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Do not copy XState other than AVX when redirecting for GC stress #65825

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Merged
VSadov merged 6 commits into from
Feb 25, 2022
Merged

Do not copy XState other than AVX when redirecting for GC stress #65825

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0ccc1c0
Do not copy XState other than AVX
VSadov Feb 24, 2022
95204ea
#if defined(TARGET_X86) || defined(TARGET_AMD64)
VSadov Feb 24, 2022
7951248
mask XState unconditionally
VSadov Feb 24, 2022
d531d3d
Ensure XSTATE_MASK_AVX is set before calling EEGetThreadContext
VSadov Feb 24, 2022
c552ae2
redundant supportsAVX, more clear comment
VSadov Feb 24, 2022
ef7bcdd
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VSadov Feb 24, 2022
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43 changes: 32 additions & 11 deletions src/coreclr/vm/threadsuspend.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -1997,15 +1997,6 @@ CONTEXT* AllocateOSContextHelper(BYTE** contextBuffer)
pfnInitializeContext2(buffer, context, &pOSContext, &contextSize, xStateCompactionMask):
InitializeContext(buffer, context, &pOSContext, &contextSize);

// if AVX is supported set the appropriate features mask in the context
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@janvorli janvorli Feb 24, 2022

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With this removed, the code above that sets the supportAVX based on the FeatureMask can be removed too.

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@VSadov VSadov Feb 24, 2022

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Right, even before supportAVX was redundant, since SetXStateFeaturesMask performs the same check - to filter out what is not supported.

if (success && supportsAVX)
{
// This should not normally fail.
// The system silently ignores any feature specified in the FeatureMask
// which is not enabled on the processor.
success = SetXStateFeaturesMask(pOSContext, XSTATE_MASK_AVX);
}

if (!success)
{
delete[] buffer;
Expand Down Expand Up @@ -2912,9 +2903,23 @@ BOOL Thread::RedirectThreadAtHandledJITCase(PFN_REDIRECTTARGET pTgt)

//////////////////////////////////////
// Get and save the thread's context
BOOL bRes = true;

// Always get complete context, pCtx->ContextFlags are set during Initialization
BOOL bRes = EEGetThreadContext(this, pCtx);

#if defined(TARGET_X86) || defined(TARGET_AMD64)
// Scenarios like GC stress may indirectly disable XState features in the pCtx
// depending on the state at the time of GC stress interrupt.
//
// Make sure that AVX feature mask is set, if supported.
//
// This should not normally fail.
// The system silently ignores any feature specified in the FeatureMask
// which is not enabled on the processor.
bRes &= SetXStateFeaturesMask(pCtx, XSTATE_MASK_AVX);
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@janvorli janvorli Feb 24, 2022

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I only have one nit - I would prefer failures / asserts on each of the failure instead of combining the results together.

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@VSadov VSadov Feb 24, 2022 •
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GetXStateFeaturesMask and SetXStateFeaturesMask basically cannot fail. They are very tolerant to the input.
To get failures you'd need something like pass context flags for a wrong architecture (like arm64). As-is that would not compile.

Realistically only CopyContext is an API with complex requirements and validation that could fail here due to some mistake.
I think in this particular case individual asserts/returns would not add a lot of value.

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@janvorli janvorli Feb 24, 2022

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I've just found those would fail also when passed context without any XSTATE present. It seems like something that could possibly happen for the usage in Thread::RedirectCurrentThreadAtHandledJITCase where we pass it a general context that we get from the vectored exception handler. So it seems that it would be nice to actually make that work in that case.

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@VSadov VSadov Feb 24, 2022

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Hmm, my read on that was that Set(Get) would be a noop, but I think you are right, if no XState is set it may fail. Arguably a bug in the validation of Set not accepting 0.

Yes, we should make this case work.

#endif //defined(TARGET_X86) || defined(TARGET_AMD64)

bRes &= EEGetThreadContext(this, pCtx);
_ASSERTE(bRes && "Failed to GetThreadContext in RedirectThreadAtHandledJITCase - aborting redirect.");

if (!bRes)
Expand Down Expand Up @@ -3029,9 +3034,25 @@ BOOL Thread::RedirectCurrentThreadAtHandledJITCase(PFN_REDIRECTTARGET pTgt, CONT

//////////////////////////////////////
// Get and save the thread's context
BOOL success = CopyContext(pCtx, pCtx->ContextFlags, pCurrentThreadCtx);
BOOL success = true;

#if defined(TARGET_X86) || defined(TARGET_AMD64)
// This method is called for GC stress interrupts in managed code.
// The current context may have various XState features, depending on what is used/dirty,
// but only AVX feature may contain live data. (that could change in the future)
// Besides pCtx may not have space to store other features.
// So we will mask out everything but AVX.
DWORD64 srcFeatures = 0;
success &= GetXStateFeaturesMask(pCurrentThreadCtx, &srcFeatures);
success &= SetXStateFeaturesMask(pCurrentThreadCtx, srcFeatures & XSTATE_MASK_AVX);
#endif //defined(TARGET_X86) || defined(TARGET_AMD64)

success &= CopyContext(pCtx, pCtx->ContextFlags, pCurrentThreadCtx);
_ASSERTE(success);

if (!success)
return FALSE;

// Ensure that this flag is set for the next time through the normal path,
// RedirectThreadAtHandledJITCase.
pCtx->ContextFlags |= CONTEXT_EXCEPTION_REQUEST;
Expand Down