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

[David Laight <david.laight.linux@gmail.com>]

On Thu, 25 Jun 2026 21:37:25 -0700
Eric Biggers <ebiggers@kernel.org> wrote:

> If the CPU declares AVX or AVX-512 support, verify that all the
> corresponding xstate bits are also set.  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 xstate bits in addition to X86_FEATURE_AVX*.
> 
...
>  static void __init parse_host_cpu_flags(char *line)
>  {
> +	u64 xcr0 = read_xcr0();
>  	int i;
> +
>  	for (i = 0; i < 32*NCAPINTS; i++) {
>  		if ((x86_cap_flags[i] != NULL) && strstr(line, x86_cap_flags[i]))

'line' comes from /proc/cpuinfo
Surely something would be terribly wrong if that included something the kernel
had disabled (or didn't support).

	David


> -			set_cpu_cap(&boot_cpu_data, i);
> +			validate_and_set_cpu_cap(i, xcr0);
>  	}
>  }
>  
>  static void __init parse_cache_line(char *line)
>  {