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

[Sean Christopherson]

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.