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

[Anton Ivanov]

On 26/06/2026 08:41, David Laight wrote:
> On Thu, 25 Jun 2026 21:37:25 -0700
> Eric Biggers <ebiggers@xxxxxxxxxx> 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)
> >   {
>
Lots of other stuff will go wrong before that. Glibc, things compiled with LLVM, python, perl, etc.

Half of the userland will go belly up, because AVX is used in string operations and hashing if it is available.

UML is just another userland application from this perspective, so there is no reason for it to behave any different from the rest of the userland.
--
Anton R. Ivanov
Cambridgegreys Limited. Registered in England. Company Number 10273661