[PATCH 1/2] x86: Skip XSAVE state size reset if ISA level requires XSAVE

[H.J. Lu]

On Thu, Mar 27, 2025 at 2:42 PM Florian Weimer <fweimer@redhat.com> wrote:
>
> If we have to use XSAVE or XSAVEC trampolines, do not adjust the size
> information they need.  Technically, it is an operator error to try to
> run with -XSAVE,-XSAVEC on such builds, but this change here disables
> some unnecessary code with higher ISA levels and simplifies testing.
>
> Related to commit befe2d3c4dec8be2cdd01a47132e47bdb7020922
> ("x86-64: Don't use SSE resolvers for ISA level 3 or above").
> ---
>  sysdeps/x86/cpu-features.c | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
>
> diff --git a/sysdeps/x86/cpu-features.c b/sysdeps/x86/cpu-features.c
> index 27abaca8b7..06876bff14 100644
> --- a/sysdeps/x86/cpu-features.c
> +++ b/sysdeps/x86/cpu-features.c
> @@ -1161,7 +1161,8 @@ no_cpuid:
>
>    bool disable_xsave_features = false;
>
> -  if (!CPU_FEATURE_USABLE_P (cpu_features, OSXSAVE))
> +  if (MINIMUM_X86_ISA_LEVEL < AVX_X86_ISA_LEVEL
> +      && !CPU_FEATURE_USABLE_P (cpu_features, OSXSAVE))
>      {
>        /* These features are usable only if OSXSAVE is usable.  */
>        CPU_FEATURE_UNSET (cpu_features, XSAVE);
> @@ -1173,9 +1174,10 @@ no_cpuid:
>        disable_xsave_features = true;
>      }
>
> -  if (disable_xsave_features
> -      || (!CPU_FEATURE_USABLE_P (cpu_features, XSAVE)
> -         && !CPU_FEATURE_USABLE_P (cpu_features, XSAVEC)))
> +  if (MINIMUM_X86_ISA_LEVEL < AVX_X86_ISA_LEVEL
> +      && (disable_xsave_features
> +         || (!CPU_FEATURE_USABLE_P (cpu_features, XSAVE)
> +             && !CPU_FEATURE_USABLE_P (cpu_features, XSAVEC))))
>      {
>        /* Clear xsave_state_size if both XSAVE and XSAVEC aren't usable.  */
>        cpu_features->xsave_state_size = 0;

Since AVX needs xsave, I prefer

if (MINIMUM_X86_ISA_LEVEL < AVX_X86_ISA_LEVEL)
  {
     Disable XSAVE
     Disable XSAVEC
  }

or

#if MINIMUM_X86_ISA_LEVEL < AVX_X86_ISA_LEVEL
...
#endif


> base-commit: be61b9493d38032519e596f282f9695667402c8d
> --
> 2.48.1
>
>


-- 
H.J.