Re: [PATCH v10 24/27] KVM: x86: Enable CET virtualization for VMX and advertise to userspace
From: Edgecombe, Rick P
Date: Wed May 01 2024 - 19:25:14 EST
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.