Messages in this thread | | | Subject | Re: FSGSBASE causing panic on 5.9-rc1 | From | Tom Lendacky <> | Date | Thu, 20 Aug 2020 10:21:31 -0500 |
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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.
Thanks, Tom
> wrmsrl(MSR_KERNEL_GS_BASE, current->thread.gsbase); > > On a related topic, we really should consolidate the VMX and SVM code for > these flows, they're both ugly. > >>> load_gs_index(svm->host.gs); >>> >>> Surely that should do load_gs_index() *before* wrmsrl(). But that's >>> not the problem at hand. >>> >>> There are also some open-coded rdmsr and wrmsrs of MSR_GS_BASE -- >>> surely these should be x86_gsbase_read_cpu() and >>> x86_gsbase_write_cpu(). (Those functions don't actually exist, but >>> the fsbase equivalents do, and we should add them.) But that's also >>> not the problem at hand. >> >> Make that cpu_kernelmode_gs_base(cpu). Perf win on all CPUs. >> >> But I still don't see the bug. > >
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