Re: FSGSBASE causing panic on 5.9-rc1

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

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.

Thanks,
Tom

> 	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.
> 
>