Re: [PATCH v10 24/27] KVM: x86: Enable CET virtualization for VMX and advertise to userspace

[Edgecombe, Rick P]

On Wed, 2024-05-01 at 16:15 -0700, Sean Christopherson wrote:
> On Sun, Feb 18, 2024, Yang Weijiang wrote:
> > @@ -696,6 +697,20 @@ void kvm_set_cpu_caps(void)
> >                 kvm_cpu_cap_set(X86_FEATURE_INTEL_STIBP);
> >         if (boot_cpu_has(X86_FEATURE_AMD_SSBD))
> >                 kvm_cpu_cap_set(X86_FEATURE_SPEC_CTRL_SSBD);
> > +       /*
> > +        * Don't use boot_cpu_has() to check availability of IBT because the
> > +        * feature bit is cleared in boot_cpu_data when ibt=off is applied
> > +        * in host cmdline.
> 
> I'm not convinced this is a good reason to diverge from the host kernel.  E.g.
> PCID and many other features honor the host setup, I don't see what makes IBT
> special.
> 
> LA57 is special because it's entirely reasonable, likely even, for a host to
> only want to use 48-bit virtual addresses, but still want to let the guest
> enable
> LA57.

Definitely. I swear we (Weijiang and I) had a back and forth at some point where
we agreed to match the host support. Plus I think the CET FPU stuff triggers off
of host support for CET. So if the host doesn't have X86_FEATURE_SHSTK or
X86_FEATURE_IBT then... hopefully it's caught later. But then don't report it's
supported.