Messages in this thread | | | Date | Thu, 20 Aug 2020 15:34:31 -0700 | From | Sean Christopherson <> | Subject | Re: FSGSBASE causing panic on 5.9-rc1 |
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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: > > > > > > > > > > On Aug 20, 2020, at 1:15 PM, Tom Lendacky <thomas.lendacky@amd.com> 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 :-)
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