Re: [PATCH v1 1/2] x86/kvm/fpu: Mask guest fpstate->xfeatures with guest_supported_xcr0

[Paolo Bonzini]

On 2/7/22 23:45, Leonardo Bras Soares Passos wrote:
> On Mon, Feb 7, 2022 at 6:00 PM Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
> > On 2/7/22 21:24, Leonardo Bras Soares Passos wrote:
> > > > With this patch,
> > > > we have to reason about the effect of calling KVM_SET_CPUID2 twice calls
> > > > back to back.  I think an "&=" would be wrong in that case.
> > >
> > > So, you suggest something like this ?
> > >
> > > vcpu->arch.guest_fpu.fpstate->xfeatures =
> > >          fpu_user_cfg.default_features & vcpu->arch.guest_supported_xcr0;
> >
> > Yes, but you need to change user_xfeatures instead of xfeatures.
> > KVM_GET_XSAVE and KVM_SET_XSAVE will take it into account automatically:
> >
> > - KVM_GET_XSAVE: fpu_copy_guest_fpstate_to_uabi -> __copy_xstate_to_uabi_buf
> >
> > - KVM_SET_XSAVE: fpu_copy_uabi_to_guest_fpstate ->
> > copy_uabi_from_kernel_to_xstate -> copy_uabi_to_xstate ->
> > validate_user_xstate_buffer
>
>
> Ok, I understand how this replaces patch 2/2, so no issue on that.
>
> About patch 1/2,  you suggest that instead of fixing what we save in
> the regs buffer, we fix only what we want to return to the user when
> they call KVM_GET_XSAVE, is that correct?

Yes, exactly.

Paolo