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

[Leonardo Bras Soares Passos]

On Mon, Feb 7, 2022 at 7:59 PM Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
>
> 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.

Thanks! I will update my patch and send a v2 shortly.

I got really curious while I was debugging this issue:
- Is it ok that the cpu has other features enabled (like PKRU), while
our vcpu does not have them?
- Should guest OS always use the cpuid for checking features available?
- Would it be better if we could have exactly the same fpu features
enabled in the cpu, as we have in the vcpu?
- Why do we xsave with a mask different from what we xrstor ?

>
> Paolo
>