Re: FSGSBASE causing panic on 5.9-rc1

[Tom Lendacky <thomas.lendacky@amd.com>]

On 8/20/20 11:30 AM, Tom Lendacky wrote:
> On 8/20/20 11:17 AM, Tom Lendacky wrote:
>> 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.

Bisecting with unsafe_fsgsbase identified:

c82965f9e530 ("x86/entry/64: Handle FSGSBASE enabled paranoid entry/exit")

But I'm thinking that could be because it starts using GET_PERCPU_BASE, 
which on Rome would use RDPID. So is SVM restoring TSC_AUX_MSR too late? 
That would explain why I don't see the issue on Naples, which doesn't 
support RDPID.

Thanks,
Tom

>>
>>>
>>> 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.
> 
> All the selftests passed.
> 
> Thanks,
> Tom
> 
>>
>> Thanks,
>> Tom
>>
>>>
>>> --Andy
>>>