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Re: [Qemu-devel] Disabling KVM "on the fly"


From: Jan Kiszka
Subject: Re: [Qemu-devel] Disabling KVM "on the fly"
Date: Thu, 18 Oct 2012 19:24:28 +0200
User-agent: Mozilla/5.0 (X11; U; Linux i686 (x86_64); de; rv:1.8.1.12) Gecko/20080226 SUSE/2.0.0.12-1.1 Thunderbird/2.0.0.12 Mnenhy/0.7.5.666

On 2012-10-18 08:29, Paolo Bonzini wrote:
> Il 17/10/2012 20:37, Jan Kiszka ha scritto:
>> On 2012-10-17 18:44, Paolo Bonzini wrote:
>>> Il 17/10/2012 18:37, Clemens Kolbitsch ha scritto:
>>>> Guys,
>>>>
>>>> I know this is question might seem a bit odd, but I'm curious:
>>>>
>>>> Has anyone ever tried to write code to disable KVM on the fly / is it
>>>> at all possible? I have a situation where I need to use TCG for
>>>> certain parts of the code, but would love to have acceleration for
>>>> everything else. My idea was to pause the VM, then use the
>>>> snapshotting mechanism to dump the state, and then to resume the
>>>> snapshot, but writing the KVM state into the non-KVM structures.
>>>
>>> As a start, you can try using "migrate exec:cat>foo.save" with a KVM
>>> machine and "-incoming 'exec:cat foo.save'" with a TCG machine.  The
>>> main problem should be that TCG doesn't implement kvmclock.
>>>
>>> If you disable the KVM interrupt controller and timer (which is just an
>>> implementation detail, not a hardware difference),
>>
>> Unnecessary. Both models (KVM in-kernel and QEMU userspace) are
>> compatible - in the absence of bugs.
> 
> He wants to really switch it on the fly---not just migrate out and
> in---and for that you need to disable the KVM-specific devices.

Well, that's even more unrealistic than via migration.

> 
>> But loading a KVM image into TCG lets non-trival guests lock up. Likely
>> due to differences in the CPU virtualization/emulation (MSRs...).
> 
> Perhaps that can be mitigated by using an older machine model.  Start
> with something simple like a pentium2 and work up from there...

Even if, there are still too many untranslated, maybe even
untranslatable states of the KVM CPU model, at least.

Jan

-- 
Siemens AG, Corporate Technology, CT RTC ITP SDP-DE
Corporate Competence Center Embedded Linux



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