[Qemu-devel] [PATCH] virtio-net: allow increasing rx queue size

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

This allows increasing the rx queue size up to 1024: unlike with tx,
guests don't put in huge S/G lists into RX so the risk of running into
the max 1024 limitation due to some off-by-one seems small.

It's helpful for users like OVS-DPDK which don't do any buffering on the
host - 1K roughly matches 500 entries in tun + 256 in the current rx
queue, which seems to work reasonably well. We could probably make do
with ~750 entries but virtio spec limits us to powers of two.
It might be a good idea to specify an s/g size limit in a future
version.

It also might be possible to make the queue size smaller down the road, 64
seems like the minimal value which will still work (as guests seem to
assume a queue full of 1.5K buffers is enough to process the largest
incoming packet, which is ~64K).  No one actually asked for this, and
with virtio 1 guests can reduce ring size without need for host
configuration, so don't bother with this for now.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
 include/hw/virtio/virtio-net.h |  1 +
 hw/net/virtio-net.c            | 22 +++++++++++++++++++++-
 2 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/include/hw/virtio/virtio-net.h b/include/hw/virtio/virtio-net.h
index 91ed97c..0ced975 100644
--- a/include/hw/virtio/virtio-net.h
+++ b/include/hw/virtio/virtio-net.h
@@ -35,6 +35,7 @@ typedef struct virtio_net_conf
     uint32_t txtimer;
     int32_t txburst;
     char *tx;
+    uint16_t rx_queue_size;
 } virtio_net_conf;
 
 /* Maximum packet size we can receive from tap device: header + 64k */
diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
index 01f1351..4e595e9 100644
--- a/hw/net/virtio-net.c
+++ b/hw/net/virtio-net.c
@@ -1412,7 +1412,8 @@ static void virtio_net_add_queue(VirtIONet *n, int index)
 {
     VirtIODevice *vdev = VIRTIO_DEVICE(n);
 
-    n->vqs[index].rx_vq = virtio_add_queue(vdev, 256, virtio_net_handle_rx);
+    n->vqs[index].rx_vq = virtio_add_queue(vdev, n->net_conf.rx_queue_size,
+                                           virtio_net_handle_rx);
     if (n->net_conf.tx && !strcmp(n->net_conf.tx, "timer")) {
         n->vqs[index].tx_vq =
             virtio_add_queue(vdev, 256, virtio_net_handle_tx_timer);
@@ -1716,10 +1717,28 @@ static void virtio_net_device_realize(DeviceState *dev, Error **errp)
     VirtIONet *n = VIRTIO_NET(dev);
     NetClientState *nc;
     int i;
+    int min_rx_queue_size;
 
     virtio_net_set_config_size(n, n->host_features);
     virtio_init(vdev, "virtio-net", VIRTIO_ID_NET, n->config_size);
 
+    /*
+     * We set a lower limit on RX queue size to what it always was.
+     * Guests that want a smaller ring can always resize it without
+     * help from us (using virtio 1 and up).
+     */
+    min_rx_queue_size = 256;
+    if (n->net_conf.rx_queue_size < min_rx_queue_size ||
+        n->net_conf.rx_queue_size > VIRTQUEUE_MAX_SIZE ||
+        (n->net_conf.rx_queue_size & (n->net_conf.rx_queue_size - 1))) {
+        error_setg(errp, "Invalid rx_queue_size (= %" PRIu16 "), "
+                   "must be a power of 2 between %d and %d.",
+                   n->net_conf.rx_queue_size, min_rx_queue_size,
+                   VIRTQUEUE_MAX_SIZE);
+        virtio_cleanup(vdev);
+        return;
+    }
+
     n->max_queues = MAX(n->nic_conf.peers.queues, 1);
     if (n->max_queues * 2 + 1 > VIRTIO_QUEUE_MAX) {
         error_setg(errp, "Invalid number of queues (= %" PRIu32 "), "
@@ -1880,6 +1899,7 @@ static Property virtio_net_properties[] = {
                        TX_TIMER_INTERVAL),
     DEFINE_PROP_INT32("x-txburst", VirtIONet, net_conf.txburst, TX_BURST),
     DEFINE_PROP_STRING("tx", VirtIONet, net_conf.tx),
+    DEFINE_PROP_UINT16("rx_queue_size", VirtIONet, net_conf.rx_queue_size, 256),
     DEFINE_PROP_END_OF_LIST(),
 };
 
-- 
MST