Messages in this thread | | | Subject | Re: FSGSBASE causing panic on 5.9-rc1 | From | Tom Lendacky <> | Date | Thu, 20 Aug 2020 11:30:50 -0500 |
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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. > >> >> Also, you're crashing when you run a guest, right? Can you try > > Right, when the guest is running. The guest boots fine and only when I put > some stress on it (kernel build) does it cause the issue. It might be > worth trying to pin all the vCPUs and see if the crash still happens. > >> running the x86 sefltests on a bad kernel without running any guests? > > I'll give that a try.
All the selftests passed.
Thanks, Tom
> > Thanks, > Tom > >> >> --Andy >>
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