[virtio-dev] Re: [RFC PATCH] docs/interop: define STANDALONE protocol feature for vhost-user

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Thu, Jul 20, 2023 at 06:22:08PM -0400, Michael S. Tsirkin wrote:
> On Thu, Jul 20, 2023 at 05:31:03PM -0400, Stefan Hajnoczi wrote:
> > On Thu, 20 Jul 2023 at 17:15, Michael S. Tsirkin <mst@redhat.com> wrote:
> > >
> > > On Thu, Jul 20, 2023 at 03:58:37PM -0400, Stefan Hajnoczi wrote:
> > > > On Thu, Jul 06, 2023 at 12:48:20PM -0400, Michael S. Tsirkin wrote:
> > > > > On Tue, Jul 04, 2023 at 01:36:00PM +0100, Alex Bennée wrote:
> > > > > > Currently QEMU has to know some details about the back-end to be able
> > > > > > to setup the guest. While various parts of the setup can be delegated
> > > > > > to the backend (for example config handling) this is a very piecemeal
> > > > > > approach.
> > > > >
> > > > > > This patch suggests a new feature flag (VHOST_USER_PROTOCOL_F_STANDALONE)
> > > > > > which the back-end can advertise which allows a probe message to be
> > > > > > sent to get all the details QEMU needs to know in one message.
> > > > >
> > > > > The reason we do piecemeal is that these existing pieces can be reused
> > > > > as others evolve or fall by wayside.
> > > > >
> > > > > For example, I can think of instances where you want to connect
> > > > > specifically to e.g. networking backend, and specify it
> > > > > on command line. Reasons could be many, e.g. for debugging,
> > > > > or to prevent connecting to wrong device on wrong channel
> > > > > (kind of like type safety).
> > > > >
> > > > > What is the reason to have 1 message? startup latency?
> > > > > How about we allow pipelining several messages then?
> > > > > Will be easier.
> > > >
> > > > This flag effectively says that the back-end is a full VIRTIO device
> > > > with a Device Status Register, Configuration Space, Virtqueues, the
> > > > device type, etc. This is different from previous vhost-user devices
> > > > which sometimes just offloaded certain virtqueues without providing the
> > > > full VIRTIO device (parts were emulated in the VMM).
> > > >
> > > > So for example, a vhost-user-net device does not support the controlq.
> > > > Alex's "standalone" device is a mode where the vhost-user protocol is
> > > > used but the back-end must implement a full virtio-net device.
> > > > Standalone devices are like vDPA device in this respect.
> > > >
> > > > I think it is important to have a protocol feature bit that advertises
> > > > that this is a standalone device, since the semantics are different for
> > > > traditional vhost-user-net devices.
> > >
> > > Not sure what that would gain as compared to a feature bit per
> > > message as we did previously.
> > 
> > Having a single feature bit makes it easier to distinguish between a
> > traditional vhost-user device and a standalone device.
> > 
> > For example, the presence of VHOST_USER_F_GET_DEVICE_ID doesn't tell
> > you whether this device is a standalone device that is appropriate for
> > a new generic QEMU --device vhost-user-device feature that Alex is
> > working on. It could be a traditional vhost-user device that is not
> > standalone but implements the VHOST_USER_GET_DEVICE_ID message.
> > 
> > How will we detect standalone devices? It will be messy if there is no
> > single feature bit that advertises that this back-end is a standalone
> > device.
> > 
> > Stefan
> 
> Looks like standalone implies some 5-6 messages to be supported.
> So just test the 6 bits are all ones.

It's not clear to me that the individual bits together mean this is
really a standalone device, but let's go with individual commands and
see if a front-end can distinguish standalone devices or not. If not,
then we can still add "standalone" feature bit before merging the code.

Stefan

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