linux-kernel - Re: [PATCH v5] x86: call smp vmxoff in smp stop

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Message-ID: <e287e628-e0c7-5fef-e113-94650228b823@redhat.com>
Date:   Thu, 20 Apr 2017 12:13:01 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     David Hildenbrand <david@...hat.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     mingo@...nel.org, x86@...nel.org,
        Tiantian Feng <fengtiantian@...wei.com>,
        Xishi Qiu <qiuxishi@...wei.com>
Subject: Re: [PATCH v5] x86: call smp vmxoff in smp stop



On 20/04/2017 12:01, David Hildenbrand wrote:
> On 19.04.2017 18:18, Paolo Bonzini wrote:
>> From: Tiantian Feng <fengtiantian@...wei.com>
>>
>> A CPU in VMX root mode will ignore INIT signals and fail to bring
>> up the APs after reboot.  Therefore, on a panic we disable VMX on all
>> CPUs before rebooting or triggering kdump.
>>
>> Do this when halting the machine as well, in case a firmware-level reboot
>> does not perform a cold reset for all processors.  Without doing this,
>> rebooting the host may hang.
>
> Guess this make sense, just wonder if something like that is actually
> expected to work? Not doing a cold reset feels like leaving a lot of
> stuff in an uninitialized state. Shouldn't that specific firmware then
> care about performing any necessary resets?

Yes, but it cannot do them if it cannot even perform the initial warm
reset ("INIT").

It's probably rare, after all the original introduction of
cpu_emergency_vmxoff is as old as 2008 and it took nine years for
someone to think of making this extra change.

Paolo