Re: [PATCH v3 1/1] x86/tdx: Handle MWAIT, MONITOR and WBINVD

From: Sean Christopherson
Date: Tue Mar 30 2021 - 12:58:43 EST


On Tue, Mar 30, 2021, Andy Lutomirski wrote:
>
> > On Mar 30, 2021, at 8:14 AM, Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
> >
> > On Mon, Mar 29, 2021, Andy Lutomirski wrote:
> >>
> >>>> On Mar 29, 2021, at 7:04 PM, Andi Kleen <ak@xxxxxxxxxxxxxxx> wrote:
> >>>
> >>> 
> >>>>
> >>>>> No, if these instructions take a #VE then they were executed at CPL=0. MONITOR
> >>>>> and MWAIT will #UD without VM-Exit->#VE. Same for WBINVD, s/#UD/#GP.
> >>>>
> >>>> Dare I ask about XSETBV?
> >>>
> >>> XGETBV does not cause a #VE, it just works normally. The guest has full
> >>> AVX capabilities.
> >>>
> >>
> >> X *SET* BV
> >
> > Heh, XSETBV also works normally, relative to the features enumerated in CPUID.
> > XSAVES/XRSTORS support is fixed to '1' in the virtual CPU model. A subset of
> > the features managed by XSAVE can be hidden by the VMM, but attempting to enable
> > unsupported features will #GP (either from hardware or injected by TDX Module),
> > not #VE.
>
> Normally in non-root mode means that every XSETBV results in a VM exit and,
> IIUC, there’s a buglet in that this happens even if CPL==3. Does something
> special happen in TDX or does the exit get reflected back to the guest as a
> #VE?

Hmm, I forgot about that quirk. I would expect the TDX Module to inject a #GP
for that case. I can't find anything in the spec that confirms or denies that,
but injecting #VE would be weird and pointless.

Andi/Sathya, the TDX Module spec should be updated to state that XSETBV will
#GP at CPL!=0. If that's not already the behavior, the module should probably
be changed...