Re: [PATCH 1/3] x86/cpufeatures: Add support for cpuid leaf 80000021/EAX (FeatureExt2Eax)

[Sean Christopherson]

On Wed, Nov 16, 2022, Borislav Petkov wrote:
> On Tue, Nov 15, 2022 at 05:10:50PM -0600, Kim Phillips wrote:
> > When trying to wire up a scattered host AUTOIBRS version up to
> > kvm, I couldn't get past all the reverse_cpuid_check()
> > BUILD_BUGs demanding exclusivity between h/w and "Linux"
> > (s/w) FEATUREs.

FWIW, it's not exclusivity per se, it's to ensure that any CPUID bits KVM wants
to advertise to userspace uses the architectural definition and not the kernel's
software defined info.  This allows KVM to do things like

	if (guest_cpuid_has(X86_FEATURE_AUTOIBRS))

and guarantee that the lookup on guest CPUID, which follows the architectural
layout, will look at the correct leaf+subleaf+reg+bit.

> I guess something like below.
> 
> Sean, can you pls check the KVM bits whether I've done them all right?

Looks correct.