Re: FSGSBASE causing panic on 5.9-rc1

[Tom Lendacky]

On 8/20/20 10:55 AM, Andy Lutomirski wrote:
> On Thu, Aug 20, 2020 at 8:21 AM Tom Lendacky <thomas.lendacky@xxxxxxx> 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@xxxxxxxxxx> wrote:
> > > > > On Wed, Aug 19, 2020 at 11:19 AM Tom Lendacky <thomas.lendacky@xxxxxxx> 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@xxxxxxxxxx>
> 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