Re: [edk2-devel] [PATCH] UefiCpuPkg: Remove ASSERT checking if FinishedCount equal to CpuCount-1

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

On 10/25/23 12:07, Yuanhao Xie wrote:
> The purpose is to fix an assertion with applying the following patch
> series:
> 
> UefiCpuPkg: Refactor the logic for placing APs in HltLoop.
> UefiCpuPkg: Refactor the logic for placing APs in Mwait/Runloop.
> UefiCpuPkg: Create MpHandOff.
> UefiCpuPkg: ApWakeupFunction directly use CpuMpData.
> UefiCpuPkg: Eliminate the second INIT-SIPI-SIPI sequence.
> UefiCpuPkg: Decouple the SEV-ES functionality.
> 
> The assertion arises from a timing discrepancy between BSP completing
> its startup signal check and the APs incrementing the FinishedCount.
> 
> Instead of assertion, use while loop to waits until all the APs have
> incremented the FinishedCount.
> 
> Cc: Ray Ni <ray.ni@intel.com>
> Cc: Eric Dong <eric.dong@intel.com>
> Cc: Rahul Kumar <rahul1.kumar@intel.com>
> Cc: Tom Lendacky <thomas.lendacky@amd.com>
> Signed-off-by: Yuanhao Xie <yuanhao.xie@intel.com>
> ---
>  UefiCpuPkg/Library/MpInitLib/MpLib.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/UefiCpuPkg/Library/MpInitLib/MpLib.c b/UefiCpuPkg/Library/MpInitLib/MpLib.c
> index 6f1456cfe1..9a6ec5db5c 100644
> --- a/UefiCpuPkg/Library/MpInitLib/MpLib.c
> +++ b/UefiCpuPkg/Library/MpInitLib/MpLib.c
> @@ -913,8 +913,8 @@ DxeApEntryPoint (
>    UINTN  ProcessorNumber;
>  
>    GetProcessorNumber (CpuMpData, &ProcessorNumber);
> -  InterlockedIncrement ((UINT32 *)&CpuMpData->FinishedCount);
>    RestoreVolatileRegisters (&CpuMpData->CpuData[0].VolatileRegisters, FALSE);
> +  InterlockedIncrement ((UINT32 *)&CpuMpData->FinishedCount);
>    PlaceAPInMwaitLoopOrRunLoop (
>      CpuMpData->ApLoopMode,
>      CpuMpData->CpuData[ProcessorNumber].StartupApSignal,
> @@ -2201,7 +2201,12 @@ MpInitLibInitialize (
>        // looping process there.
>        //
>        SwitchApContext (MpHandOff);
> -      ASSERT (CpuMpData->FinishedCount == (CpuMpData->CpuCount - 1));
> +      //
> +      // Wait for all APs finished initialization
> +      //
> +      while (CpuMpData->FinishedCount < (CpuMpData->CpuCount - 1)) {
> +        CpuPause ();
> +      }
>  
>        //
>        // Set Apstate as Idle, otherwise Aps cannot be waken-up again.

The second hunk makes sense. SwitchApContext() returns after all APs are
"live", but that doesn't guarantee that all APs are also "done" by the
time the BSP reaches the FinishedCount check.

(1) What is the justification for the first hunk? I understand that we
may want to report "finished" from an AP as late as possible. Is that
the only (general) reason for the first hunk, or is there a specific reason?

Either way, the reason for the first hunk should be documented in the
commit message.

(2) The subject line should be

  UefiCpuPkg/MpInitLib: wait for all APs to finish initialization

because

- we should also state the component name within UefiCpuPkg,

- "remove assert" is just a natural language expression of the direct
code change, so it's not useful; what's useful is naming the *goal* that
we're achieving.

(3) I suggest appending

  Fixes: 964a4f032dcd

to the commit message, just above your Signed-off-by.

Laszlo



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