Re: [PATCH] UefiCpuPkg/PiSmmCpuDxeSmm: Add volatile for mNumberToFinish

["Gao, Liming" <liming.gao@intel.com>]

Jeff:
  From those API design point, does they expect the input parameter is volatile? If yes, I agree this change.

Thanks
Liming
From: Fan, Jeff
Sent: Tuesday, November 15, 2016 11:03 AM
To: Laszlo Ersek <lersek@redhat.com>; edk2-devel@ml01.01.org
Cc: Paolo Bonzini <pbonzini@redhat.com>; Yao, Jiewen <jiewen.yao@intel.com>; Tian, Feng <feng.tian@intel.com>; Kinney, Michael D <michael.d.kinney@intel.com>; Gao, Liming <liming.gao@intel.com>
Subject: RE: [PATCH] UefiCpuPkg/PiSmmCpuDxeSmm: Add volatile for mNumberToFinish

Laszlo,

I agree to separate another patch to change the prototype of TransferApToSafeState() to reduce one cast operation. I will create v2 for it.

I also agree updating prototype of InterlockedDecrement() is compatible updating. But there are other 5 APIs as blow:
InterlockedIncrement()
InterlockedCompareExchange16()
InterlockedCompareExchange32()
InterlockedCompareExchange64()
InterlockedCompareExchangePointer()

To be consistence, we may need to update them together.

Liming & MIke, any comments on this updating.

Thanks!
Jeff

-----Original Message-----
From: Laszlo Ersek [mailto:lersek@redhat.com]
Sent: Tuesday, November 15, 2016 10:27 AM
To: Fan, Jeff; edk2-devel@ml01.01.org<mailto:edk2-devel@ml01.01.org>
Cc: Paolo Bonzini; Yao, Jiewen; Tian, Feng; Kinney, Michael D
Subject: Re: [PATCH] UefiCpuPkg/PiSmmCpuDxeSmm: Add volatile for mNumberToFinish

On 11/15/16 03:18, Jeff Fan wrote:
> The GCC 5.4 will optimize mNumberToFinish in EarlyInitializeCpu(). It
> will cause
> S3 resume failure.
>
> Adding *volatile* could make sure compiler does not so such optimization.
>
> Cc: Paolo Bonzini
> Cc: Laszlo Ersek
> Cc: Jiewen Yao
> Cc: Feng Tian
> Cc: Michael D Kinney
> Contributed-under: TianoCore Contribution Agreement 1.0
> Signed-off-by: Jeff Fan
> ---
> UefiCpuPkg/PiSmmCpuDxeSmm/CpuS3.c | 6 +++---
> 1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/UefiCpuPkg/PiSmmCpuDxeSmm/CpuS3.c
> b/UefiCpuPkg/PiSmmCpuDxeSmm/CpuS3.c
> index 3fb6864..f13ff3e 100644
> --- a/UefiCpuPkg/PiSmmCpuDxeSmm/CpuS3.c
> +++ b/UefiCpuPkg/PiSmmCpuDxeSmm/CpuS3.c
> @@ -55,7 +55,7 @@ AsmGetAddressMap (
> #define LEGACY_REGION_BASE (0xA0000 - LEGACY_REGION_SIZE)
>
> ACPI_CPU_DATA mAcpiCpuData;
> -UINT32 mNumberToFinish;
> +volatile UINT32 mNumberToFinish;
> MP_CPU_EXCHANGE_INFO *mExchangeInfo;
> BOOLEAN mRestoreSmmConfigurationInS3 = FALSE;
> VOID *mGdtForAp = NULL;
> @@ -371,7 +371,7 @@ EarlyMPRendezvousProcedure (
> //
> // Count down the number with lock mechanism.
> //
> - InterlockedDecrement (&mNumberToFinish);
> + InterlockedDecrement ((UINT32 *) &mNumberToFinish);
> }
>
> /**
> @@ -406,7 +406,7 @@ MPRendezvousProcedure (
> TopOfStack = (UINT32) (UINTN) Stack + sizeof (Stack);
> TopOfStack &= ~(UINT32) (CPU_STACK_ALIGNMENT - 1);
> CopyMem ((VOID *) (UINTN) mApHltLoopCode, mApHltLoopCodeTemplate,
> sizeof (mApHltLoopCodeTemplate));
> - TransferApToSafeState ((UINT32) (UINTN) mApHltLoopCode, TopOfStack,
> &mNumberToFinish);
> + TransferApToSafeState ((UINT32) (UINTN) mApHltLoopCode, TopOfStack,
> + (UINT32 *) &mNumberToFinish);
> }
>
> /**
>

I think I understand the idea, but the current solution requires you to cast away "volatile" in two places. That's not nice, normally it is undefined behavior.

I recommend to extend this patch, with more patches: please change the prototype of TransferApToSafeState() so that it takes a pointer-to-volatile.

I also suggest to change the prototype of InterlockedDecrement(). (You won't have to update all other call sites: it is fine to take/access a non-volatile object as a volatile, but not the other way around.)

I agree this increases the scope of the patch quite a bit, so maybe others should chime in as well.

Thanks!
Laszlo