RE: [patch 5/6] x86/fpu: Provide fpu_update_guest_xcr0/xfd()

Tian, Kevin
2021-12-16T10:21:55Z

> From: Paolo Bonzini
> Sent: Wednesday, December 15, 2021 6:41 PM
>
> There's also another important thing that hasn't been mentioned so far:
> KVM_GET_SUPPORTED_CPUID should _not_ include the dynamic bits in
> CPUID[0xD] if they have not been requested with prctl. It's okay to
> return the AMX bit, but not the bit in CPUID[0xD].

There is no vcpu in this ioctl, thus we cannot check vcpu->arch.guest_fpu.perm.

This then requires exposing xstate_get_guest_group_perm() to KVM. Thomas,
are you OK with this change given Paolo's ask? v1 included this change but
it was not necessary at the moment:

https://lore.kernel.org/lkml/87lf0ot50q.ffs@tglx/

and Paolo, do we want to document that prctl() must be done before
calling KVM_GET_SUPPORTED_CPUID? If yes, where is the proper location?

Thanks
Kevin