Re: [Qemu-devel] [PATCH] libvhost-user: Start VQs on SET_VRING_CALL

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

On Fri, Jan 13, 2017 at 03:09:46PM +0000, Felipe Franciosi wrote:
> Hi Marc-Andre,
> 
> > On 13 Jan 2017, at 07:03, Marc-André Lureau <mlureau@redhat.com> wrote:
> > 
> > Hi
> > 
> > ----- Original Message -----
> >> Currently, VQs are started as soon as a SET_VRING_KICK is received. That
> >> is too early in the VQ setup process, as the backend might not yet have
> > 
> > I think we may want to reconsider queue_set_started(), move it elsewhere, since kick/call fds aren't mandatory to process the rings.
> 
> Hmm. The fds aren't mandatory, but I imagine in that case we should still receive SET_VRING_KICK/CALL messages without an fd (ie. with the VHOST_MSG_VQ_NOFD_MASK flag set). Wouldn't that be the case?

Please look at docs/specs/vhost-user.txt, Starting and stopping rings

The spec says:
	Client must start ring upon receiving a kick (that is, detecting that
	file descriptor is readable) on the descriptor specified by
	VHOST_USER_SET_VRING_KICK, and stop ring upon receiving
	VHOST_USER_GET_VRING_BASE.


> > 
> >> a callfd to notify in case it received a kick and fully processed the
> >> request/command. This patch only starts a VQ when a SET_VRING_CALL is
> >> received.
> > 
> > I don't like that much, as soon as the kick fd is received, it should start polling it imho. callfd is optional, it may have one and not the other.
> 
> So the question is whether we should be receiving a SET_VRING_CALL anyway or not, regardless of an fd being sent. (I think we do, but I haven't done extensive testing with other device types.)

I would say not, only KICK is mandatory and that is also not enough
to process ring. You must wait for it to be readable.

> > 
> > Perhaps it's best for now to delay the callfd notification with a flag until it is received?
> 
> The other idea is to always kick when we receive the callfd. I remember discussing that alternative with you before libvhost-user went in. The protocol says both the driver and the backend must handle spurious kicks. This approach also fixes the bug.
> 
> I'm happy with whatever alternative you want, as long it makes libvhost-user usable for storage devices.
> 
> Thanks,
> Felipe
> 
> 
> > 
> > 
> >> Signed-off-by: Felipe Franciosi <felipe@nutanix.com>
> >> ---
> >> contrib/libvhost-user/libvhost-user.c | 26 +++++++++++++-------------
> >> 1 file changed, 13 insertions(+), 13 deletions(-)
> >> 
> >> diff --git a/contrib/libvhost-user/libvhost-user.c
> >> b/contrib/libvhost-user/libvhost-user.c
> >> index af4faad..a46ef90 100644
> >> --- a/contrib/libvhost-user/libvhost-user.c
> >> +++ b/contrib/libvhost-user/libvhost-user.c
> >> @@ -607,19 +607,6 @@ vu_set_vring_kick_exec(VuDev *dev, VhostUserMsg *vmsg)
> >>         DPRINT("Got kick_fd: %d for vq: %d\n", vmsg->fds[0], index);
> >>     }
> >> 
> >> -    dev->vq[index].started = true;
> >> -    if (dev->iface->queue_set_started) {
> >> -        dev->iface->queue_set_started(dev, index, true);
> >> -    }
> >> -
> >> -    if (dev->vq[index].kick_fd != -1 && dev->vq[index].handler) {
> >> -        dev->set_watch(dev, dev->vq[index].kick_fd, VU_WATCH_IN,
> >> -                       vu_kick_cb, (void *)(long)index);
> >> -
> >> -        DPRINT("Waiting for kicks on fd: %d for vq: %d\n",
> >> -               dev->vq[index].kick_fd, index);
> >> -    }
> >> -
> >>     return false;
> >> }
> >> 
> >> @@ -661,6 +648,19 @@ vu_set_vring_call_exec(VuDev *dev, VhostUserMsg *vmsg)
> >> 
> >>     DPRINT("Got call_fd: %d for vq: %d\n", vmsg->fds[0], index);
> >> 
> >> +    dev->vq[index].started = true;
> >> +    if (dev->iface->queue_set_started) {
> >> +        dev->iface->queue_set_started(dev, index, true);
> >> +    }
> >> +
> >> +    if (dev->vq[index].kick_fd != -1 && dev->vq[index].handler) {
> >> +        dev->set_watch(dev, dev->vq[index].kick_fd, VU_WATCH_IN,
> >> +                       vu_kick_cb, (void *)(long)index);
> >> +
> >> +        DPRINT("Waiting for kicks on fd: %d for vq: %d\n",
> >> +               dev->vq[index].kick_fd, index);
> >> +    }
> >> +
> >>     return false;
> >> }
> >> 
> >> --
> >> 1.9.4
> >> 
> >> 
>