Re: [edk2-devel] [Patch 1/1] UefiCpuPkg/Library/MpInitLib: Fix AP VolatileRegisters race condition

["Zeng, Star" <star.zeng@intel.com>]

Mike,

Oh, see it.
There is no sequence dependence between FinishedCount increment and NumApsExecuting decrement, right?

     InterlockedIncrement ((UINT32 *) &CpuMpData->FinishedCount);
 
     InterlockedDecrement ((UINT32 *) &CpuMpData->MpCpuExchangeInfo->NumApsExecuting);


Thanks,
Star

> -----Original Message-----
> From: Kinney, Michael D <michael.d.kinney@intel.com>
> Sent: Monday, January 25, 2021 1:16 PM
> To: Zeng, Star <star.zeng@intel.com>; devel@edk2.groups.io; Kinney,
> Michael D <michael.d.kinney@intel.com>
> Cc: Dong, Eric <eric.dong@intel.com>; Ni, Ray <ray.ni@intel.com>; Laszlo
> Ersek <lersek@redhat.com>; Kumar, Rahul1 <rahul1.kumar@intel.com>
> Subject: RE: [edk2-devel] [Patch 1/1] UefiCpuPkg/Library/MpInitLib: Fix AP
> VolatileRegisters race condition
> 
> Hi Star,
> 
> That line is only active when (CpuMpData->SevEsIsEnabled) is TRUE.
> 
> The race condition addressed by this BZ is for systems with SecEsIsEnabled
> FALSE.
> 
> From comments in this file the SecEsIsEnabled cases have already been
> handled.
> 
> Mike
> 
> > -----Original Message-----
> > From: Zeng, Star <star.zeng@intel.com>
> > Sent: Sunday, January 24, 2021 7:15 PM
> > To: devel@edk2.groups.io; Kinney, Michael D
> > <michael.d.kinney@intel.com>
> > Cc: Dong, Eric <eric.dong@intel.com>; Ni, Ray <ray.ni@intel.com>;
> > Laszlo Ersek <lersek@redhat.com>; Kumar, Rahul1
> > <rahul1.kumar@intel.com>; Zeng, Star <star.zeng@intel.com>
> > Subject: RE: [edk2-devel] [Patch 1/1] UefiCpuPkg/Library/MpInitLib:
> > Fix AP VolatileRegisters race condition
> >
> > Does
> >
> https://github.com/tianocore/edk2/blob/master/UefiCpuPkg/Library/MpIni
> > tLib/MpLib.c#L909 (also decrements
> > NumApsExecuting) also need be handled?
> >
> > Thanks,
> > Star
> >
> > > -----Original Message-----
> > > From: devel@edk2.groups.io <devel@edk2.groups.io> On Behalf Of
> > > Michael D Kinney
> > > Sent: Saturday, January 23, 2021 1:10 AM
> > > To: devel@edk2.groups.io
> > > Cc: Dong, Eric <eric.dong@intel.com>; Ni, Ray <ray.ni@intel.com>;
> > > Laszlo Ersek <lersek@redhat.com>; Kumar, Rahul1
> > > <rahul1.kumar@intel.com>
> > > Subject: [edk2-devel] [Patch 1/1] UefiCpuPkg/Library/MpInitLib: Fix
> > > AP VolatileRegisters race condition
> > >
> > > REF: https://bugzilla.tianocore.org/show_bug.cgi?id=3182
> > >
> > > Fix the order of operations in ApWakeupFunction() when
> > > PcdCpuApLoopMode is set to HLT mode that uses INIT-SIPI-SIPI to wake
> > > APs.  In this mode, volatile state is restored and saved each time a
> > > INIT-SIPI-SIPI is sent to an AP to request a function to be executed
> > > on the AP.  When the function is completed the volatile state of the
> > > AP is saved.  However, the counters NumApsExecuting and
> > > FinishedCount are updated before the volatile state is saved.  This
> > > allows for a race condition window for the BSP that is waiting on
> > > these counters to request a new INIT-SIPI-SIPI before all the APs
