Re: [PATCH v2 1/1] x86/tdx: Handle MWAIT, MONITOR and WBINVD

[Kuppuswamy, Sathyanarayanan]

On 3/29/21 3:02 PM, Dave Hansen wrote:
> On 3/29/21 2:55 PM, Kuppuswamy, Sathyanarayanan wrote:
> > > MONITOR is a privileged instruction, right?  So we can only end up in
> > > here if the kernel screws up and isn't reading CPUID correctly, right?
> > >
> > > That dosen't seem to me like something we want to suppress.  This needs
> > > a warning, at least.  I assume that having a MONITOR instruction
> > > immediately return doesn't do any harm.
> > Agree. Since we are not supposed to come here, I will use BUG.
>
> "This is unexpected" is a WARN()able offense.
>
> "This is unexpected and might be corrupting data" is where we want to
> use BUG().
>
> Does broken MONITOR risk data corruption?
We will be reaching this point only if something is buggy in kernel. I am
not sure about impact of this buggy state. But MONITOR instruction by
itself, should not cause data corruption.

> > > > +    case EXIT_REASON_MWAIT_INSTRUCTION:
> > > > +        /* MWAIT is supressed, not supposed to reach here. */
> > > > +        WARN(1, "MWAIT unexpected #VE Exception\n");
> > > > +        return -EFAULT;
> > >
> > > How is MWAIT "supppressed"?
> > I am clearing the MWAIT feature flag in early init code. We should also
> > disable this feature in firmware. setup_clear_cpu_cap(X86_FEATURE_MWAIT);
>
> I'd be more explicit about that.  Maybe even reference the code that
> clears the X86_FEATURE.
This change is part of the same patch.
>

--
Sathyanarayanan Kuppuswamy
Linux Kernel Developer