RE: [PATCH v19 130/130] RFC: KVM: x86, TDX: Add check for KVM_SET_CPUID2

Huang, Kai
2024-03-25T22:32:06Z

> On Mon, 2024-03-25 at 11:14 +0000, Huang, Kai wrote:
> > To confirm, I mean you want to simply make KVM_SET_CPUID2 return error
> > for TDX guest?
> >
> > It is acceptable to me, and I don't see any conflict with Sean's comments.
> >
> > But I don't know Sean's perference.  As he said, I think  the
> > consistency checking is quite straight-forward:
> >
> > "
> > It's not complicated at all.  Walk through the leafs defined during
> > TDH.MNG.INIT, reject KVM_SET_CPUID if a leaf isn't present or doesn't
> > match exactly.
> > "
> >
> Yea, I'm just thinking if we could take two patches down to one small one it
> might be a way to essentially break off this work to another series without
> affecting the ability to boot a TD. It
> *seems* to be the way things are going.
>
> > So to me it's not a big deal.
> >
> > Either way, we need a patch to handle SET_CPUID2:
> >
> > 1) if we go option 1) -- that is reject SET_CPUID2 completely -- we
> > need to make vcpu's CPUID point to KVM's saved CPUID during
> TDH.MNG.INIT.
>
> Ah, I missed this part. Can you elaborate? By dropping these two patches it
> doesn't prevent a TD boot. If we then reject SET_CPUID, this will break things
> unless we make other changes? And they are not small?
>

(sorry replying from outlook due to some issue to my linux box environment)

It booted because Qemu does sane thing, i.e., it always passes the correct CPUIDs in KVM_SET_CPUID2.

Per-Sean's comments, KVM should guarantee the consistency between CPUIDs done in TDH.MNG.INIT and KVM_SET_CPUID2, otherwise if Qemu passes in-consistent CPUIDs KVM can easily fail to work with TD.

To guarantee the consistency, KVM could do two options as we discussed:

1) reject KVM_SET_CPUID2 completely.
2) Still allow KVM_SET_CPUID2 but manually check the CPUID consistency between the one done in TDH.MNG.INIT and the one passed in KVM_SET_CPUID2.

1) can obviously guarantee consistency. But KVM maintains CPUIDs in 'vcpu', so to make the existing KVM code continue to work, we need to manually set 'vcpu->cpuid' to the one that is done in TDH.MNG.INIT.

2) you need to check the consistency and reject KVM_SET_CPUID2 if in-consistency found. But other than that, KVM doesn't need to anything more because if we allow KVM_SET_CPUID2, the 'vcpu' will have its own CPUIDs populated anyway.