Re: [PATCH 25/25] KVM: x86: Add CPUID bits missing from KVM_GET_SUPPORTED_CPUID

Paolo Bonzini
2024-09-12T18:29:43Z

On Thu, Sep 12, 2024 at 6:42 PM Sean Christopherson <seanjc@google.com> wrote:
>
> On Thu, Sep 12, 2024, Paolo Bonzini wrote:
> > On Thu, Sep 12, 2024 at 4:45 PM Xiaoyao Li <xiaoyao.li@intel.com> wrote:
> > > > KVM is not going to have any checks, it's only going to pass the
> > > > CPUID to the TDX module and return an error if the check fails
> > > > in the TDX module.
> > >
> > > If so, new feature can be enabled for TDs out of KVM's control.
> > >
> > > Is it acceptable?
> >
> > It's the same as for non-TDX VMs, I think it's acceptable.
>
> No? IIUC, it's not the same.
>
> E.g. KVM doesn't yet support CET, and while userspace can enumerate CET support
> to VMs all it wants, guests will never be able to set CR4.CET and thus can't
> actually enable CET.
>
> IIUC, the proposal here is to allow userspace to configure the features that are
> exposed _and enabled_ for a TDX VM without any enforcement from KVM.

Yeah, that's correct, on the other hand a lot of features are just
new instructions and no new registers. Those pass under the radar
and in fact you can even use them if the CPUID bit is 0 (of course).
Others are just data, and again you can pass any crap you'd like.

And for SNP we had the case where we are forced to leave features
enabled if their state is in the VMSA, because we cannot block
writes to XCR0 and XSS that we'd like to be invalid.

> CET might be a bad example because it looks like it's controlled by TDCS.XFAM, but
> presumably there are other CPUID-based features that would actively enable some
> feature for a TDX VM.

XFAM is controlled by userspace though, not KVM, so we've got no
control on that either.

> For HYPERVISOR and TSC_DEADLINE_TIMER, I would much prefer to fix those KVM warts,
> and have already posted patches[1][2] to do exactly that.
>
> With those out of the way, are there any other CPUID-based features that KVM
> supports, but doesn't advertise? Ignore MWAIT, it's a special case and isn't
> allowed in TDX VMs anyways.

I don't think so.

Paolo