[PATCH net-next] virtio-net: Set dev->needed_headroom for virtio-net device when it has the feature VIRTIO_F_ANY_LAYOUT

["Zhangjie (HZ)" <zhangjie14@huawei.com>]

This is a small supplement for commit
e7428e95a06fb516fac1308bd0e176e27c0b9287("virtio-net: put virtio-net header
inline with data"). TCP packages have enough room to put virtio-net header
in, but UDP packages do not. By setting dev->needed_headroom for virtio-net
device, UDP packages could have enough room.

For UDP packages, sk_buff is alloced in fun __ip_append_data. The size
is "alloclen + hh_len + 15", and "hh_len = LL_RESERVED_SPACE(rt->dst.dev);".
The Macro is defined as follows:
#define LL_RESERVED_SPACE(dev) \
      ((((dev)->hard_header_len+(dev)->needed_headroom)\
      &~(HH_DATA_MOD - 1)) + HH_DATA_MOD)
By default, for UDP packages, after skb is allocated, only 16 bytes
reserved. And 2 bytes remained after mac header is set. That is not
enough to put virtio-net header in. If we set dev->needed_headroom
to 12 or 10 (according to mergeable_rx_bufs is on or off ), more room
can be reserved. Then there is enough room for UDP packages to put
the header in.

test result list as below:
guest and host: suse11sp3, netperf, intel 2.4GHz
+-------+---------+---------+---------+---------+
|       |   old   |         |   new   |         |
+-------+---------+---------+---------+---------+
|  UDP  | Gbit/s  |   PPs   | Gbit/s  |   PPs   |
|  64   |  0.573  | 692232  |  0.614  | 742420  |
|  256  |   1.6   | 686860  |  1.71   | 733331  |
|  512  |  2.92   | 674576  |  3.07   | 710446  |
| 1024  |  4.99   | 598977  |  5.17   | 620821  |
| 1460  |  5.68   | 483757  |  7.16   | 610519  |
+-------+---------+---------+---------+---------+


Signed-off-by: ZhangJie <zhangjie14@huawei.com>
---
  drivers/net/virtio_net.c |    7 +++++++
  1 files changed, 7 insertions(+), 0 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 8a852b5..e18acc3 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -1724,6 +1724,13 @@ static int virtnet_probe(struct virtio_device *vdev)
  	if (virtio_has_feature(vdev, VIRTIO_NET_F_CTRL_VQ))
  		vi->has_cvq = true;

+        if (vi->any_header_sg) {
+                if (vi->mergeable_rx_bufs)
+                        dev->needed_headroom = sizeof(struct 
virtio_net_hdr_mrg_rxbuf);
+                else
+                        dev->needed_headroom = sizeof(struct 
virtio_net_hdr);
+        }
+
  	/* Use single tx/rx queue pair as default */
  	vi->curr_queue_pairs = 1;
  	vi->max_queue_pairs = max_queue_pairs;
-- 
1.7.1