Messages in this thread | | | Date | Mon, 21 Nov 2022 20:50:24 +0100 | From | Borislav Petkov <> | Subject | Re: [PATCH v4 1/6] x86: KVM: Advertise CMPccXADD CPUID to user space |
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On Mon, Nov 21, 2022 at 05:28:39PM +0000, Sean Christopherson wrote: > Yes. Most userspace VMMs sanitize their CPUID models based on KVM_GET_SUPPORTED_CPUID, > e.g. by default, QEMU will refuse to enable features in guest CPUID that aren't > reported as supported by KVM. > > Another use case is for userspace to blindly use the result of KVM_GET_SUPPORTED_CPUID > as the guest's CPUID model, e.g. for using KVM to isolate code as opposed to standing > up a traditional virtual machine. For that use case, userspace again relies on KVM to > enumerate support.
Ah ok, thx.
/me makes a mental note.
> What I was trying to call out in the above is that the KVM "enabling" technically > doesn't expose the feature to the guest. E.g. a clever guest could ignore CPUID > and probe the relevant instructions manually by seeing whether or not they #UD.
As can a nasty userspace on baremetal. That's why /proc/cpuinfo is not really the authority of what's supported and we're going away from treating it that way.
-- Regards/Gruss, Boris.
https://people.kernel.org/tglx/notes-about-netiquette
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