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Message-ID: <20080818140355.GA30511@elte.hu>
Date: Mon, 18 Aug 2008 16:03:55 +0200
From: Ingo Molnar <mingo@...e.hu>
To: Avi Kivity <avi@...ranet.com>
Cc: bugme-daemon@...zilla.kernel.org,
linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [Bug 11219] KVM modules break emergency reboot
* Avi Kivity <avi@...ranet.com> wrote:
> (copying mingo)
>
> (context: sysrq-B with kvm-intel.ko loaded doesn't work. on my machine,
> it kills the sata interface, but the processor and network keeps working)
>
> Strangely, the specs say:
>
>> • The INIT signal is blocked whenever a logical processor is in VMX
>> root operation.
>> It is not blocked in VMX non-root operation. Instead, INITs cause VM
>> exits (see
>> Section 21.3, “Other Causes of VM Exits”).
>
> So INIT (which is wired to the triple-fault processor output, it seems,
> rather than RESET) is blocked and the machine is not reset completely.
>
> So we need to disable vmx during native_machine_emergency_restart().
> There are at least three ways of doing this:
>
> - add a vmxoff sequence (with an exception handler) to
> native_machine_emergency_restart(). while simplest, this will not
> unblock INIT for other cpus
>
> - add an emergency_restart notifier_block, and have kvm subscribe. This
> has the disadvantage of being slightly complex, opening a tiny race
> (emergency restart during kvm module initialization), and requiring IPIs
> during emergency restart.
>
> - move vmxon/vmxoff management out of the kvm module and into x86 core.
> Bloats the core but reduces complexity. IPIs still required.
>
> I think the notifier block is the way to go. Ingo, let me know what you
> prefer.
notifier should be OK i think - sysrq-b is an emergency mechanism after
all.
btw., "echo b > /proc/sysrq-trigger" never worked reliably for me with
KVM also loaded.
Ingo
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