Re: FSGSBASE causing panic on 5.9-rc1

[Sean Christopherson]

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();
	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.