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

[Sean Christopherson <seanjc@google.com>]

On Mon, Nov 21, 2022, Borislav Petkov wrote:
> On Mon, Nov 21, 2022 at 03:48:06PM +0000, Sean Christopherson wrote:
> > Actually, for these features that don't require additional KVM enabling, KVM isn't
> > making the feature avaiable to the guest.  KVM is just advertising to userspace
> > that KVM "supports" these features.  Userspace ultimately controls guest CPUID;
> > outside of a few special cases, KVM neither rejects nor filters unsupported bits
> > in CPUID.
> 
> So is there any point to those "enable it in KVM" patches streaming constantly?

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.

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.