Messages in this thread | ![/](/images/icornerl.gif) | | Subject | Re: FSGSBASE causing panic on 5.9-rc1 | From | Tom Lendacky <> | Date | Thu, 20 Aug 2020 11:17:00 -0500 |
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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.
Thanks, Tom
> > --Andy >
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