Re: [PATCH v3 1/1] x86/kvm/fpu: Mask guest fpstate->xfeatures with guest_supported_xcr0

[Paolo Bonzini <pbonzini@redhat.com>]

On 2/14/22 10:43, David Edmondson wrote:
> Sorry if this is a daft question:
> 
> In what situations will there be bits set in
> vcpu->arch.guest_supported_xcr0 that are not set in
> vcpu->arch.guest_fpu.fpstate->xfeatures ?
> 
> guest_supported_xcr0 is filtered based on supported_xcr0, which I would
> expect to weed out all bits that are not set in ->xfeatures.

Good point, so we can do just

	vcpu->arch.guest_fpu.fpstate->user_xfeatures =
		vcpu->arch.guest_supported_xcr0;

On top of this patch, we can even replace vcpu->arch.guest_supported_xcr0
with vcpu->arch.guest_fpu.fpstate->user_xfeatures.  Probably with local
variables or wrapper functions though, so as to keep the code readable.
For example:

static inline u64 kvm_guest_supported_xfd()
{
	u64 guest_supported_xcr0 = vcpu->arch.guest_fpu.fpstate->user_xfeatures;

	return guest_supported_xcr0 & XFEATURE_MASK_USER_DYNAMIC;
}

Also, already in this patch fpstate_realloc should do

         newfps->user_xfeatures = curfps->user_xfeatures | xfeatures;

only if !guest_fpu.  In other words, the user_xfeatures of the guest FPU
should be controlled exclusively by KVM_SET_CPUID2.

Thanks,

Paolo