Re: [Qemu-devel] [RFC v2 1/2] docs: vhost-user: add in-band kick/call messages

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

On Mon, Sep 16, 2019 at 01:40:35PM +0200, Johannes Berg wrote:
> Hi Michael,
> 
> I had just wanted to prepare a resend, but
> 
> > > Hmm I don't like this. I propose that with VHOST_USER_PROTOCOL_F_IN_BAND_NOTIFICATIONS
> > > we just don't allow VHOST_USER_SET_VRING_CALL (if you think it's
> > > important to allow them, we can say that we do not require them).
> > 
> > You can't actually skip SET_VRING_CALL, it's necessary to start a vring,
> > so libvhost-user for example calls dev->iface->queue_set_started() only
> > in this case. The docs in the "Starting and stopping rings" section also
> > explain this.
> 
> [...]
> 
> > See above. But I guess we could put a flag into bit 9 indicating that
> > you want to use messages instead of polling or a file descriptor, if you
> > prefer.
> 
> Personally, I don't think it matters since right now I can see the in-
> band notification as being really necessary/useful only for simulation
> work, and in that case no polling will be doable.
> 
> If you do think it's important to not make the two mutually exclusive,
> how would you prefer to have this handled? With a new flag, e.g. in bit
> 9, indicating "use inband signalling instead of polling or eventfd"?
> 
> Thanks,
> johannes


So first we really need to fix up Starting and stopping section,
explaining that if the FD is invalid, this means ring
is immediately started, right?

If we want to keep it simple, my proposal is this, if
VHOST_USER_PROTOCOL_F_IN_BAND_NOTIFICATIONS is set then
VHOST_USER_SET_VRING_CALL and VHOST_USER_SET_VRING_KICK are not valid.
Starting/stopping ring needs to be updated, teaching it
that ring is started after it gets a kick.


-- 
MST