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 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?

> 
> >
> >>>      net->dev.nvqs = 2;
> >>>      net->dev.vqs = net->vqs;
> >>>  
> >>> @@ -196,11 +203,6 @@ fail:
> >>>      return NULL;
> >>>  }
> >>>  
> >>> -static void vhost_net_set_vq_index(struct vhost_net *net, int vq_index)
> >>> -{
> >>> -    net->dev.vq_index = vq_index;
> >>> -}
> >>> -
> >>>  static int vhost_net_set_vnet_endian(VirtIODevice *dev, NetClientState *peer,
> >>>                                       bool set)
> >>>  {
> >>> @@ -325,10 +327,6 @@ int vhost_net_start(VirtIODevice *dev, NetClientState *ncs,
> >>>          goto err;
> >>>      }
> >>>  
> >>> -    for (i = 0; i < total_queues; i++) {
> >>> -        vhost_net_set_vq_index(get_vhost_net(ncs[i].peer), i * 2);
> >>> -    }
> >>> -
> >>>      r = k->set_guest_notifiers(qbus->parent, total_queues * 2, true);
> >>>      if (r < 0) {
> >>>          error_report("Error binding guest notifier: %d", -r);
> >>> diff --git a/net/vhost-user.c b/net/vhost-user.c
> >>> index 93dcecd..2d6bbe5 100644
> >>> --- a/net/vhost-user.c
> >>> +++ b/net/vhost-user.c
> >>> @@ -146,6 +146,7 @@ static int net_vhost_user_init(NetClientState *peer, const char *device,
> >>>      /* We don't provide a receive callback */
> >>>      s->nc.receive_disabled = 1;
> >>>      s->chr = chr;
> >>> +    nc->queue_index = 0;
> >>>  
> >> Fail to understand why this is needed. It will be set to correct value
> >> in set_netdev().
> > Right, it's unnecessary.
> >
> > BTW, May I ask you a quick question: why set_netdev is invoked after
> > tap init, while it is invoked before vhost-user init?
> >
> > 	--yliu
> 
> I think the reason is that you must initialize all netdevs before a
> device tries to connect them.

Thanks!

	--yliu