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Subject[PATCH] virtio-net: put virtio net header inline with data
For small packets we can simplify xmit processing by linearizing buffers
with the header: most packets seem to have enough head room we can use
for this purpose.

Since some older hypervisors (e.g. qemu before version 1.5)
required that header is the first s/g element,
we need a feature bit for this.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---

This is a repost of my old patch, rebased to latest kernel.

Before:
Thu Jun 6 05:24:59 EDT 2013
TCP REQUEST/RESPONSE TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET to
11.0.0.4 (11.0.0.4) port 0 AF_INET : demo
Local /Remote
Socket Size Request Resp. Elapsed Trans.
Send Recv Size Size Time Rate
bytes Bytes bytes bytes secs. per sec

16384 87380 1 1 10.00 12931.13


After:
TCP REQUEST/RESPONSE TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET to
11.0.0.4 (11.0.0.4) port 0 AF_INET : demo
Local /Remote
Socket Size Request Resp. Elapsed Trans.
Send Recv Size Size Time Rate
bytes Bytes bytes bytes secs. per sec

16384 87380 1 1 10.00 14151.12


drivers/net/virtio_net.c | 42 +++++++++++++++++++++++++++++++++--------
include/uapi/linux/virtio_net.h | 6 +++++-
2 files changed, 39 insertions(+), 9 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index c9e0038..d35a097 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -106,6 +106,9 @@ struct virtnet_info {
/* Has control virtqueue */
bool has_cvq;

+ /* Host can handle any s/g split between our header and packet data */
+ bool any_header_sg;
+
/* enable config space updates */
bool config_enable;

@@ -668,12 +671,28 @@ static void free_old_xmit_skbs(struct send_queue *sq)

static int xmit_skb(struct send_queue *sq, struct sk_buff *skb)
{
- struct skb_vnet_hdr *hdr = skb_vnet_hdr(skb);
+ struct skb_vnet_hdr *hdr;
const unsigned char *dest = ((struct ethhdr *)skb->data)->h_dest;
struct virtnet_info *vi = sq->vq->vdev->priv;
unsigned num_sg;
+ unsigned hdr_len;
+ bool can_push;

pr_debug("%s: xmit %p %pM\n", vi->dev->name, skb, dest);
+ if (vi->mergeable_rx_bufs)
+ hdr_len = sizeof hdr->mhdr;
+ else
+ hdr_len = sizeof hdr->hdr;
+
+ can_push = vi->any_header_sg &&
+ !((unsigned long)skb->data & (__alignof__(*hdr) - 1)) &&
+ !skb_header_cloned(skb) && skb_headroom(skb) >= hdr_len;
+ /* Even if we can, don't push here yet as this would skew
+ * csum_start offset below. */
+ if (can_push)
+ hdr = (struct skb_vnet_hdr *)(skb->data - hdr_len);
+ else
+ hdr = skb_vnet_hdr(skb);

if (skb->ip_summed == CHECKSUM_PARTIAL) {
hdr->hdr.flags = VIRTIO_NET_HDR_F_NEEDS_CSUM;
@@ -702,15 +721,18 @@ static int xmit_skb(struct send_queue *sq, struct sk_buff *skb)
hdr->hdr.gso_size = hdr->hdr.hdr_len = 0;
}

- hdr->mhdr.num_buffers = 0;
-
- /* Encode metadata header at front. */
if (vi->mergeable_rx_bufs)
- sg_set_buf(sq->sg, &hdr->mhdr, sizeof hdr->mhdr);
- else
- sg_set_buf(sq->sg, &hdr->hdr, sizeof hdr->hdr);
+ hdr->mhdr.num_buffers = 0;

- num_sg = skb_to_sgvec(skb, sq->sg + 1, 0, skb->len) + 1;
+ if (can_push) {
+ __skb_push(skb, hdr_len);
+ num_sg = skb_to_sgvec(skb, sq->sg, 0, skb->len);
+ /* Pull header back to avoid skew in tx bytes calculations. */
+ __skb_pull(skb, hdr_len);
+ } else {
+ sg_set_buf(sq->sg, hdr, hdr_len);
+ num_sg = skb_to_sgvec(skb, sq->sg + 1, 0, skb->len) + 1;
+ }
return virtqueue_add_outbuf(sq->vq, sq->sg, num_sg, skb, GFP_ATOMIC);
}

@@ -1554,6 +1576,9 @@ static int virtnet_probe(struct virtio_device *vdev)
if (virtio_has_feature(vdev, VIRTIO_NET_F_MRG_RXBUF))
vi->mergeable_rx_bufs = true;

+ if (virtio_has_feature(vdev, VIRTIO_NET_F_ANY_HEADER_SG))
+ vi->any_header_sg = true;
+
if (virtio_has_feature(vdev, VIRTIO_NET_F_CTRL_VQ))
vi->has_cvq = true;

@@ -1729,6 +1754,7 @@ static unsigned int features[] = {
VIRTIO_NET_F_CTRL_RX, VIRTIO_NET_F_CTRL_VLAN,
VIRTIO_NET_F_GUEST_ANNOUNCE, VIRTIO_NET_F_MQ,
VIRTIO_NET_F_CTRL_MAC_ADDR,
+ VIRTIO_NET_F_ANY_HEADER_SG,
};

static struct virtio_driver virtio_net_driver = {
diff --git a/include/uapi/linux/virtio_net.h b/include/uapi/linux/virtio_net.h
index c520203..9c98b7d 100644
--- a/include/uapi/linux/virtio_net.h
+++ b/include/uapi/linux/virtio_net.h
@@ -55,6 +55,8 @@
* Steering */
#define VIRTIO_NET_F_CTRL_MAC_ADDR 23 /* Set MAC address */

+#define VIRTIO_NET_F_ANY_HEADER_SG 25 /* Host can handle any header s/g */
+
#define VIRTIO_NET_S_LINK_UP 1 /* Link is up */
#define VIRTIO_NET_S_ANNOUNCE 2 /* Announcement is needed */

@@ -70,7 +72,9 @@ struct virtio_net_config {
__u16 max_virtqueue_pairs;
} __attribute__((packed));

-/* This is the first element of the scatter-gather list. If you don't
+/* This header comes first in the scatter-gather list.
+ * If VIRTIO_NET_F_ANY_HEADER_SG is not negotiated, it must
+ * be the first element of the scatter-gather list. If you don't
* specify GSO or CSUM features, you can simply ignore the header. */
struct virtio_net_hdr {
#define VIRTIO_NET_HDR_F_NEEDS_CSUM 1 // Use csum_start, csum_offset
--
MST

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