xen-netback: improve ring effeciency for guest RX

There was a bug that netback routines netbk/xenvif_skb_count_slots and
netbk/xenvif_gop_frag_copy disagreed with each other, which caused
netback to push wrong number of responses to netfront, which caused
netfront to eventually crash. The bug was fixed in 6e43fc04a
("xen-netback: count number required slots for an skb more carefully").

Commit 6e43fc04a focused on backport-ability. The drawback with the
existing packing scheme is that the ring is not used effeciently, as
stated in 6e43fc04a.

skb->data like:
    |        1111|222222222222|3333        |

is arranged as:
    |1111        |222222222222|3333        |

If we can do this:
    |111122222222|22223333    |
That would save one ring slot, which improves ring effeciency.

This patch effectively reverts 6e43fc04a. That patch made count_slots
agree with gop_frag_copy, while this patch goes the other way around --
make gop_frag_copy agree with count_slots. The end result is that they
still agree with each other, and the ring is now arranged like:
    |111122222222|22223333    |

The patch that improves packing was first posted by Xi Xong and Matt
Wilson. I only rebase it on top of net-next and rewrite commit message,
so I retain all their SoBs. For more infomation about the original bug
please refer to email listed below and commit message of 6e43fc04a.

Original patch:
http://lists.xen.org/archives/html/xen-devel/2013-07/msg00760.html

Signed-off-by: Xi Xiong <xixiong@amazon.com>
Reviewed-by: Matt Wilson <msw@amazon.com>
[ msw: minor code cleanups, rewrote commit message, adjusted code
  to count RX slots instead of meta structures ]
Signed-off-by: Matt Wilson <msw@amazon.com>
Cc: Annie Li <annie.li@oracle.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Cc: Ian Campbell <Ian.Campbell@citrix.com>
[ liuw: rebased on top of net-next tree, rewrote commit message, coding
  style cleanup. ]
Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Cc: David Vrabel <david.vrabel@citrix.com>
Acked-by: Ian Campbell <Ian.Campbell@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/drivers/net/xen-netback/netback.c b/drivers/net/xen-netback/netback.c
index f3e591c..d0b0feb 100644 (file)
--- a/drivers/net/xen-netback/netback.c
+++ b/drivers/net/xen-netback/netback.c
@@ -47,6 +47,14 @@
 #include <asm/xen/hypercall.h>
 #include <asm/xen/page.h>
 
+/* SKB control block overlay is used to store useful information when
+ * doing guest RX.
+ */
+struct skb_cb_overlay {
+       int meta_slots_used;
+       int peek_slots_count;
+};
+
 /* Provide an option to disable split event channels at load time as
  * event channels are limited resource. Split event channels are
  * enabled by default.
@@ -212,49 +220,6 @@ static bool start_new_rx_buffer(int offset, unsigned long size, int head)
        return false;
 }
 
-struct xenvif_count_slot_state {
-       unsigned long copy_off;
-       bool head;
-};
-
-unsigned int xenvif_count_frag_slots(struct xenvif *vif,
-                                    unsigned long offset, unsigned long size,
-                                    struct xenvif_count_slot_state *state)
-{
-       unsigned count = 0;
-
-       offset &= ~PAGE_MASK;
-
-       while (size > 0) {
-               unsigned long bytes;
-
-               bytes = PAGE_SIZE - offset;
-
-               if (bytes > size)
-                       bytes = size;
-
-               if (start_new_rx_buffer(state->copy_off, bytes, state->head)) {
-                       count++;
-                       state->copy_off = 0;
-               }
-
-               if (state->copy_off + bytes > MAX_BUFFER_OFFSET)
-                       bytes = MAX_BUFFER_OFFSET - state->copy_off;
-
-               state->copy_off += bytes;
-
-               offset += bytes;
-               size -= bytes;
-
-               if (offset == PAGE_SIZE)
-                       offset = 0;
-
-               state->head = false;
-       }
-
-       return count;
-}
-
 /*
  * Figure out how many ring slots we're going to need to send @skb to
  * the guest. This function is essentially a dry run of
@@ -262,40 +227,53 @@ unsigned int xenvif_count_frag_slots(struct xenvif *vif,
  */
 unsigned int xenvif_count_skb_slots(struct xenvif *vif, struct sk_buff *skb)
 {
-       struct xenvif_count_slot_state state;
        unsigned int count;
-       unsigned char *data;
-       unsigned i;
+       int i, copy_off;
+       struct skb_cb_overlay *sco;
 
-       state.head = true;
-       state.copy_off = 0;
+       count = DIV_ROUND_UP(skb_headlen(skb), PAGE_SIZE);
 
-       /* Slot for the first (partial) page of data. */
-       count = 1;
+       copy_off = skb_headlen(skb) % PAGE_SIZE;
 
-       /* Need a slot for the GSO prefix for GSO extra data? */
        if (skb_shinfo(skb)->gso_size)
                count++;
 
-       data = skb->data;
-       while (data < skb_tail_pointer(skb)) {
-               unsigned long offset = offset_in_page(data);
-               unsigned long size = PAGE_SIZE - offset;
+       for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) {
+               unsigned long size = skb_frag_size(&skb_shinfo(skb)->frags[i]);
+               unsigned long offset = skb_shinfo(skb)->frags[i].page_offset;
+               unsigned long bytes;
 
-               if (data + size > skb_tail_pointer(skb))
-                       size = skb_tail_pointer(skb) - data;
+               offset &= ~PAGE_MASK;
 
-               count += xenvif_count_frag_slots(vif, offset, size, &state);
+               while (size > 0) {
+                       BUG_ON(offset >= PAGE_SIZE);
+                       BUG_ON(copy_off > MAX_BUFFER_OFFSET);
 
