Re: FSGSBASE causing panic on 5.9-rc1

[Andy Lutomirski]

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);
        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.

I haven't actually spotted the bug yet...

--Andy