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

["Michael S. Tsirkin" <mst@redhat.com>]

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.

> However, I think having a single message is inflexible and duplicates
> existing vhost-user protocol messages like VHOST_USER_GET_QUEUE_NUM. I
> would prefer VHOST_USER_GET_DEVICE_ID and other messages.
> 
> Stefan

Exactly.

-- 
MST