Re: [RFC v2 16/32] x86/tdx: Handle MWAIT, MONITOR and WBINVD

[Andi Kleen]

On 5/10/2021 7:44 PM, Kuppuswamy, Sathyanarayanan wrote:
> On 5/10/21 7:17 PM, Andi Kleen wrote:
> > > > To prevent TD guest from using MWAIT/MONITOR instructions,
> > > > support for these instructions are already disabled by TDX
> > > > module (SEAM). So CPUID flags for these instructions should
> > > > be in disabled state.
> > > Why does this not result in a #UD if the instruction is disabled by
> > > SEAM?
> >
> > It's just the TDX module (SEAM is the execution mode used by the TDX 
> > module)
>
> If it is disabled by the TDX Module, we should never execute it. But 
> for some
> reason, if we still come across this instruction (buggy TDX module?), 
> we add
> appropriate warning in  #VE handler.

I think the only case where it could happen is if the kernel jumps to a 
random address due to a bug and the destination happens to be these 
instruction bytes. Of course it is exceedingly unlikely.

Or we make some mistake, but that's hopefully fixed quickly.


-Andi