Messages in this thread | | | Date | Thu, 20 Aug 2020 15:05:07 -0700 | From | Sean Christopherson <> | Subject | Re: FSGSBASE causing panic on 5.9-rc1 |
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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.
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