Re: [PATCH v2 1/8] x86/fpu: Check for missing AVX and AVX-512 xstate bits

[Thomas Gleixner <tglx@kernel.org>]

On Mon, Jul 27 2026 at 19:15, Eric Biggers wrote:
> If the CPU declares AVX or AVX-512 support, verify that the
> corresponding xstate bits are also set.  If not, warn and clear them.

That's clearly a firmware bug which has to be treated as all other
inconsistencies of that sort.

> This eliminates the perceived need for AVX and AVX-512 optimized code in
> the kernel to call cpu_has_xfeatures().  That has never been universally
> done, which strongly suggests that it has never really been needed in
> practice, but this should remove any remaining doubt.

Indeed it is inconsistent all over the place. While looking at the
follow up patches I wondered how many of these remaining nested
conditions are redundant. Most of them should be covered by the
dependency checks. If they are not, then we should add them.

> Signed-off-by: Eric Biggers <ebiggers@kernel.org>
> ---
>  arch/x86/kernel/fpu/xstate.c | 21 +++++++++++++++++++++
>  1 file changed, 21 insertions(+)
>
> diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
> index a7b6524a9dea2..904ff933c0d88 100644
> --- a/arch/x86/kernel/fpu/xstate.c
> +++ b/arch/x86/kernel/fpu/xstate.c
> @@ -799,6 +799,23 @@ static u64 __init guest_default_mask(void)
>  	return ~(u64)XFEATURE_MASK_USER_DYNAMIC;
>  }
>  
> +/* Clear any X86_FEATURE_* used by the kernel whose xfeatures are missing. */
> +static void __init clear_cpu_caps_with_missing_xfeatures(u64 xfeatures)
> +{
> +	u64 mask;
> +
> +	mask = XFEATURE_MASK_FPSSE | XFEATURE_MASK_YMM;
> +	if (boot_cpu_has(X86_FEATURE_AVX) && (xfeatures & mask) != mask) {
> +		pr_err("x86/fpu: Disabling AVX support due to missing xstate features\n");

Please make these

       pr_err(FW_BUG "...")

As that's what it is.

Thanks,

        tglx