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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 09:28:32 +1000

On Wed, 2012-06-20 at 01:11 +0200, Juan Quintela wrote:
> > 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.
> It does, but I have no clue how the hashed page tables are implemented
> on ppc, i.e. if there is anything specific for bare metal.  Alex?

We support the paravirtualized -M pseries in full emu as well, in which
case the hashed page table is handled by qemu itself who implements the
H_ENTER & co hypercalls. So it's very similar, except that qemu doesn't
have to ask the kernel to get a snapshot :-)

So I can flush out the storage format and two stage process inside qemu,
and then bother with the kvm/kernel interface.

Normal "bare metal" operation in qemu (or even KVM "PR") doesn't require
this as in that case the hash table is just a normal part of the guest
memory, it's only an issue when doing paravirtualized guest such as
pseries (aka PAPR).


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