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


From: Clemens Kolbitsch
Subject: Re: [Qemu-devel] Disabling KVM "on the fly"
Date: Thu, 18 Oct 2012 10:38:50 -0700

On Thu, Oct 18, 2012 at 10:24 AM, Jan Kiszka <address@hidden> wrote:
> 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.

I am just looking for a way to do it, regardless whether it causes a
big performance penalty due to migration, etc. Even if this means
preparing a special environment (emulating specific hardware, etc),
this is fine with me.

Thanks a lot for all the feedback, now I know what I will have to look into!

-Clemens



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