Re: [PATCH V11.1 1/3] libxl: Add support for Virtio disk configuration

[Anthony PERARD <anthony.perard@xxxxxxxxxx>]

On Sat, Jul 16, 2022 at 07:37:45PM +0300, Oleksandr Tyshchenko wrote:
> From: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
> 
> This patch adds basic support for configuring and assisting virtio-mmio
> based virtio-disk backend (emulator) 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's point of view:
>  - as the frontend is virtio-blk which is not a Xenbus driver, nothing
>    written to Xenstore are fetched by the frontend currently ("vdev"
>    is not passed to the frontend). But this might need to be revised
>    in future, so frontend data might be written to Xenstore in order to
>    support hotplugging virtio devices or passing the backend domain id
>    on arch where the device-tree is not available.
>  - 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.
> 
> For the immediate purpose and an ability to extend that support for
> other use-cases in future (Qemu, virtio-pci, etc) perform the following
> actions:
> - Add new disk backend type (LIBXL_DISK_BACKEND_STANDALONE) and reflect
>   that in the configuration
> - Introduce new disk "specification" and "transport" fields to struct
>   libxl_device_disk. Both are written to the Xenstore. The transport
>   field is only used for the specification "virtio" and it assumes
>   only "mmio" value for now.
> - Introduce new "specification" option with "xen" communication
>   protocol being default value.
> - Add new device kind (LIBXL__DEVICE_KIND_VIRTIO_DISK) as current
>   one (LIBXL__DEVICE_KIND_VBD) doesn't fit into Virtio disk model
> 
> An example of domain configuration for Virtio disk:
> disk = [ 'phy:/dev/mmcblk0p3, xvda1, backendtype=standalone, 
> specification=virtio']
> 
> Nothing has changed for default Xen disk configuration.
> 
> Please note, this patch is not enough for virtio-disk to work
> on Xen (Arm), as for every Virtio device (including disk) we need
> to allocate Virtio MMIO params (IRQ and memory region) and pass
> them to the backend, also update Guest device-tree. The subsequent
> patch will add these missing bits. For the current patch,
> the default "irq" and "base" are just written to the Xenstore.
> This is not an ideal splitting, but this way we avoid breaking
> the bisectability.
> 
> Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>

Reviewed-by: Anthony PERARD <anthony.perard@xxxxxxxxxx>

Thanks,

-- 
Anthony PERARD