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

Edgecombe, Rick P
2024-09-12T18:42:46Z

On Thu, 2024-09-12 at 20:29 +0200, Paolo Bonzini wrote:
> > 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.

There are some ATTRIBUTES (the non-xsave features like PKS get bucketed in
there), which can affect the host. So we have to filter this config in KVM. I'd
just assume not trust future XFAM bits because it's easy to implement.

>
> > 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.