Re: FSGSBASE causing panic on 5.9-rc1

[Tom Lendacky]

On 8/20/20 3:07 PM, Dave Hansen wrote:
> On 8/20/20 12:05 PM, Tom Lendacky wrote:
> > > I added a quick hack to save TSC_AUX to a new variable in the SVM
> > > struct and then restore it right after VMEXIT (just after where GS is
> > > restored in svm_vcpu_enter_exit()) and my guest is no longer crashing.
> >
> > Sorry, I mean my host is no longer crashing.
>
> Just to make sure I've got this:
> 1. Older CPUs didn't have X86_FEATURE_RDPID
> 2. FSGSBASE patches started using RDPID in the NMI entry path when
>     supported *AND* FSGSBASE was enabled
> 3. There was a latent SVM bug which did not restore the RDPID data
>     before NMIs were reenabled after VMEXIT
> 4. If an NMI comes in the window between VMEXIT and the
>     wrmsr(TSC_AUX)... boom

Right, which means that the setting of TSC_AUX to the guest value needs to 
be moved, too.

Thanks,
Tom

> If FSGSBASE reverted is disabled (as Tom did on the command-line), then
> the RDPID path isn't hit.
>
> Fun.