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

[Sean Christopherson]

On Thu, Sep 12, 2024, Paolo Bonzini wrote:
> On Thu, Sep 12, 2024 at 6:42 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
> >
> > On Thu, Sep 12, 2024, Paolo Bonzini wrote:
> > > On Thu, Sep 12, 2024 at 4:45 PM Xiaoyao Li <xiaoyao.li@xxxxxxxxx> 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.

Right, I don't care about those precisely because there's nothing KVM can or
_needs_ to do for features that don't have interception controls.

> 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.

Oh, I'm well aware :-)

> > 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.

I assume it's plain text though?  I.e. whatever ioctl() sets TDCS.XFAM can be
rejected by KVM if it attempts to enable unsupported features?

I don't expect that we'll want KVM to gatekeep many, if any features, but I do
think we should require explicit enabling in KVM whenever possible, even if the
enabling is boring and largely ceremonial.