RE: [PATCH v3 05/22] kvm: x86: Check permitted dynamic xfeatures at KVM_SET_CPUID2

[Tian, Kevin]

> From: Sean Christopherson <seanjc@xxxxxxxxxx>
> Sent: Wednesday, December 29, 2021 7:39 AM
> 
> On Wed, Dec 22, 2021, Jing Liu wrote:
> > Guest xstate permissions should be set by userspace VMM before vcpu
> > creation. Extend KVM_SET_CPUID2 to verify that every feature reported
> > in CPUID[0xD] has proper permission set.
> 
> Why?  Nothing in the changelog, code, or comments explains why KVM
> _needs_ to prevent
> userspace from advertising bogus features to the guest.  E.g. the virtual
> address
> width check exists because _KVM_ will do the wrong thing given a width
> other than 48
> or 57, and explicity says as much in a comment.

Advertising a known bogus feature due to lacking of permission does
no good compared to failing it early even before the guest is running. This
also avoids tons of complexity at run-time to deal with permission
violation (e.g. introducing new kvm exit reason if you tracked the v2
discussion).

But yes, we should add a clear comment here.

Thanks
Kevin