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

[Xiaoyao Li]

On 9/13/2024 12:42 AM, Sean Christopherson 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.
>
> 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.

Actually MWAIT becoems allowed by TDX and it's configurable.

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