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Subject: Re: [virtio-dev] Re: [PATCH v2 1/2] virtio-net: enable configurable tx queue size

On 06/22/2017 10:00 PM, Michael S. Tsirkin wrote:
On Sat, Jun 17, 2017 at 04:38:03PM +0800, Wei Wang wrote:
On 06/16/2017 10:31 PM, Michael S. Tsirkin wrote:
On Fri, Jun 16, 2017 at 06:48:38PM +0800, Wei Wang wrote:
This patch enables the virtio-net tx queue size to be configurable
between 256 (the default queue size) and 1024 by the user when the
vhost-user backend is used.

Currently, the maximum tx queue size for other backends is 512 due
to the following limitations:
- QEMU backend: the QEMU backend implementation in some cases may
send 1024+1 iovs to writev.
- Vhost_net backend: there are possibilities that the guest sends
a vring_desc of memory which corsses a MemoryRegion thereby
generating more than 1024 iovs in total after translattion from
guest-physical address in the backend.

Signed-off-by: Wei Wang <wei.w.wang@intel.com>
   hw/net/virtio-net.c            | 46 ++++++++++++++++++++++++++++++++++--------
   include/hw/virtio/virtio-net.h |  1 +
   2 files changed, 39 insertions(+), 8 deletions(-)

diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
index 7d091c9..e1a08fd 100644
--- a/hw/net/virtio-net.c
+++ b/hw/net/virtio-net.c
@@ -33,8 +33,11 @@
   /* previously fixed value */
   /* for now, only allow larger queues; with virtio-1, guest can downsize */
    * Calculate the number of bytes up to and including the given 'field' of
@@ -1491,18 +1494,33 @@ static void virtio_net_tx_bh(void *opaque)
   static void virtio_net_add_queue(VirtIONet *n, int index)
       VirtIODevice *vdev = VIRTIO_DEVICE(n);
+    NetClientState *nc = qemu_get_queue(n->nic);
       n->vqs[index].rx_vq = virtio_add_queue(vdev, n->net_conf.rx_queue_size,
+    /*
+     * Currently, backends other than vhost-user don't support 1024 queue
+     * size.
+     */
+    if (n->net_conf.tx_queue_size == VIRTQUEUE_MAX_SIZE &&
+        nc->peer->info->type != NET_CLIENT_DRIVER_VHOST_USER) {
+        fprintf(stderr, "warning: %s: queue size %d not supported\n",
+                __func__, n->net_conf.tx_queue_size);
+        n->net_conf.tx_queue_size = VIRTIO_NET_TX_QUEUE_DEFAULT_SIZE;
+    }
Also, I suspect we can get here with no peer, and above will crash.
It seems ugly to do this on each virtio_net_add_queue.
How about moving this to realize?
The code has been re-arranged to make sure nc->peer is ready before
it's used, but I agree that it looks better to move the above to realize().


The issues left are minor, let's make progress and merge this asap

OK. I'll send out the new version soon.


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