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Re: [Qemu-devel] [PATCH] net/virtio: fix multi-queue negotiation


From: Marcel Apfelbaum
Subject: Re: [Qemu-devel] [PATCH] net/virtio: fix multi-queue negotiation
Date: Thu, 25 Jun 2015 19:54:17 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.5.0

On 06/24/2015 11:00 AM, Jason Wang wrote:


On 06/19/2015 02:05 AM, Marcel Apfelbaum wrote:
Clear host multi-queue related features if the peer
doesn't support it.

Signed-off-by: Marcel Apfelbaum <address@hidden>
---
Notes:
  This fixes a guest CPU soft lock, however the virtio-net
  device will not work correctly. It seems that is
  peer's "fault", not knowing how to handle the situation.
  However, I submit this patch since it corrects the negotiation
  and saves us from a guest crash (!).

Could you please describe how to reproduce this issue?
Hi Jason,
Sorry for the late reply.
I was hoping that a vhost-user multi-queue "insider" will ask questions :)

This happens when we have OVS as backend without multi-queue support
and qemu/guest with multi-queue-support.




  Any ideas from the virtio/multi-queue developers on how to debug
  this further are welcomed.

  hw/net/virtio-net.c | 13 ++++++++++++-
  1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
index 9281aa1..63e59e8 100644
--- a/hw/net/virtio-net.c
+++ b/hw/net/virtio-net.c
@@ -367,6 +367,11 @@ static int peer_has_ufo(VirtIONet *n)
      return n->has_ufo;
  }

+static int peer_has_multiqueue(VirtIONet *n)
+{
+    return n->multiqueue;
+}

The name is confusing, this is in fact whether or not guest support
multiqueue. To check peer's ability, you need check
n->nic_conf.peers.queues instead.
I just wanted to mimic similar code, I have no issue against
this approach.


+
  static void virtio_net_set_mrg_rx_bufs(VirtIONet *n, int mergeable_rx_bufs,
                                         int version_1)
  {
@@ -469,6 +474,13 @@ static uint64_t virtio_net_get_features(VirtIODevice 
*vdev, uint64_t features)
          virtio_clear_feature(&features, VIRTIO_NET_F_HOST_UFO);
      }

+    if (!peer_has_multiqueue(n)) {
+        virtio_clear_feature(&features, VIRTIO_NET_F_MQ);

I'm not quite understand this, VIRTIO_NET_F_MQ should work if peer has
only 1 queue.
Please explain, I thought  VIRTIO_NET_F_MQ flag *enables*
multi-queue, and in our case we pass queue=2 to vhost.


+        virtio_clear_feature(&features, VIRTIO_NET_F_GUEST_ANNOUNCE);
+        virtio_clear_feature(&features, VIRTIO_NET_F_CTRL_VQ);
+        virtio_clear_feature(&features, VIRTIO_NET_F_CTRL_RX);
+    }
+

Those features don't depend on multiqueue, why clear them?
Once VIRTIO_NET_F_MQ is cleared, the virtio driver from guest is complaining
about those other flags too.

Any explanations about how *it should* work are welcomed.
Again, the scenario is: backend doesn't support multi-queue, QEMU/guest do,
and queues=2 is passed on command line.


Thanks,
Marcel

      if (!get_vhost_net(nc->peer)) {
          virtio_add_feature(&features, VIRTIO_F_VERSION_1);
          return features;
@@ -1314,7 +1326,6 @@ static void virtio_net_tx_bh(void *opaque)
  static void virtio_net_set_multiqueue(VirtIONet *n, int multiqueue)
  {
      n->multiqueue = multiqueue;
-
      virtio_net_set_queues(n);
  }






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