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

[Paolo Bonzini]

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

Paolo