-               data += size;
-       }
+                       bytes = PAGE_SIZE - offset;
 
-       for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) {
-               unsigned long size = skb_frag_size(&skb_shinfo(skb)->frags[i]);
-               unsigned long offset = skb_shinfo(skb)->frags[i].page_offset;
+                       if (bytes > size)
+                               bytes = size;
+
+                       if (start_new_rx_buffer(copy_off, bytes, 0)) {
+                               count++;
+                               copy_off = 0;
+                       }
 
-               count += xenvif_count_frag_slots(vif, offset, size, &state);
+                       if (copy_off + bytes > MAX_BUFFER_OFFSET)
+                               bytes = MAX_BUFFER_OFFSET - copy_off;
+
+                       copy_off += bytes;
+
+                       offset += bytes;
+                       size -= bytes;
+
+                       if (offset == PAGE_SIZE)
+                               offset = 0;
+               }
        }
+
+       sco = (struct skb_cb_overlay *)skb->cb;
+       sco->peek_slots_count = count;
        return count;
 }
 
@@ -327,14 +305,11 @@ static struct xenvif_rx_meta *get_next_rx_buffer(struct xenvif *vif,
        return meta;
 }
 
-/*
- * Set up the grant operations for this fragment. If it's a flipping
- * interface, we also set up the unmap request from here.
- */
+/* Set up the grant operations for this fragment. */
 static void xenvif_gop_frag_copy(struct xenvif *vif, struct sk_buff *skb,
                                 struct netrx_pending_operations *npo,
                                 struct page *page, unsigned long size,
-                                unsigned long offset, int *head)
+                                unsigned long offset, int head, int *first)
 {
        struct gnttab_copy *copy_gop;
        struct xenvif_rx_meta *meta;
@@ -358,12 +333,12 @@ static void xenvif_gop_frag_copy(struct xenvif *vif, struct sk_buff *skb,
                if (bytes > size)
                        bytes = size;
 
-               if (start_new_rx_buffer(npo->copy_off, bytes, *head)) {
+               if (start_new_rx_buffer(npo->copy_off, bytes, head)) {
                        /*
                         * Netfront requires there to be some data in the head
                         * buffer.
                         */
-                       BUG_ON(*head);
+                       BUG_ON(*first);
 
                        meta = get_next_rx_buffer(vif, npo);
                }
@@ -397,10 +372,10 @@ static void xenvif_gop_frag_copy(struct xenvif *vif, struct sk_buff *skb,
                }
 
                /* Leave a gap for the GSO descriptor. */
-               if (*head && skb_shinfo(skb)->gso_size && !vif->gso_prefix)
+               if (*first && skb_shinfo(skb)->gso_size && !vif->gso_prefix)
                        vif->rx.req_cons++;
 
-               *head = 0; /* There must be something in this buffer now. */
+               *first = 0; /* There must be something in this buffer now. */
 
        }
 }
@@ -426,7 +401,7 @@ static int xenvif_gop_skb(struct sk_buff *skb,
        struct xen_netif_rx_request *req;
        struct xenvif_rx_meta *meta;
        unsigned char *data;
-       int head = 1;
+       int first = 1;
        int old_meta_prod;
 
        old_meta_prod = npo->meta_prod;
@@ -462,7 +437,7 @@ static int xenvif_gop_skb(struct sk_buff *skb,
                        len = skb_tail_pointer(skb) - data;
 
                xenvif_gop_frag_copy(vif, skb, npo,
-                                    virt_to_page(data), len, offset, &head);
+                                    virt_to_page(data), len, offset, 1, &first);
                data += len;
        }
 
@@ -471,7 +446,7 @@ static int xenvif_gop_skb(struct sk_buff *skb,
                                     skb_frag_page(&skb_shinfo(skb)->frags[i]),
                                     skb_frag_size(&skb_shinfo(skb)->frags[i]),
                                     skb_shinfo(skb)->frags[i].page_offset,
-                                    &head);
+                                    0, &first);
        }
 
        return npo->meta_prod - old_meta_prod;
@@ -529,10 +504,6 @@ static void xenvif_add_frag_responses(struct xenvif *vif, int status,
        }
 }
 
-struct skb_cb_overlay {
-       int meta_slots_used;
-};
-
 static void xenvif_kick_thread(struct xenvif *vif)
 {
        wake_up(&vif->wq);
@@ -563,19 +534,26 @@ void xenvif_rx_action(struct xenvif *vif)
        count = 0;
 
        while ((skb = skb_dequeue(&vif->rx_queue)) != NULL) {
+               RING_IDX old_rx_req_cons;
+
                vif = netdev_priv(skb->dev);
                nr_frags = skb_shinfo(skb)->nr_frags;
 
+               old_rx_req_cons = vif->rx.req_cons;
                sco = (struct skb_cb_overlay *)skb->cb;
                sco->meta_slots_used = xenvif_gop_skb(skb, &npo);
 
-               count += nr_frags + 1;
+               count += vif->rx.req_cons - old_rx_req_cons;
 
                __skb_queue_tail(&rxq, skb);
 
+               skb = skb_peek(&vif->rx_queue);
+               if (skb == NULL)
+                       break;
+               sco = (struct skb_cb_overlay *)skb->cb;
+
                /* Filled the batch queue? */
-               /* XXX FIXME: RX path dependent on MAX_SKB_FRAGS */
-               if (count + MAX_SKB_FRAGS >= XEN_NETIF_RX_RING_SIZE)
+               if (count + sco->peek_slots_count >= XEN_NETIF_RX_RING_SIZE)
                        break;
        }