Re: [net-next PATCH v2 4/5] virtio_net: add dedicated XDP transmit queues

On 16-11-21 03:45 AM, Daniel Borkmann wrote:

On 11/20/2016 03:51 AM, John Fastabend wrote:

quoted

XDP requires using isolated transmit queues to avoid interference
with normal networking stack (BQL, NETDEV_TX_BUSY, etc). This patch
adds a XDP queue per cpu when a XDP program is loaded and does not
expose the queues to the OS via the normal API call to
netif_set_real_num_tx_queues(). This way the stack will never push
an skb to these queues.

However virtio/vhost/qemu implementation only allows for creating
TX/RX queue pairs at this time so creating only TX queues was not
possible. And because the associated RX queues are being created I
went ahead and exposed these to the stack and let the backend use
them. This creates more RX queues visible to the network stack than
TX queues which is worth mentioning but does not cause any issues as
far as I can tell.

Signed-off-by: John Fastabend <redacted>
---

[...]

quoted

      }

+    curr_qp = vi->curr_queue_pairs - vi->xdp_queue_pairs;
+    if (prog)
+        xdp_qp = nr_cpu_ids;
+
+    /* XDP requires extra queues for XDP_TX */
+    if (curr_qp + xdp_qp > vi->max_queue_pairs) {
+        netdev_warn(dev, "request %i queues but max is %i\n",
+                curr_qp + xdp_qp, vi->max_queue_pairs);
+        return -ENOMEM;
+    }
+
+    err = virtnet_set_queues(vi, curr_qp + xdp_qp);
+    if (err) {
+        dev_warn(&dev->dev, "XDP Device queue allocation failure.\n");
+        return err;
+    }
+
      if (prog) {
-        prog = bpf_prog_add(prog, vi->max_queue_pairs - 1);
-        if (IS_ERR(prog))
+        prog = bpf_prog_add(prog, vi->max_queue_pairs);

I think this change is not correct, it would be off by one now.
The previous 'vi->max_queue_pairs - 1' was actually correct here.
dev_change_xdp_fd() already gives you a reference (see the doc on
enum xdp_netdev_command in netdevice.h).

Right, this was an error thanks for checking it I'll send a v3. And
maybe draft a test for XDP ref counting to test it in the future.

.John