Re: FSGSBASE causing panic on 5.9-rc1

[Dave Hansen]

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

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

Fun.