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

[Paolo Bonzini]

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), the differences
between KVM and TCG are just that KVM doesn't initialize some TCG-only
data structure, and that KVM uses many CPU threads; TCG uses one that
goes through CPUs round-robin.  The CPU threads of course execute
different code.

So no, in theory there is nothing that prevents this from working in
principle, except for kvmclock.

Paolo

> I know nothing of that sort is implemented at this point. Leaving
> aside the issue of certain CPUID instructions suddenly returning
> different results, are there obvious problems even trying to go down
> that road?
> 
> Would love to hear some thoughts on this - don't hesitate to tell me
> "that's stupid and impossible because XYZ" (assuming you also fill in
> the blanks ;) )
> 
> Thanks!
> Clemens
>