Re: [Qemu-devel] Qemu disaggregation in Xen environment

[Stefano Stabellini]

On Tue, 6 Mar 2012, Anthony Liguori wrote:
> On 03/05/2012 04:53 PM, Stefano Stabellini wrote:
> > On Mon, 5 Mar 2012, Anthony Liguori wrote:
> >> On 02/28/2012 05:46 AM, Julien Grall wrote:
> >>> Hello,
> >>>
> >>> In the current model, only one instance of qemu is running for each 
> >>> running HVM
> >>> domain.
> >>>
> >>> We are looking at disaggregating qemu to have, for example, an instance to
> >>> emulate only
> >>> network controllers, another to emulate block devices, etc...
> >>
> >> Why would you want to do this?
> >
> > We are trying to disaggregate QEMU, the same way we do with Linux.
> >
> > On Xen we can run a Linux guest to drive the network card, another Linux
> > guest to drive the SATA controller, another one for the management
> > stack, etc. This helps both with scalability and isolation.
> >
> > In this scenario is only natural that we run a QEMU that only emulates
> > a SATA controller in the storage domain, a QEMU that only emulates the
> > network card in the network domain and everything else in a stubdom.
> >
> > What's better than using QEMU as emulator? Using three QEMUs per guest
> > as emulators! :-)
> 
> My concern is that this moves the Xen use case pretty far from what the 
> typical 
> QEMU use case would be (running one emulator per guest).
> 
> If it was done in a non-invasive way, maybe it would be acceptable but at a 
> high 
> level, I don't see how that's possible.
>
> I almost think you would be better off working to build a second front end 
> (reusing the device model, and nothing else) specifically for Xen.
> 
> Almost like qemu-io but instead of using the block layer, use the device 
> model.

(sorry for the reply, I was traveling and sick: bad combination)


Ideally what we would like is a way to run a QEMU emulator that only
builds a machine with the devices we want, let's say just a SATA
controller. My understanding of "Machine description as data" is that it
would perfectly fit this use case, so if we had it in QEMU, we
wouldn't have any need for a separate "qemu-dev".
Also, considering the way QEMU hooks into Xen, it is rather simple for
us to run multiple QEMUs for a single domain, the changes on the QEMU
side to do that would be minimal: we just need to introduce a
registration mechanism for QEMU to tell Xen what IO events it is the
handler of.
Finally it would also nice to have a way to restrict at compile time the
amount of supported emulators so that we can have a QEMU binary tailored
to the machine it is going to emulate.
This last step is probably a bit harder than the others but it makes
perfect sense as a follow up of "Machine description as data".


Now, if you are opposed to having "Machine description as data" in QEMU,
then we can do all this in "qemu-dev", even though I am a bit concerned
about code duplication between vl.c and the future qemu-dev.c.