linux-kernel - Re: [Bug 11219] KVM modules break emergency reboot

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Message-ID: <48A97B9A.1040800@qumranet.com>
Date:	Mon, 18 Aug 2008 16:39:38 +0300
From:	Avi Kivity <avi@...ranet.com>
To:	bugme-daemon@...zilla.kernel.org
CC:	Ingo Molnar <mingo@...e.hu>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [Bug 11219] KVM modules break emergency reboot

(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.

-- 
error compiling committee.c: too many arguments to function

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