Re: FSGSBASE causing panic on 5.9-rc1

[Andy Lutomirski]

> On Aug 20, 2020, at 1:15 PM, Tom Lendacky <thomas.lendacky@xxxxxxx> wrote:
> 
> 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.
> 

Depending on how much of a perf hit this is, we could also skip using RDPID in the paranoid path on SVM-capable CPUs.