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

[David Laight]

On Fri, 26 Jun 2026 09:21:49 +0100
Anton Ivanov <anton.ivanov at cambridgegreys.com> wrote:

> On 26/06/2026 08:41, David Laight wrote:
> > On Thu, 25 Jun 2026 21:37:25 -0700
> > Eric Biggers <ebiggers at 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)
> >>   {  
> > 
> > 
> > 
>  >  
> 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.

And glibc will check xcr0.

> 
> 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.