On Thu, 20 Jul 2023 at 17:15, Michael S. Tsirkin <m...@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
