected by the callback. The kernel also provides functions to replace the fast version (with no callback) with the slower version that supports the callback hook. The kernel publishes the following functions:
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NKern::SchedulerHooks(TLinAddr &start, TLinAddr &end)
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This returns the address in the scheduler where the callback trampoline should be placed. (This is located towards the end of the scheduler, after it has selected the new thread to run.)
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NKern::InsertSchedulerHooks()
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This is the function for patching-in the callback trampoline. It constructs a branch-and-link instruction to the callback trampoline and inserts it at
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THE KERNEL DEBUG INTERFACE
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the address returned by NKern::SchedulerHooks. It performs a write to an instruction in the scheduler code, which is usually in ROM so you must shadow the ROM rst. In the general case in which no software is using the event, this implementation has zero speed overhead. Because of this, the API is a little more awkward to use, but this is clearly a compromise worth making for an API that has only a few specialized clients, and affects such a performance critical area of the nanokernel. Tools that wish to use the context switch event are responsible for shadowing the appropriate area of scheduler code, and calling the function to patch the scheduler. Let s look at some example code. First we install the scheduler hooks by shadowing the appropriate area of ROM, and then we call the NKern::InsertSchedulerHooks() function to patch the scheduler:
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TInt InsertSchedulerHooks() { // Get range of memory used by hooks TLinAddr start,end; NKern::SchedulerHooks(start,end); // Create shadow pages for hooks TUint32 pageSize=Kern::RoundToPageSize(1); for(TLinAddr a=start; a<end; a+=pageSize) { NKern::ThreadEnterCS(); TInt r=Epoc::AllocShadowPage(a); NKern::ThreadLeaveCS(); if(r!=KErrNone && r!=KErrAlreadyExists) { RemoveSchedulerHooks(); return r; } } // Put hooks in NKern::InsertSchedulerHooks(); // Make I and D caches consistent for hook region Cache::IMB_Range(start,end-start); return KErrNone; }
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Now the hooks are installed, we can ask to be called back on every context switch:
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// Ask for callback NKern::SetRescheduleCallback(MyRescheduleCallback);
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KERNEL-SIDE DEBUG
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The callback function that you provide must have the following prototype:
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void MyRescheduleCallback(NThread* aNThread);
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You can temporarily disable context switch callbacks by passing a NULL function pointer to NKern::SetRescheduleCallback(). To completely remove the rescheduling hooks, we do the following:
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void RemoveSchedulerHooks() { // Prevent rescheduling whilst we // disable the callback NKern::Lock(); // Disable Callback NKern::SetRescheduleCallback(NULL); // Invalidate CurrentThread CurrentThread() = NULL; // Callback now disabled... NKern::Unlock(); // Get range of memory used by hooks TLinAddr start,end; NKern::SchedulerHooks(start,end); // Free shadow pages which cover hooks TUint32 pageSize=Kern::RoundToPageSize(1); NKern::ThreadEnterCS(); for(TLinAddr a=start; a<end; a+=pageSize) Epoc::FreeShadowPage(a); NKern::ThreadLeaveCS(); }
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Your callback function will be called with the kernel preemption lock held, during a reschedule. (For more information on rescheduling see Section 3.6.) As I ve said, this area of code is performance sensitive, and your callback function should therefore use as few processor cycles as possible. The callback function you provide should be of the type TRescheduleCallback (see INCLUDE\NKERN\NKERN.H). The kernel passes a pointer to the newly scheduled NThread as a parameter to your function. In some cases you will need to nd the Symbian OS thread that corresponds to this nanokernel thread. You can construct it as follows:
DThread* pT = _LOFF(aNThread, DThread, iNThread);