Re: [PATCH v2 2/8] um: Check for missing AVX and AVX-512 xstate bits

On Mon, Jul 27, 2026 at 07:15:57PM -0700, Eric Biggers wrote:
> If the CPU declares AVX or AVX-512 support, verify that all the
> corresponding bits are set in the XCR0 register.  If any are missing,
> warn and don't set the corresponding X86_FEATURE_* flags.
> 
> This eliminates the perceived need for UML-supporting AVX and AVX-512
> optimized code in the kernel (that is, lib/raid/ currently) to start
> checking the XCR0 bits in addition to X86_FEATURE_AVX*.
> 
> This aligns UML with the vast majority of userspace programs, which
> check the XCR0 bits before considering AVX and AVX-512 to be supported,
> as per the procedure documented in Intel's CPU manual.
> 
> Signed-off-by: Eric Biggers <[email protected]>
> ---
>  arch/um/kernel/um_arch.c | 78 +++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 77 insertions(+), 1 deletion(-)

./scripts/get_maintainer.pl -f arch/um/kernel/um_arch.c
Richard Weinberger <[email protected]> (maintainer:USER-MODE LINUX (UML))
Anton Ivanov <[email protected]> (maintainer:USER-MODE LINUX (UML))
Johannes Berg <[email protected]> (maintainer:USER-MODE LINUX (UML),commit_signer:8/10=80%,authored:1/10=10%,removed_lines:2/40=5%)
...

Please use that script. CCed now.

> diff --git a/arch/um/kernel/um_arch.c b/arch/um/kernel/um_arch.c
> index 2141f5f1f5a20..aafbaef2ae82f 100644
> --- a/arch/um/kernel/um_arch.c
> +++ b/arch/um/kernel/um_arch.c
> @@ -264,12 +264,88 @@ unsigned long brk_start;
>  
>  #define MIN_VMALLOC (32 * 1024 * 1024)
>  
> +static u64 __init read_xcr0(void)
> +{
> +	u32 a, b, c, d;
> +
> +	asm volatile("cpuid"
> +		     : "=a"(a), "=b"(b), "=c"(c), "=d"(d)
> +		     : "a"(0), "c"(0));
> +	if (a >= 1) { /* max_leaf >= 1 */
> +		asm volatile("cpuid"
> +			     : "=a"(a), "=b"(b), "=c"(c), "=d"(d)
> +			     : "a"(1), "c"(0));
> +		if (c & (1 << 27)) { /* XSAVE enabled by OS */
> +			asm volatile("xgetbv" : "=d"(d), "=a"(a) : "c"(0));
> +			return ((u64)d << 32) | a;
> +		}
> +	}

I don't know - I probably would pull in some helpers for cpuid and xgetbv
instead of doing this ad-hoc. UML maintainers' call.

> +	return 0;
> +}

Thx.

-- 
Regards/Gruss,
    Boris.

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