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

[Sean Christopherson <seanjc@google.com>]

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.

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.

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.

[1] https://lore.kernel.org/all/20240517173926.965351-34-seanjc@google.com
[2] https://lore.kernel.org/all/20240517173926.965351-35-seanjc@google.com