Re: FSGSBASE causing panic on 5.9-rc1

[Sean Christopherson <sean.j.christopherson@intel.com>]

On Thu, Aug 20, 2020 at 07:00:16PM -0500, Tom Lendacky wrote:
> On 8/20/20 5:34 PM, Sean Christopherson wrote:
> > 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@intel.com> wrote:
> > > > 
> > > > On Thu, Aug 20, 2020 at 01:36:46PM -0700, Andy Lutomirski wrote:
> > > > > 
> > > > > 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 :-)
> 
> I can do it, but won't be able to get to it until sometime tomorrow.

Confirmed VMX goes kaboom when running perf with a VM.  Patch incoming.