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

[Juergen Gross]

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_tyshche...@epam.com>

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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