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Re: [Qemu-devel] vm state save/restore question

From: Benjamin Herrenschmidt
Subject: Re: [Qemu-devel] vm state save/restore question
Date: Wed, 20 Jun 2012 09:00:33 +1000

On Wed, 2012-06-20 at 00:55 +0200, Juan Quintela wrote:
> This was going to be my question.
> If we can do something like: send hash register, and get a bitmap of
> the
> ones that get changed, we should be good.  Perhaps we need something
> "interesting" like removing old entries (no clue if they got just
> overwritten, or how they are replaced), and we should be good. 

Right, we can do an initial "snapshot" and have the kernel start
tracking changes from there. On the final step, we can then request from
the kernel a new snapshot with a dirty bitmap.

Or we can be even simpler, just do two snapshots and have qemu diff them
itself :-)

I am confident I can come up with something as far as the kernel and
qemu <-> kernel interface goes. I need to get my head around the details
on how to implement that two stage save process in qemu though and the
corresponding restore which will need to read both snapshots and apply
the diff before shooting it back to the kernel.

BTW. Does migration in pure qemu (full system emu) works similarily, ie,
two stage ? If it does I can easily prototype everything there.


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