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

[Paolo Bonzini]

On 12/16/21 11:21, Tian, Kevin wrote:
> > 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.

Right, this is a generic /dev/kvm ioctl therefore it has to check the 
process state.

> 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?

You can document it under the KVM_GET_SUPPORTED_CPUID ioctl.

Paolo