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

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