Messages in this thread | | | From | Jim Mattson <> | Date | Thu, 20 Aug 2020 11:38:02 -0700 | Subject | Re: FSGSBASE causing panic on 5.9-rc1 |
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On Thu, Aug 20, 2020 at 11:34 AM Tom Lendacky <thomas.lendacky@amd.com> wrote: > > On 8/20/20 11:30 AM, Tom Lendacky wrote: > > On 8/20/20 11:17 AM, Tom Lendacky wrote: > >> On 8/20/20 10:55 AM, Andy Lutomirski wrote: > >>> On Thu, Aug 20, 2020 at 8:21 AM Tom Lendacky <thomas.lendacky@amd.com> > >>> wrote: > >>>> > >>>> On 8/20/20 10:10 AM, Sean Christopherson wrote: > >>>>> On Wed, Aug 19, 2020 at 05:21:33PM -0700, Andy Lutomirski wrote: > >>>>>> On Wed, Aug 19, 2020 at 2:25 PM Andy Lutomirski <luto@kernel.org> > >>>>>> wrote: > >>>>>>> > >>>>>>> On Wed, Aug 19, 2020 at 11:19 AM Tom Lendacky > >>>>>>> <thomas.lendacky@amd.com> wrote: > >>>>>>>> > >>>>>>>> On 8/19/20 1:07 PM, Tom Lendacky wrote: > >>>>>>>>> It looks like the FSGSBASE support is crashing my second > >>>>>>>>> generation EPYC > >>>>>>>>> system. I was able to bisect it to: > >>>>>>>>> > >>>>>>>>> b745cfba44c1 ("x86/cpu: Enable FSGSBASE on 64bit by default and > >>>>>>>>> add a chicken bit") > >>>>>>>>> > >>>>>>>>> The panic only happens when using KVM. Doing kernel builds or stress > >>>>>>>>> on bare-metal appears fine. But if I fire up, in this case, a > >>>>>>>>> 64-vCPU > >>>>>>>>> guest and do a kernel build within the guest, I get the following: > >>>>>>>> > >>>>>>>> I should clarify that this panic is on the bare-metal system, not > >>>>>>>> in the > >>>>>>>> guest. And that specifying nofsgsbase on the bare-metal command > >>>>>>>> line fixes > >>>>>>>> the issue. > >>>>>>> > >>>>>>> I certainly see some oddities: > >>>>>>> > >>>>>>> We have this code: > >>>>>>> > >>>>>>> static void svm_vcpu_put(struct kvm_vcpu *vcpu) > >>>>>>> { > >>>>>>> struct vcpu_svm *svm = to_svm(vcpu); > >>>>>>> int i; > >>>>>>> > >>>>>>> avic_vcpu_put(vcpu); > >>>>>>> > >>>>>>> ++vcpu->stat.host_state_reload; > >>>>>>> kvm_load_ldt(svm->host.ldt); > >>>>>>> #ifdef CONFIG_X86_64 > >>>>>>> loadsegment(fs, svm->host.fs); > >>>>>>> wrmsrl(MSR_KERNEL_GS_BASE, current->thread.gsbase); > >>>>> > >>>>> Pretty sure current->thread.gsbase can be stale, i.e. this needs: > >>>>> > >>>>> current_save_fsgs(); > >>>> > >>>> I did try adding current_save_fsgs() in svm_vcpu_load(), saving the > >>>> current->thread.gsbase value to a new variable in the svm struct. I then > >>>> used that variable in the wrmsrl below, but it still crashed. > >>> > >>> Can you try bisecting all the way back to: > >>> > >>> commit dd649bd0b3aa012740059b1ba31ecad28a408f7f > >>> Author: Andy Lutomirski <luto@kernel.org> > >>> Date: Thu May 28 16:13:48 2020 -0400 > >>> > >>> x86/cpu: Add 'unsafe_fsgsbase' to enable CR4.FSGSBASE > >>> > >>> and adding the unsafe_fsgsbase command line option while you bisect. > >> > >> I'll give that a try. > > Bisecting with unsafe_fsgsbase identified: > > c82965f9e530 ("x86/entry/64: Handle FSGSBASE enabled paranoid entry/exit") > > But I'm thinking that could be because it starts using GET_PERCPU_BASE, > which on Rome would use RDPID. So is SVM restoring TSC_AUX_MSR too late? > That would explain why I don't see the issue on Naples, which doesn't > support RDPID.
It looks to me like SVM loads the guest TSC_AUX from vcpu_load to vcpu_put, with this comment:
/* This assumes that the kernel never uses MSR_TSC_AUX */ if (static_cpu_has(X86_FEATURE_RDTSCP)) wrmsrl(MSR_TSC_AUX, svm->tsc_aux);
We are talking about mainline here, right?
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