RE: [PATCH V2] [X86] Workaround possible CPUID bug in Sandy Bridge.

[Liu, Hongtao via Gcc-patches]

> -----Original Message-----
> From: Uros Bizjak <ubiz...@gmail.com>
> Sent: Wednesday, August 9, 2023 2:33 PM
> To: Liu, Hongtao <hongtao....@intel.com>
> Cc: gcc-patches@gcc.gnu.org
> Subject: Re: [PATCH V2] [X86] Workaround possible CPUID bug in Sandy
> Bridge.
> 
> On Wed, Aug 9, 2023 at 3:48 AM liuhongt <hongtao....@intel.com> wrote:
> >
> > > Please rather do it in a more self-descriptive way, as proposed in
> > > the attached patch. You won't need a comment then.
> > >
> >
> > Adjusted in V2 patch.
> >
> > Don't access leaf 7 subleaf 1 unless subleaf 0 says it is supported
> > via EAX.
> >
> > Intel documentation says invalid subleaves return 0. We had been
> > relying on that behavior instead of checking the max sublef number.
> >
> > It appears that some Sandy Bridge CPUs return at least the subleaf 0
> > EDX value for subleaf 1. Best guess is that this is a bug in a
> > microcode patch since all of the bits we're seeing set in EDX were
> > introduced after Sandy Bridge was originally released.
> >
> > This is causing avxvnniint16 to be incorrectly enabled with
> > -march=native on these CPUs.
> >
> > gcc/ChangeLog:
> >
> >         * common/config/i386/cpuinfo.h (get_available_features): Check
> >         EAX for valid subleaf before use CPUID.
> > ---
> >  gcc/common/config/i386/cpuinfo.h | 82
> > +++++++++++++++++---------------
> >  1 file changed, 43 insertions(+), 39 deletions(-)
> >
> > diff --git a/gcc/common/config/i386/cpuinfo.h
> > b/gcc/common/config/i386/cpuinfo.h
> > index 30ef0d334ca..9fa4dec2a7e 100644
> > --- a/gcc/common/config/i386/cpuinfo.h
> > +++ b/gcc/common/config/i386/cpuinfo.h
> > @@ -663,6 +663,7 @@ get_available_features (struct __processor_model
> *cpu_model,
> >    unsigned int max_cpuid_level = cpu_model2->__cpu_max_level;
> >    unsigned int eax, ebx;
> >    unsigned int ext_level;
> > +  unsigned int subleaf_level;
> 
> Oh, I failed this in my previous review. This variable should be named
> max_subleaf_level, as it represents the maximum supported ECX value.
I've committed previous patch ,but not backport yet.
Guess I can just commit another patch to change the name?
For backport, I'll merge the change together with just 1 commit.
> 
> Uros.
> 
> >
> >    /* Get XCR_XFEATURE_ENABLED_MASK register with xgetbv.  */
> >  #define XCR_XFEATURE_ENABLED_MASK      0x0
> > @@ -762,7 +763,7 @@ get_available_features (struct __processor_model
> *cpu_model,
> >    /* Get Advanced Features at level 7 (eax = 7, ecx = 0/1). */
> >    if (max_cpuid_level >= 7)
> >      {
> > -      __cpuid_count (7, 0, eax, ebx, ecx, edx);
> > +      __cpuid_count (7, 0, subleaf_level, ebx, ecx, edx);
> >        if (ebx & bit_BMI)
> >         set_feature (FEATURE_BMI);
> >        if (ebx & bit_SGX)
> > @@ -874,45 +875,48 @@ get_available_features (struct
> __processor_model *cpu_model,
> >             set_feature (FEATURE_AVX512FP16);
> >         }
> >
> > -      __cpuid_count (7, 1, eax, ebx, ecx, edx);
> > -      if (eax & bit_HRESET)
> > -       set_feature (FEATURE_HRESET);
> > -      if (eax & bit_CMPCCXADD)
> > -       set_feature(FEATURE_CMPCCXADD);
> > -      if (edx & bit_PREFETCHI)
> > -       set_feature (FEATURE_PREFETCHI);
> > -      if (eax & bit_RAOINT)
> > -       set_feature (FEATURE_RAOINT);
> > -      if (avx_usable)
> > -       {
> > -         if (eax & bit_AVXVNNI)
> > -           set_feature (FEATURE_AVXVNNI);
> > -         if (eax & bit_AVXIFMA)
> > -           set_feature (FEATURE_AVXIFMA);
> > -         if (edx & bit_AVXVNNIINT8)
> > -           set_feature (FEATURE_AVXVNNIINT8);
> > -         if (edx & bit_AVXNECONVERT)
> > -           set_feature (FEATURE_AVXNECONVERT);
> > -         if (edx & bit_AVXVNNIINT16)
> > -           set_feature (FEATURE_AVXVNNIINT16);
> > -         if (eax & bit_SM3)
> > -           set_feature (FEATURE_SM3);
> > -         if (eax & bit_SHA512)
> > -           set_feature (FEATURE_SHA512);
> > -         if (eax & bit_SM4)
> > -           set_feature (FEATURE_SM4);
> > -       }
> > -      if (avx512_usable)
> > -       {
> > -         if (eax & bit_AVX512BF16)
> > -           set_feature (FEATURE_AVX512BF16);
> > -       }
> > -      if (amx_usable)
> > +      if (subleaf_level >= 1)
> >         {
> > -         if (eax & bit_AMX_FP16)
> > -           set_feature (FEATURE_AMX_FP16);
> > -         if (edx & bit_AMX_COMPLEX)
> > -           set_feature (FEATURE_AMX_COMPLEX);
> > +         __cpuid_count (7, 1, eax, ebx, ecx, edx);
> > +         if (eax & bit_HRESET)
> > +           set_feature (FEATURE_HRESET);
> > +         if (eax & bit_CMPCCXADD)
> > +           set_feature(FEATURE_CMPCCXADD);
> > +         if (edx & bit_PREFETCHI)
> > +           set_feature (FEATURE_PREFETCHI);
> > +         if (eax & bit_RAOINT)
> > +           set_feature (FEATURE_RAOINT);
> > +         if (avx_usable)
> > +           {
> > +             if (eax & bit_AVXVNNI)
> > +               set_feature (FEATURE_AVXVNNI);
> > +             if (eax & bit_AVXIFMA)
> > +               set_feature (FEATURE_AVXIFMA);
> > +             if (edx & bit_AVXVNNIINT8)
> > +               set_feature (FEATURE_AVXVNNIINT8);
> > +             if (edx & bit_AVXNECONVERT)
> > +               set_feature (FEATURE_AVXNECONVERT);
> > +             if (edx & bit_AVXVNNIINT16)
> > +               set_feature (FEATURE_AVXVNNIINT16);
> > +             if (eax & bit_SM3)
> > +               set_feature (FEATURE_SM3);
> > +             if (eax & bit_SHA512)
> > +               set_feature (FEATURE_SHA512);
> > +             if (eax & bit_SM4)
> > +               set_feature (FEATURE_SM4);
> > +           }
> > +         if (avx512_usable)
> > +           {
> > +             if (eax & bit_AVX512BF16)
> > +               set_feature (FEATURE_AVX512BF16);
> > +           }
> > +         if (amx_usable)
> > +           {
> > +             if (eax & bit_AMX_FP16)
> > +               set_feature (FEATURE_AMX_FP16);
> > +             if (edx & bit_AMX_COMPLEX)
> > +               set_feature (FEATURE_AMX_COMPLEX);
> > +           }
> >         }
> >      }
> >
> > --
> > 2.31.1
> >