linux-kernel - Re: [PATCH v3 0/3] x86/crash: Fix double NMI shootdown bug

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Message-ID: <cbcb6f35-e5d7-c1c9-4db9-fe5cc4de579a@amd.com>
Date:   Tue, 15 Nov 2022 12:34:36 -0600
From:   Tom Lendacky <thomas.lendacky@....com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Andrew Cooper <Andrew.Cooper3@...rix.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Guilherme G . Piccoli" <gpiccoli@...lia.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH v3 0/3] x86/crash: Fix double NMI shootdown bug

On 11/15/22 10:56, Sean Christopherson wrote:
> On Tue, Nov 15, 2022, Andrew Cooper wrote:
>> On 14/11/2022 23:34, Sean Christopherson wrote:
>>> Tom,
>>>
>>> I Cc'd you this time around because the APM doesn't explicitly state that
>>> GIF is set when EFER.SVME is disabled.  KVM's nSVM emulation does set GIF
>>> in this case, but it's not clear whether or not KVM is making up behavior.
>>> If clearing EFER.SVME doesn't set GIF, then patch 1 needs to be modified
>>> to try STGI before clearing EFER.SVME, e.g. if a crash is initiated from
>>> KVM between CLGI and STGI.  Responding CPUs are "safe" because GIF=0 also
>>> blocks NMIs, but the initiating CPU might leave GIF=0 when jumping into
>>> the new kernel.
>>
>> GIF exists independently of EFER.SVME.
>>
>> It is also gets set by the SKINIT instruction, which is why there is an
>> asymmetry on the #UD conditions of STGI and CLGI.
>>
>> STGI is also intended to be used by the DLME once critical
>> initialisation is done, hence why it's dependent on EFER.SVME || SKINIT.
> 
> Gah, stupid APM.  The pseudocode states "EFER.SVME || CPUID.SKINIT", but the
> description and the comment both say that SVM must be enabled to execute SKINIT,
> which made me hope that disabling SVM would reset everything.
> 
>    This instruction generates a #UD exception if SVM is not enabled. See “Enabling
>    SVM” in AMD64 Architecture Programmer’s Manual Volume 2: System Instructions,
>    order# 24593.
> 
> ...
> 
>    IF ((EFER.SVME == 0) && !(CPUID 8000_0001.ECX[SKINIT]) || (!PROTECTED_MODE))
>            EXCEPTION [#UD]  // This instruction can only be executed
>                             // in protected mode with SVM enabled
>                                                       ^^^^^^^^^^^

I'll see if I can't get someone to update the psuedo-code comment here.

But, to your original question, as far as I understand, GIF is not 
automatically set if SVME is cleared from EFER.

Thanks,
Tom