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

[Paolo Bonzini]

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.

> 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...

Paolo