Re: [PATCH] UefiCpuPkg/PiSmmCpu: Don't allocate Token for SmmStartupThisAp

["Laszlo Ersek" <lersek@redhat.com>]

On 03/08/21 03:16, Ray Ni wrote:
> REF: https://bugzilla.tianocore.org/show_bug.cgi?id=3199
> 
> When Token points to mSmmStartupThisApToken, this routine is called
> from SmmStartupThisAp() in non-blocking mode due to
> PcdCpuSmmBlockStartupThisAp == FALSE.
> 
> In this case, caller wants to startup AP procedure in non-blocking
> mode and cannot get the completion status from the Token because there
> is no way to return the Token to caller from SmmStartupThisAp().
> Caller needs to use its specific way to query the completion status.
> 
> There is no need to allocate a token for such case so the 2 overheads
> can be avoided:
> 1. Call AllocateTokenBuffer() when there is no free token.
> 2. Get a free token from the token buffer.
> 
> Signed-off-by: Ray Ni <ray.ni@intel.com>
> Cc: Eric Dong <eric.dong@intel.com>
> Cc: Laszlo Ersek <lersek@redhat.com>
> Cc: Rahul Kumar <rahul1.kumar@intel.com>
> ---
>  UefiCpuPkg/PiSmmCpuDxeSmm/MpService.c | 27 ++++++++++++++++++++-------
>  1 file changed, 20 insertions(+), 7 deletions(-)
> 
> diff --git a/UefiCpuPkg/PiSmmCpuDxeSmm/MpService.c b/UefiCpuPkg/PiSmmCpuDxeSmm/MpService.c
> index 6227b2428a..efb89832ca 100644
> --- a/UefiCpuPkg/PiSmmCpuDxeSmm/MpService.c
> +++ b/UefiCpuPkg/PiSmmCpuDxeSmm/MpService.c
> @@ -1,7 +1,7 @@
>  /** @file
>  SMM MP service implementation
>  
> -Copyright (c) 2009 - 2020, Intel Corporation. All rights reserved.<BR>
> +Copyright (c) 2009 - 2021, Intel Corporation. All rights reserved.<BR>
>  Copyright (c) 2017, AMD Incorporated. All rights reserved.<BR>
>  
>  SPDX-License-Identifier: BSD-2-Clause-Patent
> @@ -22,6 +22,7 @@ UINTN                                       mSemaphoreSize;
>  SPIN_LOCK                                   *mPFLock = NULL;
>  SMM_CPU_SYNC_MODE                           mCpuSmmSyncMode;
>  BOOLEAN                                     mMachineCheckSupported = FALSE;
> +MM_COMPLETION                               mSmmStartupThisApToken;
>  
>  extern UINTN mSmmShadowStackSize;
>  
> @@ -1240,9 +1241,23 @@ InternalSmmStartupThisAp (
>    mSmmMpSyncData->CpuData[CpuIndex].Procedure = Procedure;
>    mSmmMpSyncData->CpuData[CpuIndex].Parameter = ProcArguments;
>    if (Token != NULL) {
> -    ProcToken= GetFreeToken (1);
> -    mSmmMpSyncData->CpuData[CpuIndex].Token = ProcToken;
> -    *Token = (MM_COMPLETION)ProcToken->SpinLock;
> +    if (Token != &mSmmStartupThisApToken) {
> +      //
> +      // When Token points to mSmmStartupThisApToken, this routine is called
> +      // from SmmStartupThisAp() in non-blocking mode (PcdCpuSmmBlockStartupThisAp == FALSE).
> +      // 
> +      // In this case, caller wants to startup AP procedure in non-blocking
> +      // mode and cannot get the completion status from the Token because there
> +      // is no way to return the Token to caller from SmmStartupThisAp().
> +      // Caller needs to use its implementation specific way to query the completion status.
> +      // 
> +      // There is no need to allocate a token for such case so the overhead of SMRAM and
> +      // the allocation operation can be avoided.
> +      //
> +      ProcToken= GetFreeToken (1);

(1) please fix the whitespace error here (it comes from the pre-patch
code, but it's just one character, so we can fix it in this patch)


> +      mSmmMpSyncData->CpuData[CpuIndex].Token = ProcToken;

(2) It seems like this patch introduces a new code path.
InternalSmmStartupThisAp() will continue thinking that this invocation
is non-blocking:

  if (Token == NULL) {
    AcquireSpinLock (mSmmMpSyncData->CpuData[CpuIndex].Busy);
    ReleaseSpinLock (mSmmMpSyncData->CpuData[CpuIndex].Busy);
  }

but APHandler() will think that the invocation was blocking, and it will
not call ReleaseToken() any longer:

    if (mSmmMpSyncData->CpuData[CpuIndex].Token != NULL) {
      ReleaseToken (CpuIndex);
    }

This behavior seems OK to me.

However, please modify the commit message: I think we should list the
3rd step we are omitting (after AllocateTokenBuffer / GetFreeToken) --
namely, ReleaseToken().

With these two superficial updates:

Reviewed-by: Laszlo Ersek <lersek@redhat.com>

Thanks
Laszlo

> +      *Token = (MM_COMPLETION)ProcToken->SpinLock;
> +    }
>    }
>    mSmmMpSyncData->CpuData[CpuIndex].Status    = CpuStatus;
>    if (mSmmMpSyncData->CpuData[CpuIndex].Status != NULL) {
> @@ -1474,8 +1489,6 @@ SmmStartupThisAp (
>    IN OUT  VOID                      *ProcArguments OPTIONAL
>    )
>  {
> -  MM_COMPLETION               Token;
> -
>    gSmmCpuPrivate->ApWrapperFunc[CpuIndex].Procedure = Procedure;
>    gSmmCpuPrivate->ApWrapperFunc[CpuIndex].ProcedureArgument = ProcArguments;
>  
> @@ -1486,7 +1499,7 @@ SmmStartupThisAp (
>      ProcedureWrapper,
>      CpuIndex,
>      &gSmmCpuPrivate->ApWrapperFunc[CpuIndex],
> -    FeaturePcdGet (PcdCpuSmmBlockStartupThisAp) ? NULL : &Token,
> +    FeaturePcdGet (PcdCpuSmmBlockStartupThisAp) ? NULL : &mSmmStartupThisApToken,
>      0,
>      NULL
>      );
>