Re: [PATCH V10 1/3] libxl: Add support for Virtio disk configuration

[Juergen Gross <jgross@xxxxxxxx>]

On 30.06.22 14:18, Oleksandr wrote:
> Dear all.
>
>
> On 25.06.22 17:32, Oleksandr wrote:
> > On 24.06.22 15:59, George Dunlap wrote:
> >
> > Hello George
> >
> > > > On 17 Jun 2022, at 17:14, Oleksandr Tyshchenko <olekstysh@xxxxxxxxx> 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_OTHER) 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=other, 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>
> > > OK, I am *really* sorry for coming in here at the last minute and quibbling 
> > > about things.
> >
> > no problem
> >
> >
> > >   But this introduces a public interface which looks really wrong to me.  
> > > I’ve replied to the mail from December where Juergen proposed the “Other” 
> > > protocol.
> > > Hopefully this will be a simple matter of finding a better name than 
> > > “other”.  (Or you guys convincing me that “other” is really the best name for 
> > > this value; or even Anthony asserting his right as a maintainer to overrule 
> > > my objection if he thinks I’m out of line.)
> >
> > I saw your reply to V6 and Juergen's answer. I share Juergen's opinion as well 
> > as I understand your concern. I think, this is exactly the situation when 
> > finding a perfect name (obvious, short, etc) for the backendtype (in our 
> > particular case) is really difficult.
> > Personally I tend to leave "other", because there is no better alternative 
> > from my PoV. Also I might be completely wrong here, but I don't think we will 
> > have to extend the "backendtype" for supporting other possible virtio backend 
> > implementations in the foreseeable future:
> > - when Qemu gains the required support we will choose qdisk: backendtype qdisk 
> > specification virtio
> > - for the possible virtio alternative of the blkback we will choose phy: 
> > backendtype phy specification virtio
> > If there will be a need to keep various implementation, we will be able to 
> > describe that by using "transport" (mmio, pci, xenbus, argo, whatever).
> > Actually this is why we also introduced "specification" and "transport".
> >
> > IIRC, there were other (suggested?) names except "other" which are "external" 
> > and "daemon". If you think that one of them is better than "other", I will 
> > happy to use it.
>
> Could we please make a decision on this?
>
> If "other" is not unambiguous, then maybe we could choose "daemon" to describe 
> arbitrary user-level backends, any thought?
IMO this would exclude other cases, like special kernel drivers.

Maybe "standalone"? "only-relying-on-xenstore-data" would be a bit
exaggerated, while conveying the idea quite nicely.


Juergen

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