Re: [PATCH v4 1/6] x86: KVM: Advertise CMPccXADD CPUID to user space

[Borislav Petkov]

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