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

["Fan, Jeff" <jeff.fan@intel.com>]

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
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 <pbonzini@redhat.com>
> Cc: Laszlo Ersek <lersek@redhat.com>
> Cc: Jiewen Yao <jiewen.yao@intel.com>
> Cc: Feng Tian <feng.tian@intel.com>
> Cc: Michael D Kinney <michael.d.kinney@intel.com>
> Contributed-under: TianoCore Contribution Agreement 1.0
> Signed-off-by: Jeff Fan <jeff.fan@intel.com>
> ---
>  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