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

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Johannes Berg (johannes@sipsolutions.net) wrote:
> On Wed, 2019-09-11 at 20:15 +0100, Dr. David Alan Gilbert wrote:
> 
> > > Extend the protocol slightly, so that a message can be used for kick
> > > and call instead, if VHOST_USER_PROTOCOL_F_IN_BAND_NOTIFICATIONS is
> > > negotiated. This in itself doesn't guarantee synchronisation, but both
> > > sides can also negotiate VHOST_USER_PROTOCOL_F_REPLY_ACK and thus get
> > > a reply to this message by setting the need_reply flag, and ensure
> > > synchronisation this way.
> > 
> > I'm confused; if you've already got REPLY_ACK, why do we need anything
> > else?  We already require the reply on set_mem_table as part of
> > postcopy.
> 
> Hmm? How's this related to set_mem_table?
> 
> For simulation purposes, I need the kick and call (and error perhaps
> though it's not really used by anyone now it seems) to be synchronous
> messages instead of asynchronous event FD pushes.
> 
> But I think enough words have been expended on explaining it already, if
> I may kindly ask you to read the discussions with Stefan and Michael
> here:
> 
> https://lore.kernel.org/qemu-devel/20190902121233.13382-1-johannes@sipsolutions.net/

Ah OK.

You're actually using the same trick of using
REPLY_ACK/need_reply  to make it synchronous that set_mem_table does;
that makes sense - but then new calls are getting it to actually process
some data/commands on the ring itself?

Dave

> Thanks,
> johannes
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK