Re: FSGSBASE causing panic on 5.9-rc1

[Sean Christopherson]

On Thu, Aug 20, 2020 at 03:07:10PM -0700, Andy Lutomirski wrote:
> On Thu, Aug 20, 2020 at 3:05 PM Sean Christopherson
> <sean.j.christopherson@xxxxxxxxx> wrote:
> >
> > On Thu, Aug 20, 2020 at 01:36:46PM -0700, Andy Lutomirski wrote:
> > >
> > >
> > > > 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.
> >
> > Doesn't this affect VMX as well?  KVM+VMX doesn't restore TSC_AUX until the
> > kernel returns to userspace.  I don't see anything that prevents the NMI
> > RDPID path from affecting Intel CPUs.
> >
> > Assuming that's the case, I would strongly prefer this be handled in the
> > paranoid path.  NMIs are unblocked immediately on VMX VM-Exit, which means
> > using the MSR load lists in the VMCS, and I hate those with a vengeance.
> >
> > Perf overhead on VMX would be 8-10% for VM-Exits that would normally stay
> > in KVM's run loop, e.g. ~125 cycles for the WMRSR, ~1300-1500 cycles to
> > handle the most common VM-Exits.  It'd be even higher overhead for the
> > VMX preemption timer, which is handled without even enabling IRQs and is
> > a hot path as it's used to emulate the TSC deadline timer for the guest.
> 
> I'm fine with that -- let's get rid of RDPID unconditionally in the
> paranoid path.  Want to send a patch that also adds as comment
> explaining why we're not using RDPID?

Sure, though I won't object if Tom beats me to the punch :-)