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

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Message-Id: <5756198D-C8BD-4290-BFCA-04424EB230A6@amacapital.net>
Date:   Thu, 20 Aug 2020 13:36:46 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Tom Lendacky <thomas.lendacky@....com>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        Jim Mattson <jmattson@...gle.com>,
        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 Aug 20, 2020, at 1:15 PM, Tom Lendacky <thomas.lendacky@....com> wrote:
> 
> 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.
> 

Depending on how much of a perf hit this is, we could also skip using RDPID in the paranoid path on SVM-capable CPUs.