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.kin...@intel.com> > Sent: Friday, January 19, 2024 7:11 AM > To: Tom Lendacky <thomas.lenda...@amd.com>; devel@edk2.groups.io; > Gao, Liming <liming....@intel.com>; Liu, Zhiguang <zhiguang....@intel.com>; > Dong, Eric <eric.d...@intel.com>; Ni, Ray <ray...@intel.com>; Kumar, Rahul R > <rahul.r.ku...@intel.com>; Gerd Hoffmann <kra...@redhat.com>; Ard > Biesheuvel <ardb+tianoc...@kernel.org> > Cc: Michael Roth <michael.r...@amd.com>; Kinney, Michael D > <michael.d.kin...@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.lenda...@amd.com> > > Sent: Wednesday, January 17, 2024 1:26 PM > > To: devel@edk2.groups.io; Kinney, Michael D > > <michael.d.kin...@intel.com>; Gao, Liming <liming....@intel.com>; Liu, > > Zhiguang <zhiguang....@intel.com>; Dong, Eric <eric.d...@intel.com>; Ni, > > Ray <ray...@intel.com>; Kumar, Rahul R <rahul.r.ku...@intel.com>; Gerd > > Hoffmann <kra...@redhat.com>; Ard Biesheuvel > <ardb+tianoc...@kernel.org> > > Cc: Michael Roth <michael.r...@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 -=-=-=-=-=-=-=-=-=-=-=- Groups.io Links: You receive all messages sent to this group. View/Reply Online (#114039): https://edk2.groups.io/g/devel/message/114039 Mute This Topic: https://groups.io/mt/102432782/21656 Group Owner: devel+ow...@edk2.groups.io Unsubscribe: https://edk2.groups.io/g/devel/leave/9847357/21656/1706620634/xyzzy [arch...@mail-archive.com] -=-=-=-=-=-=-=-=-=-=-=-
