Re: [PATCH] UefiCpuPkg/Library/MpInitLib: Remove BSP index == 0 Assumption.

["Dong, Eric" <eric.dong@intel.com>]

Hi Laszlo,

> -----Original Message-----
> From: Laszlo Ersek [mailto:lersek@redhat.com]
> Sent: Wednesday, January 15, 2020 6:05 PM
> To: Dong, Eric <eric.dong@intel.com>; devel@edk2.groups.io
> Cc: Ni, Ray <ray.ni@intel.com>
> Subject: Re: [PATCH] UefiCpuPkg/Library/MpInitLib: Remove BSP index == 0
> Assumption.
> 
> On 01/15/20 07:06, Eric Dong wrote:
> > REF: https://bugzilla.tianocore.org/show_bug.cgi?id=2392
> >
> > Current code implementation assumes BSP index is 0 at the begin.
> > This code change removes this assumption. It get BSP index from the
> > saved data structure if it existed.
> >
> > Cc: Ray Ni <ray.ni@intel.com>
> > Cc: Laszlo Ersek <lersek@redhat.com>
> > Signed-off-by: Eric Dong <eric.dong@intel.com>
> > ---
> >  UefiCpuPkg/Library/MpInitLib/MpLib.c | 10 ++++++----
> >  1 file changed, 6 insertions(+), 4 deletions(-)
> >
> > diff --git a/UefiCpuPkg/Library/MpInitLib/MpLib.c
> > b/UefiCpuPkg/Library/MpInitLib/MpLib.c
> > index 6ec9b172b8..922c87b766 100644
> > --- a/UefiCpuPkg/Library/MpInitLib/MpLib.c
> > +++ b/UefiCpuPkg/Library/MpInitLib/MpLib.c
> > @@ -636,7 +636,7 @@ ApWakeupFunction (
> >        //   to initialize AP in InitConfig path.
> >        // NOTE: IDTR.BASE stored in CpuMpData-
> >CpuData[0].VolatileRegisters points to a different IDT shared by all APs.
> >        //
> > -      RestoreVolatileRegisters (&CpuMpData->CpuData[0].VolatileRegisters,
> FALSE);
> > +      RestoreVolatileRegisters
> > + (&CpuMpData->CpuData[CpuMpData->BspNumber].VolatileRegisters,
> > + FALSE);
> >        InitializeApData (CpuMpData, ProcessorNumber, BistData,
> ApTopOfStack);
> >        ApStartupSignalBuffer =
> > CpuMpData->CpuData[ProcessorNumber].StartupApSignal;
> >
> > @@ -1615,6 +1615,7 @@ MpInitLibInitialize (
> >    UINTN                    ApResetVectorSize;
> >    UINTN                    BackupBufferAddr;
> >    UINTN                    ApIdtBase;
> > +  UINT64                   BspTopOfStack;
> >
> >    OldCpuMpData = GetCpuMpDataFromGuidedHob ();
> >    if (OldCpuMpData == NULL) {
> > @@ -1677,7 +1678,7 @@ MpInitLibInitialize (
> >    CpuMpData->BackupBufferSize = ApResetVectorSize;
> >    CpuMpData->WakeupBuffer     = (UINTN) -1;
> >    CpuMpData->CpuCount         = 1;
> > -  CpuMpData->BspNumber        = 0;
> > +  CpuMpData->BspNumber        = OldCpuMpData != NULL ?
> OldCpuMpData->BspNumber : 0;
> >    CpuMpData->WaitEvent        = NULL;
> >    CpuMpData->SwitchBspFlag    = FALSE;
> >    CpuMpData->CpuData          = (CPU_AP_DATA *) (CpuMpData + 1);
> > @@ -1704,11 +1705,12 @@ MpInitLibInitialize (
> >    // Don't pass BSP's TR to APs to avoid AP init failure.
> >    //
> >    VolatileRegisters.Tr = 0;
> > -  CopyMem (&CpuMpData->CpuData[0].VolatileRegisters,
> > &VolatileRegisters, sizeof (VolatileRegisters));
> > +  CopyMem
> > + (&CpuMpData->CpuData[CpuMpData->BspNumber].VolatileRegisters,
> > + &VolatileRegisters, sizeof (VolatileRegisters));
> >    //
> >    // Set BSP basic information
> >    //
> > -  InitializeApData (CpuMpData, 0, 0, CpuMpData->Buffer +
> > ApStackSize);
> > +  BspTopOfStack = CpuMpData->Buffer + (CpuMpData->BspNumber + 1)
> *
> > + CpuMpData->CpuApStackSize;  InitializeApData (CpuMpData,
> > + CpuMpData->BspNumber, 0, BspTopOfStack);
> >    //
> >    // Save assembly code information
> >    //
> >
> 
> The patch seems reasonable to me (although I have not tried verifying that
> all necessary spots are updated).
> 
> However, there is one thing I certainly don't understand, and the commit
> message doesn't explain it. In the "BspTopOfStack" calculation, why do we
> change the *second* factor, when we change the multiplication from:
> 
>   (0                    + 1) * ApStackSize
> 
> (where the (0 + 1) is implied in the old code), to:
> 
>   (CpuMpData->BspNumber + 1) * CpuMpData->CpuApStackSize
> 
> ?
> 
> I understand why the *first* factor is changed -- we basically replace "0" with
> "CpuMpData->BspNumber" --; what I don't understand is why we replace
> "ApStackSize" with "CpuMpData->CpuApStackSize", in the second factor.
> 
> ... Higher up in the code, we have:
> 
>   CpuMpData->CpuApStackSize   = ApStackSize;
> 
> so this part of the patch might actually have no effect. But, even then, I think
> it makes the patch harder to understand. So in that case, I'd suggest sticking
> with "ApStackSize", just for keeping the patch simpler.
> 
[[Eric]] driver has two places to call InitializeApData (). Here is one and the other in ApWakeupFunction().
      InitializeApData (CpuMpData, ProcessorNumber, BistData, ApTopOfStack);
At that function, it calculates the ApTopOfStack like below:
      ApTopOfStack  = CpuMpData->Buffer + (ProcessorNumber + 1) * CpuMpData->CpuApStackSize;

I update new code to follow this coding style. I think after this change, the exit two code pieces are follow
the same coding style.  So I think we can keep my original change.

Thanks,
Eric

> Thanks
> Laszlo