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.

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