Re: [PATCH v19 023/130] KVM: TDX: Initialize the TDX module when loading the KVM intel kernel module

Sean Christopherson
2024-04-25T16:31:02Z

On Tue, Apr 23, 2024, Kai Huang wrote:
> On Tue, 2024-04-23 at 22:59 +0000, Huang, Kai wrote:
> > > Right, but that doesn't say why the #UD occurred.  The macro dresses it up in
> > > TDX_SW_ERROR so that KVM only needs a single parser, but at the end of the day
> > > KVM is still only going to see that SEAMCALL hit a #UD.
> >
> > Right.  But is there any problem here?  I thought the point was we can
> > just use the error code to tell what went wrong.
>
> Oh, I guess I was replying too quickly. From the spec, #UD happens when
>
> IF not in VMX operation or inSMM or inSEAM or
> ((IA32_EFER.LMA & CS.L) == 0)
> THEN #UD;
>
> Are you worried about #UD was caused by other cases rather than "not in
> VMX operation"?

Yes.

> But it's quite obvious the other 3 cases are not possible, correct?

The spec I'm looking at also has:

If IA32_VMX_PROCBASED_CTLS3[5] is 0.

And anecdotally, I know of at least one crash in our production environment where
a VMX instruction hit a seemingly spurious #UD, i.e. it's not impossible for a
ucode bug or hardware defect to cause problems. That's obviously _extremely_
unlikely, but that's why I emphasized that sanity checking CR4.VMXE is cheap.
Practically speaking it costs nothing, so IMO it's worth adding even if the odds
of it ever being helpful are one-in-and-million.