Messages in this thread | | | Date | Thu, 20 Aug 2020 08:10:49 -0700 | From | Sean Christopherson <> | Subject | Re: FSGSBASE causing panic on 5.9-rc1 |
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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(); 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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