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

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Date:   Tue, 15 Nov 2022 00:32:24 +0000
From:   Andrew Cooper <Andrew.Cooper3@...rix.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        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>
CC:     "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>,
        Tom Lendacky <thomas.lendacky@....com>,
        Andrew Cooper <Andrew.Cooper3@...rix.com>
Subject: Re: [PATCH v3 0/3] x86/crash: Fix double NMI shootdown bug

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.

~Andrew