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Date:   Thu, 20 Aug 2020 13:07:36 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Tom Lendacky <thomas.lendacky@....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 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

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

Fun.

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