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SubjectRe: [PATCH] virtio-net: put virtio net header inline with data
On 06/06/2013 05:55 PM, Michael S. Tsirkin wrote:
> 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>
> ---

Hi Michael:

The idea looks good, but there are some questions:

- What if we just use direct descriptors with sgs <=2 and double the
ring size?
- I believe we may add more things into vnet header in the future, so
this trick may not work because of limited header room.

Thanks
> 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



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