Re: [PATCH V6 1/2] libxl: Add support for Virtio disk configuration

[Juergen Gross <jgross@xxxxxxxx>]

On 14.12.21 18:44, Oleksandr wrote:
> On 14.12.21 18:03, Anthony PERARD wrote:
>
> Hi Anthony
>
>
> > On Wed, Dec 08, 2021 at 06:59:43PM +0200, Oleksandr Tyshchenko wrote:
> > > From: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
> > >
> > > This patch adds basic support for configuring and assisting virtio-disk
> > > backend (emualator) which is intended to run out of Qemu and could be
> > > run in any domain.
> > > Although the Virtio block device is quite different from traditional
> > > Xen PV block device (vbd) from the toolstack point of view:
> > >   - as the frontend is virtio-blk which is not a Xenbus driver, nothing
> > >     written to Xenstore are fetched by the frontend (the vdev is not
> > >     passed to the frontend)
> > >   - the ring-ref/event-channel are not used for the backend<->frontend
> > >     communication, the proposed IPC for Virtio is IOREQ/DM
> > > it is still a "block device" and ought to be integrated in existing
> > > "disk" handling. So, re-use (and adapt) "disk" parsing/configuration
> > > logic to deal with Virtio devices as well.
> > How backend are intended to be created? Is there something listening on
> > xenstore?
> >
> > You mention QEMU as been the backend, do you intend to have QEMU
> > listening on xenstore to create a virtio backend? Or maybe it is on the
> > command line? There is QMP as well, but it's probably a lot more
> > complicated as I think libxl needs refactoring for that.
>
> No, QEMU is not involved there. The backend is a standalone application,
> it is launched from the command line. The backend reads the Xenstore to get
> the configuration and to detect when guest with the frontend is 
> created/destroyed.
I think this should be reflected somehow in the configuration, as I
expect qemu might gain this functionality in the future.

I'm wondering whether we shouldn't split the backend from the protocol
(or specification?). Something like "protocol=virtio" (default would be
e.g. "xen") and then you could add "backend=external" for your use case?
We could later expand that to "backend=qemu" or "backend=<script-path>".


Juergen

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