Re: Is the PEI Core MP Safe? UefiCpuPkg seems to think so calling GetFirstGuidHob on the APs?

["Ni, Ruiyu" <ruiyu.ni@intel.com>]

Andrew,
Today's MpInitLib does depend on physical memory (the waking up vector sits below 1MB).
So the MpPei module can use RegisterForShadow() to make sure it is loaded in memory before running.
Then the module global variable can be used.
But when system is in S3 boot path, it seems PeiCore doesn't follow RegisterForShadow() to load the PEIM
in memory.

Using IDT entry as the storage of global variable is a good idea. We will evaluate.

Thanks/Ray

> -----Original Message-----
> From: afish@apple.com <afish@apple.com>
> Sent: Thursday, June 21, 2018 11:25 AM
> To: Ni, Ruiyu <ruiyu.ni@intel.com>
> Cc: edk2-devel <edk2-devel@lists.01.org>
> Subject: Re: [edk2] Is the PEI Core MP Safe? UefiCpuPkg seems to think so
> calling GetFirstGuidHob on the APs?
> 
> 
> 
> > On Jun 20, 2018, at 8:06 PM, Ni, Ruiyu <ruiyu.ni@intel.com> wrote:
> >
> > Andrew,
> > Good catch! It does break the rule that AP shouldn't call PEI services.
> > But considering the specific case, it should be safe.
> > Because:
> > 1. HOB only stores a pointer to the buffer that contains all the MP
> information.
> > 2. BSP modifies the HOB by only appending data to the end. It may
> > modifies some HOB content. But MpInitLib implementation itself doesn't
> modify the pointer value stored in HOB.
> >
> 
> Ray,
> 
> I think the should be safe is also making assumptions about the debug and a
> few other library instances that got tested against. It is not really an
> architectural guaranty against the library classes that could be linked. We
> don't use the default
> 
> It would be good to use an IDT entry, or optionally a global variable if memory
> is present, longer term.
> 
> Thanks,
> 
> Andrew Fish
> 
> > In PEI environment where global variable is read-only, it's hard to not rely
> on HOB.
> > I guess that's the reason of today's tricky implementation.
> >
> > Thanks/Ray
> >
> >> -----Original Message-----
> >> From: edk2-devel <edk2-devel-bounces@lists.01.org> On Behalf Of
> >> Andrew Fish
> >> Sent: Thursday, June 21, 2018 1:23 AM
> >> To: edk2-devel <edk2-devel@lists.01.org>
> >> Subject: [edk2] Is the PEI Core MP Safe? UefiCpuPkg seems to think so
> >> calling GetFirstGuidHob on the APs?
> >>
> >> Is there some MP safe contract with the PEI Core implementation I
> >> don't know about?
> >>
> >> Looks like the APs are calling PeiServicesGetHobList() to figure out
> >> "who they are"? How does this work? Or am I missing something?
> >>
> >>
> >>
> https://github.com/tianocore/edk2/blob/master/UefiCpuPkg/Library/MpIn
> >> i
> >> tLib/MpLib.c#L1945
> >>
> >> /**
> >>  This return the handle number for the calling processor.  This
> >> service may be  called from the BSP and APs.
> >>  @param[out] ProcessorNumber  Pointer to the handle number of AP.
> >>                               The range is from 0 to the total number of
> >>                               logical processors minus 1. The total number of
> >>                               logical processors can be retrieved by
> >>                               MpInitLibGetNumberOfProcessors().
> >>  @retval EFI_SUCCESS             The current processor handle number was
> >> returned
> >>                                  in ProcessorNumber.
> >>  @retval EFI_INVALID_PARAMETER   ProcessorNumber is NULL.
> >>  @retval EFI_NOT_READY           MP Initialize Library is not initialized.
> >> **/
> >> EFI_STATUS
> >> EFIAPI
> >> MpInitLibWhoAmI (
> >>  OUT UINTN                    *ProcessorNumber
> >>  )
> >> {
> >>  CPU_MP_DATA           *CpuMpData;
> >>
> >>  if (ProcessorNumber == NULL) {
> >>    return EFI_INVALID_PARAMETER;
> >>  }
> >>
> >>  CpuMpData = GetCpuMpData ();
> >>
> >>  return GetProcessorNumber (CpuMpData, ProcessorNumber); }
> >>
> >>
> >>
> https://github.com/tianocore/edk2/blob/master/UefiCpuPkg/Library/MpIn
> >> i
> >> tLib/PeiMpLib.c#L34
> >>
> >> /**
> >>  Get pointer to CPU MP Data structure.
> >>  @return  The pointer to CPU MP Data structure.
> >> **/
> >> CPU_MP_DATA *
> >> GetCpuMpData (
> >>  VOID
> >>  )
> >> {
> >>  CPU_MP_DATA      *CpuMpData;
> >>
> >>  CpuMpData = GetCpuMpDataFromGuidedHob ();  ASSERT
> (CpuMpData !=
> >> NULL);  return CpuMpData; }
> >>
> >>
> https://github.com/tianocore/edk2/blob/master/UefiCpuPkg/Library/MpIn
> >> i
> >> tLib/MpLib.c#L2302
> >>
> >>
> >> /**
> >>  Get pointer to CPU MP Data structure from GUIDed HOB.
> >>  @return  The pointer to CPU MP Data structure.
> >> **/
> >> CPU_MP_DATA *
> >> GetCpuMpDataFromGuidedHob (
> >>  VOID
> >>  )
> >> {
> >>  EFI_HOB_GUID_TYPE       *GuidHob;
> >>  VOID                    *DataInHob;
> >>  CPU_MP_DATA             *CpuMpData;
> >>
> >>  CpuMpData = NULL;
> >>  GuidHob = GetFirstGuidHob (&mCpuInitMpLibHobGuid);  if (GuidHob !=
> >> NULL) {
> >>    DataInHob = GET_GUID_HOB_DATA (GuidHob);
> >>    CpuMpData = (CPU_MP_DATA *) (*(UINTN *) DataInHob);  }  return
> >> CpuMpData; }
> >>
> >> Thanks,
> >>
> >> Andrew Fish
> >> _______________________________________________
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> >> edk2-devel@lists.01.org
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