Re: [PATCH] KVM: SVM: Propagate Translation Cache Extensions to the guest

[Sean Christopherson <seanjc@google.com>]

On Fri, Mar 06, 2026, Yosry Ahmed wrote:
> > Hrm, I think we should handle all of the kvm_enable_efer_bits() calls that are
> > conditioned only on CPU support in common code.  While it's highly unlikely Intel
> > CPUs will ever support more EFER-based features, if they do, then KVM will
> > over-report support since kvm_initialize_cpu_caps() will effectively enable the
> > feature, but VMX won't enable the corresponding EFER bit.
> >
> > I can't think anything that will go sideways if we rely purely on KVM caps, so
> > get to something like this as prep work, and then land TCE in common x86?
> 
> Taking a second look here, doesn't this break the changes introduced
> by commit 11988499e62b ("KVM: x86: Skip EFER vs. guest CPUID checks
> for host-initiated writes")? Userspace writes may fail if the
> corresponding CPUID feature is not enabled.

No, because kvm_cpu_cap_has() == boot_cpu_has() filtered by what KVM supports.
All of these EFER updates subtly rely on KVM enabling the associated CPUID
feature in kvm_set_cpu_caps().

If we used guest_cpu_cap_has(), then yes, that would be a problem.