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

[Alexander Graf]

On 20.06.2012, at 01:28, Benjamin Herrenschmidt wrote:

> 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).

IIRC we still allocate it outside of normal guest memory, so you don't get the 
migration for free :).


Alex