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Subject: Re: FSGSBASE causing panic on 5.9-rc1
To:     Andy Lutomirski <luto@kernel.org>
Cc:     Sean Christopherson <sean.j.christopherson@intel.com>,
        Joerg Roedel <joro@8bytes.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        X86 ML <x86@kernel.org>,
        "Chang S. Bae" <chang.seok.bae@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Sasha Levin <sashal@kernel.org>,
        Borislav Petkov <bp@alien8.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>
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From:   Tom Lendacky <thomas.lendacky@amd.com>
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Date:   Thu, 20 Aug 2020 11:17:00 -0500
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On 8/20/20 10:55 AM, Andy Lutomirski wrote:
> On Thu, Aug 20, 2020 at 8:21 AM Tom Lendacky <thomas.lendacky@amd.com> 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@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.
> 
> Can you try bisecting all the way back to:
> 
> commit dd649bd0b3aa012740059b1ba31ecad28a408f7f
> Author: Andy Lutomirski <luto@kernel.org>
> 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
>