Re: [edk2-devel] [PATCH 1/2] UefiCpuPkg/MpInitLib: Use AsmCpuidEx() for CPUID_EXTENDED_TOPOLOGY leaf

["Ni, Ray" <ray.ni@intel.com>]

Mike,
I agree with your words after "However".
Zeroing ECX in AsmCpuid() is confusing to future code maintainer: If CPUID instruction does
not consume "ECX", why is it needed to zero "ECX"?

Thanks,
Ray
> -----Original Message-----
> From: Kinney, Michael D <michael.d.kinney@intel.com>
> Sent: Friday, January 19, 2024 7:11 AM
> To: Tom Lendacky <thomas.lendacky@amd.com>; devel@edk2.groups.io;
> Gao, Liming <liming.gao@intel.com>; Liu, Zhiguang <zhiguang.liu@intel.com>;
> Dong, Eric <eric.dong@intel.com>; Ni, Ray <ray.ni@intel.com>; Kumar, Rahul R
> <rahul.r.kumar@intel.com>; Gerd Hoffmann <kraxel@redhat.com>; Ard
> Biesheuvel <ardb+tianocore@kernel.org>
> Cc: Michael Roth <michael.roth@amd.com>; Kinney, Michael D
> <michael.d.kinney@intel.com>
> Subject: RE: [edk2-devel] [PATCH 1/2] UefiCpuPkg/MpInitLib: Use
> AsmCpuidEx() for CPUID_EXTENDED_TOPOLOGY leaf
> 
> Hi Tom,
> 
> I do not see any harm in zeroing ECX in AsmCpuid().
> 
> If it is not zeroed, then it would have an undefined value.
> 
> However, calling AsmCpuid() for any Index that evaluates ECX
> (including a check for 0) should never be done.  If ECX is
> evaluated for a given Index, then AsmCpuIdEx() must be used.
> 
> Mike
> 
> > -----Original Message-----
> > From: Tom Lendacky <thomas.lendacky@amd.com>
> > Sent: Wednesday, January 17, 2024 1:26 PM
> > To: devel@edk2.groups.io; Kinney, Michael D
> > <michael.d.kinney@intel.com>; Gao, Liming <liming.gao@intel.com>; Liu,
> > Zhiguang <zhiguang.liu@intel.com>; Dong, Eric <eric.dong@intel.com>; Ni,
> > Ray <ray.ni@intel.com>; Kumar, Rahul R <rahul.r.kumar@intel.com>; Gerd
> > Hoffmann <kraxel@redhat.com>; Ard Biesheuvel
> <ardb+tianocore@kernel.org>
> > Cc: Michael Roth <michael.roth@amd.com>
> > Subject: Re: [edk2-devel] [PATCH 1/2] UefiCpuPkg/MpInitLib: Use
> > AsmCpuidEx() for CPUID_EXTENDED_TOPOLOGY leaf
> >
> > On 11/28/23 08:35, Lendacky, Thomas via groups.io wrote:
> > > On 11/6/23 17:15, Tom Lendacky wrote:
> > >> On 11/6/23 16:45, Lendacky, Thomas via groups.io wrote:
> > >>> The CPUID_EXTENDED_TOPOLOGY CPUID leaf takes a subleaf as input
> when
> > >>> returning CPUID information. However, the AsmCpuid() function does
> > not
> > >>> zero out ECX before the CPUID instruction, so the input leaf is used
> > as
> > >>> the sub-leaf for the CPUID request and returns erroneous/invalid
> > CPUID
> > >>> data, since the intent of the request was to get data related to
> > sub-leaf
> > >>> 0. Instead, use AsmCpuidEx() for the CPUID_EXTENDED_TOPOLOGY leaf.
> > >>
> > >> Alternatively, the AsmCpuid() function could be changed to XOR ECX
> > >> before invoking the CPUID instruction. This would ensure that the 0
> > >> sub-leaf is returned for any CPUID leaves that support sub-leaves.
> > >> Thoughts?
> > >>
> > >> Adding some additional maintainers for their thoughts, too.
> > >
> > > Any thoughts on this approach (as a separate, unrelated patch) to
> > > eliminate future issues that could pop up?
> > >
> > > Seems like zeroing out ECX before calling CPUID would be an
> > appropriate
> > > thing to do, but I'm not sure if that will have any impact on the
> > existing
> > > code base... it shouldn't, but you never know.
> >
> > Just a re-ping for thoughts on this.
> >
> > Thanks,
> > Tom
> >
> > >
> > > Thanks,
> > > Tom
> > >
> > >>
> > >> Thanks,
> > >> Tom


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