Re: [PATCH v3 17/27] KVM: x86: Mark CR4.FRED as not reserved when guest can use FRED

[Sean Christopherson]

On Thu, Dec 12, 2024, Xin Li wrote:
> On 10/24/2024 12:18 AM, Chao Gao wrote:
> > > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> > > index 03f42b218554..bfdd10773136 100644
> > > --- a/arch/x86/kvm/vmx/vmx.c
> > > +++ b/arch/x86/kvm/vmx/vmx.c
> > > @@ -8009,6 +8009,10 @@ void vmx_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
> > > 	kvm_governed_feature_check_and_set(vcpu, X86_FEATURE_LAM);
> > > 	kvm_governed_feature_check_and_set(vcpu, X86_FEATURE_FRED);
> > > 
> > > +	/* Don't allow CR4.FRED=1 before all of FRED KVM support is in place. */
> > > +	if (!guest_can_use(vcpu, X86_FEATURE_FRED))
> > > +		vcpu->arch.cr4_guest_rsvd_bits |= X86_CR4_FRED;
> > 
> > is this necessary? __kvm_is_valid_cr4() ensures that guests cannot set any bit
> > which isn't supported by the hardware.
> > 
> > To account for hardware/KVM caps, I think the following changes will work. This
> > will fix all other bits besides X86_CR4_FRED.
> 
> This seems a generic infra improvement, maybe it's better for you to
> send it as an individual patch to Sean and the KVM mailing list?

Already ahead of y'all :-)  (I think, I didn't look closely at this).

https://lore.kernel.org/all/20241128013424.4096668-6-seanjc@xxxxxxxxxx