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xen-netback: remove unconditional __pskb_pull_tail() in guest Tx path

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Unconditionally pulling 128 bytes into the linear area is not required
for:

- security: Every protocol demux starts with pskb_may_pull() to pull
  frag data into the linear area, if necessary, before looking at
  headers.

- performance: Netback has already grant copied up-to 128 bytes from
  the first slot of a packet into the linear area. The first slot
  normally contain all the IPv4/IPv6 and TCP/UDP headers.

The unconditional pull would often copy frag data unnecessarily.  This
is a performance problem when running on a version of Xen where grant
unmap avoids TLB flushes for pages which are not accessed.  TLB
flushes can now be avoided for > 99% of unmaps (it was 0% before).

Grant unmap TLB flush avoidance will be available in a future version
of Xen (probably 4.6).

Signed-off-by: Malcolm Crossley <malcolm.crossley@citrix.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Malcolm Crossley 2014-11-05 10:50:22 +00:00 • committed by David S. Miller
parent 17007aa53a
commit 7e5d775395
1 changed files with 12 additions and 14 deletions

26
drivers/net/xen-netback/netback.c
View file

@ -82,6 +82,16 @@ MODULE_PARM_DESC(max_queues,
static unsigned int fatal_skb_slots = FATAL_SKB_SLOTS_DEFAULT; static unsigned int fatal_skb_slots = FATAL_SKB_SLOTS_DEFAULT;
module_param(fatal_skb_slots, uint, 0444); module_param(fatal_skb_slots, uint, 0444);
/* The amount to copy out of the first guest Tx slot into the skb's
* linear area. If the first slot has more data, it will be mapped
* and put into the first frag.
*
* This is sized to avoid pulling headers from the frags for most
* TCP/IP packets.
*/
#define XEN_NETBACK_TX_COPY_LEN 128
static void xenvif_idx_release(struct xenvif_queue *queue, u16 pending_idx, static void xenvif_idx_release(struct xenvif_queue *queue, u16 pending_idx,
u8 status); u8 status);
@ -125,13 +135,6 @@ static inline struct xenvif_queue *ubuf_to_queue(const struct ubuf_info *ubuf)
pending_tx_info[0]); pending_tx_info[0]);
} }
/* This is a miniumum size for the linear area to avoid lots of
* calls to __pskb_pull_tail() as we set up checksum offsets. The
* value 128 was chosen as it covers all IPv4 and most likely
* IPv6 headers.
*/
#define PKT_PROT_LEN 128
static u16 frag_get_pending_idx(skb_frag_t *frag) static u16 frag_get_pending_idx(skb_frag_t *frag)
{ {
return (u16)frag->page_offset; return (u16)frag->page_offset;
@ -1446,9 +1449,9 @@ static void xenvif_tx_build_gops(struct xenvif_queue *queue,
index = pending_index(queue->pending_cons); index = pending_index(queue->pending_cons);
pending_idx = queue->pending_ring[index]; pending_idx = queue->pending_ring[index];
data_len = (txreq.size > PKT_PROT_LEN && data_len = (txreq.size > XEN_NETBACK_TX_COPY_LEN &&
ret < XEN_NETBK_LEGACY_SLOTS_MAX) ? ret < XEN_NETBK_LEGACY_SLOTS_MAX) ?
PKT_PROT_LEN : txreq.size; XEN_NETBACK_TX_COPY_LEN : txreq.size;
skb = xenvif_alloc_skb(data_len); skb = xenvif_alloc_skb(data_len);
if (unlikely(skb == NULL)) { if (unlikely(skb == NULL)) {
@ -1653,11 +1656,6 @@ static int xenvif_tx_submit(struct xenvif_queue *queue)
} }
} }
if (skb_is_nonlinear(skb) && skb_headlen(skb) < PKT_PROT_LEN) {
int target = min_t(int, skb->len, PKT_PROT_LEN);
__pskb_pull_tail(skb, target - skb_headlen(skb));
}
skb->dev = queue->vif->dev; skb->dev = queue->vif->dev;
skb->protocol = eth_type_trans(skb, skb->dev); skb->protocol = eth_type_trans(skb, skb->dev);
skb_reset_network_header(skb); skb_reset_network_header(skb);