Re: [PATCH 09/10] x86/fpu: Allow restoring signal frames with larger xstate_size

[Chang S. Bae]

On 8/7/2026 4:12 PM, Andrei Vagin wrote:
> We definitely can trigger enablement of dynamic features from
> user-space, but the issue is that userspace currently has no way to
> query what features are actually enabled for a task (the kernel only
> exposes fpstate->user_xfeatures through signal frames, which is not
> viable for C/R). 

https://docs.kernel.org/arch/x86/xstate.html

-ARCH_GET_XCOMP_SUPP
arch_prctl(ARCH_GET_XCOMP_SUPP, &features);

ARCH_GET_XCOMP_SUPP stores the supported features in userspace storage 
of type uint64_t. The second argument is a pointer to that storage.

-ARCH_GET_XCOMP_PERM
arch_prctl(ARCH_GET_XCOMP_PERM, &features);

ARCH_GET_XCOMP_PERM stores the features for which the userspace process 
has permission in userspace storage of type uint64_t. The second 
argument is a pointer to that storage.

Do you need something else than these?

Thanks,
Chang