Re: FSGSBASE causing panic on 5.9-rc1

[Tom Lendacky]

On 8/20/20 1:41 PM, Tom Lendacky wrote:
> On 8/20/20 1:39 PM, Jim Mattson wrote:
> > On Thu, Aug 20, 2020 at 11:38 AM Jim Mattson <jmattson@xxxxxxxxxx> wrote:
> > > On Thu, Aug 20, 2020 at 11:34 AM Tom Lendacky <thomas.lendacky@xxxxxxx> 
> > > wrote:
> > > > Bisecting with unsafe_fsgsbase identified:
> > > >
> > > > c82965f9e530 ("x86/entry/64: Handle FSGSBASE enabled paranoid 
> > > > entry/exit")
> > > >
> > > > But I'm thinking that could be because it starts using GET_PERCPU_BASE,
> > > > which on Rome would use RDPID. So is SVM restoring TSC_AUX_MSR too late?
> > > > That would explain why I don't see the issue on Naples, which doesn't
> > > > support RDPID.
> > >
> > > It looks to me like SVM loads the guest TSC_AUX from vcpu_load to
> > > vcpu_put, with this comment:
> > >
> > > /* This assumes that the kernel never uses MSR_TSC_AUX */
> > > if (static_cpu_has(X86_FEATURE_RDTSCP))
> > >          wrmsrl(MSR_TSC_AUX, svm->tsc_aux);
> >
> > Correction: It never restores TSC_AUX, AFAICT.
>
> It does, it's in the host_save_user_msrs array.

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.

Thanks,
Tom

> Thanks,
> Tom
>
> >