linux-kernel - Re: FSGSBASE causing panic on 5.9-rc1

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Message-ID: <f6694ee6-d672-1cf9-deaf-4d600bc4e9eb@amd.com>
Date:   Thu, 20 Aug 2020 15:15:45 -0500
From:   Tom Lendacky <thomas.lendacky@....com>
To:     Dave Hansen <dave.hansen@...el.com>,
        Jim Mattson <jmattson@...gle.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Joerg Roedel <joro@...tes.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        X86 ML <x86@...nel.org>,
        "Chang S. Bae" <chang.seok.bae@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sasha Levin <sashal@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: FSGSBASE causing panic on 5.9-rc1

On 8/20/20 3:07 PM, Dave Hansen wrote:
> On 8/20/20 12:05 PM, Tom Lendacky wrote:
>>> I added a quick hack to save TSC_AUX to a new variable in the SVM
>>> struct and then restore it right after VMEXIT (just after where GS is
>>> restored in svm_vcpu_enter_exit()) and my guest is no longer crashing.
>>
>> Sorry, I mean my host is no longer crashing.
> 
> Just to make sure I've got this:
> 1. Older CPUs didn't have X86_FEATURE_RDPID
> 2. FSGSBASE patches started using RDPID in the NMI entry path when
>     supported *AND* FSGSBASE was enabled
> 3. There was a latent SVM bug which did not restore the RDPID data
>     before NMIs were reenabled after VMEXIT
> 4. If an NMI comes in the window between VMEXIT and the
>     wrmsr(TSC_AUX)... boom

Right, which means that the setting of TSC_AUX to the guest value needs to 
be moved, too.

Thanks,
Tom

> 
> If FSGSBASE reverted is disabled (as Tom did on the command-line), then
> the RDPID path isn't hit.
> 
> Fun.
>