Re: [Qemu-devel] [PATCH 5/7] vhost_net: move vhost_net_set_vq_index ahead at vhost_net_init

[Yuanhan Liu <yuanhan.liu@linux.intel.com>]

On Thu, Sep 10, 2015 at 01:52:30PM +0800, Jason Wang wrote:
> 
> 
> On 09/10/2015 01:17 PM, Yuanhan Liu wrote:
> > On Thu, Sep 10, 2015 at 12:46:00PM +0800, Jason Wang wrote:
> >> > 
> >> > 
> >> > On 09/10/2015 11:57 AM, Yuanhan Liu wrote:
> >>> > > On Thu, Sep 10, 2015 at 11:14:27AM +0800, Jason Wang wrote:
> >>>> > >>
> >>>> > >> On 09/08/2015 03:38 PM, Yuanhan Liu wrote:
> >>>>> > >>> So that we could use the `vq_index' as well in the vhost_net_init
> >>>>> > >>> stage, which is required when adding vhost-user multiple-queue support,
> >>>>> > >>> where we need the vq_index to indicate which queue pair we are gonna
> >>>>> > >>> initiate.
> >>>>> > >>>
> >>>>> > >>> vhost-user has no multiple queue support yet, hence no queue_index set
> >>>>> > >>> before. Here is a quick set to 0 at net_vhost_user_init() stage, and it
> >>>>> > >>> will be set properly soon in the next patch.
> >>>>> > >>>
> >>>>> > >>> Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
> >>>>> > >>> ---
> >>>>> > >>>  hw/net/vhost_net.c | 16 +++++++---------
> >>>>> > >>>  net/vhost-user.c   |  1 +
> >>>>> > >>>  2 files changed, 8 insertions(+), 9 deletions(-)
> >>>>> > >>>
> >>>>> > >>> diff --git a/hw/net/vhost_net.c b/hw/net/vhost_net.c
> >>>>> > >>> index f9441e9..141b557 100644
> >>>>> > >>> --- a/hw/net/vhost_net.c
> >>>>> > >>> +++ b/hw/net/vhost_net.c
> >>>>> > >>> @@ -138,6 +138,11 @@ static int vhost_net_get_fd(NetClientState *backend)
> >>>>> > >>>      }
> >>>>> > >>>  }
> >>>>> > >>>  
> >>>>> > >>> +static void vhost_net_set_vq_index(struct vhost_net *net, int vq_index)
> >>>>> > >>> +{
> >>>>> > >>> +    net->dev.vq_index = vq_index;
> >>>>> > >>> +}
> >>>>> > >>> +
> >>>>> > >>>  struct vhost_net *vhost_net_init(VhostNetOptions *options)
> >>>>> > >>>  {
> >>>>> > >>>      int r;
> >>>>> > >>> @@ -167,6 +172,8 @@ struct vhost_net *vhost_net_init(VhostNetOptions *options)
> >>>>> > >>>      }
> >>>>> > >>>      net->nc = options->net_backend;
> >>>>> > >>>  
> >>>>> > >>> +    vhost_net_set_vq_index(net, net->nc->queue_index * 2);
> >>>>> > >>> +
> >>>> > >> This breaks vhost kernel multiqueue since queue_index was not
> >>>> > >> initialized at this time.
> >>> > > Right, thanks for pointing it out.
> >>> > >
> >>>> > >> We do this in set_netdev() instead of setting
> >>>> > >> it in each kind of netdev.
> >>> > > Can we move it to net_init_tap() for setting the right queue_index
> >>> > > for each nc?
> >>> > >
> >>> > > Or, can we call vhost_net_set_vq_index twice, one at vhost_net_init(for
> >>> > > vhost-user mq support), another one at vhost_net_start(for vhost kernel
> >>> > > mq support)?
> >>> > >
> >>> > > Or, do you have better ideas?
> >> > 
> >> > I think setting queue_index in net_init_tap() looks ok.
> > Good to know.
> >
> >> > But a question
> >> > is that why need we do this at so early stage? ( Even before its peers
> >> > is connected.)
> > For vhost-user multiple queues support, we will invoke vhost_net_init()
> > N times for each queue pair, and hence we need to distinguish which
> > queue it is while sending messages like VHOST_SET_VRING_CALL for
> > initializing corresponding queue pair.
> >
> > Does that make sense to you?
> >
> 
> Not sure. Since current codes works for vhost-kernel. (vhost_net_init()
> was also called N times). We don't want to break existed vhost-kernel
> API when developing multiqueue. For each virtqueue TX/RX pair, we have
> one vhost net device and it has no knowledge for the others (which was
> hide by qemu). So VHOST_SET_VRING_CALL works without any change here.
> 
> For the case here, since you still have multiple instances of vhost_net
> structure. Maybe the vhost-user backend can distinguish form this?

Yeah, I guess that's the difference between vhost-user and vhost-kernel.
Vhost-kernel opens a char device(/dev/vhost-net) for each vhost_dev,
hence it's distinguishable. But for vhost-user, all vhost_dev share one
char device(a socket) for communication, hence, it's not distinguishable.

I was thinking maybe we could export vhost_net_set_vq_index() and invoke
it at net/vhost-user.c, so that we break nothing, and in the meantime,
it keeps the logic inside vhost-user.

What do you think?

	--yliu