> > > have completely saved their volatile state.  The fix is to save the
> > > AP volatile state before updating the NumApsExecuting and
> > > FinishedCount counters.
> > >
> > > Cc: Eric Dong <eric.dong@intel.com>
> > > Cc: Ray Ni <ray.ni@intel.com>
> > > Cc: Laszlo Ersek <lersek@redhat.com>
> > > Cc: Rahul Kumar <rahul1.kumar@intel.com>
> > > Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>
> > > ---
> > >  UefiCpuPkg/Library/MpInitLib/MpLib.c | 31
> > > ++++++++++++++++------------
> > >  1 file changed, 18 insertions(+), 13 deletions(-)
> > >
> > > diff --git a/UefiCpuPkg/Library/MpInitLib/MpLib.c
> > > b/UefiCpuPkg/Library/MpInitLib/MpLib.c
> > > index 681fa79b4cff..8b1f7f84bad6 100644
> > > --- a/UefiCpuPkg/Library/MpInitLib/MpLib.c
> > > +++ b/UefiCpuPkg/Library/MpInitLib/MpLib.c
> > > @@ -769,15 +769,6 @@ ApWakeupFunction (
> > >        RestoreVolatileRegisters
> > > (&CpuMpData->CpuData[0].VolatileRegisters,
> > > FALSE);
> > >        InitializeApData (CpuMpData, ProcessorNumber, BistData,
> > > ApTopOfStack);
> > >        ApStartupSignalBuffer = CpuMpData-
> > > >CpuData[ProcessorNumber].StartupApSignal;
> > > -
> > > -      //
> > > -      // Delay decrementing the APs executing count when SEV-ES is
> enabled
> > > -      // to allow the APs to issue an AP_RESET_HOLD before the BSP
> possibly
> > > -      // performs another INIT-SIPI-SIPI sequence.
> > > -      //
> > > -      if (!CpuMpData->SevEsIsEnabled) {
> > > -        InterlockedDecrement ((UINT32 *) &CpuMpData-
> > > >MpCpuExchangeInfo->NumApsExecuting);
> > > -      }
> > >      } else {
> > >        //
> > >        // Execute AP function if AP is ready @@ -866,19 +857,33 @@
> > > ApWakeupFunction (
> > >        }
> > >      }
> > >
> > > +    if (CpuMpData->ApLoopMode == ApInHltLoop) {
> > > +      //
> > > +      // Save AP volatile registers
> > > +      //
> > > +      SaveVolatileRegisters (&CpuMpData-
> > > >CpuData[ProcessorNumber].VolatileRegisters);
> > > +    }
> > > +
> > >      //
> > >      // AP finished executing C code
> > >      //
> > >      InterlockedIncrement ((UINT32 *) &CpuMpData->FinishedCount);
> > >
> > > +    if (CpuMpData->InitFlag == ApInitConfig) {
> > > +      //
> > > +      // Delay decrementing the APs executing count when SEV-ES is
> enabled
> > > +      // to allow the APs to issue an AP_RESET_HOLD before the BSP
> possibly
> > > +      // performs another INIT-SIPI-SIPI sequence.
> > > +      //
> > > +      if (!CpuMpData->SevEsIsEnabled) {
> > > +        InterlockedDecrement ((UINT32 *) &CpuMpData-
> > > >MpCpuExchangeInfo->NumApsExecuting);
> > > +      }
> > > +    }
> > > +
> > >      //
> > >      // Place AP is specified loop mode
> > >      //
> > >      if (CpuMpData->ApLoopMode == ApInHltLoop) {
> > > -      //
> > > -      // Save AP volatile registers
> > > -      //
> > > -      SaveVolatileRegisters (&CpuMpData-
> > > >CpuData[ProcessorNumber].VolatileRegisters);
> > >        //
> > >        // Place AP in HLT-loop
> > >        //
> > > --
> > > 2.29.2.windows.2
> > >
> > >
> > >
> > > 
> > >