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Commit 4f0581d2 authored by Wei Liu's avatar Wei Liu Committed by David S. Miller
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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 6e43fc04
("xen-netback: count number required slots for an skb more carefully").

Commit 6e43fc04 focused on backport-ability. The drawback with the
existing packing scheme is that the ring is not used effeciently, as
stated in 6e43fc04.

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 6e43fc04. 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 6e43fc04.

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



Signed-off-by: default avatarXi Xiong <xixiong@amazon.com>
Reviewed-by: default avatarMatt Wilson <msw@amazon.com>
[ msw: minor code cleanups, rewrote commit message, adjusted code
  to count RX slots instead of meta structures ]
Signed-off-by: default avatarMatt 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: default avatarWei Liu <wei.liu2@citrix.com>
Cc: David Vrabel <david.vrabel@citrix.com>
Acked-by: default avatarIan Campbell <Ian.Campbell@citrix.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent 6459082a

drivers/net/xen-netback/netback.c

+61 −83
Original line number Diff line number Diff line
@@ -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.
@@ -212,90 +220,60 @@ 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)

/*

 * 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

 * xenvif_gop_frag_copy.

 */

unsigned int xenvif_count_skb_slots(struct xenvif *vif, struct sk_buff *skb)

{

	unsigned count = 0;

	unsigned int count;

	int i, copy_off;

	struct skb_cb_overlay *sco;



	count = DIV_ROUND_UP(skb_headlen(skb), PAGE_SIZE);



	copy_off = skb_headlen(skb) % PAGE_SIZE;



	if (skb_shinfo(skb)->gso_size)

		count++;



	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;



		offset &= ~PAGE_MASK;



		while (size > 0) {

		unsigned long bytes;

			BUG_ON(offset >= PAGE_SIZE);

			BUG_ON(copy_off > MAX_BUFFER_OFFSET);



			bytes = PAGE_SIZE - offset;



			if (bytes > size)

				bytes = size;



		if (start_new_rx_buffer(state->copy_off, bytes, state->head)) {

			if (start_new_rx_buffer(copy_off, bytes, 0)) {

				count++;

			state->copy_off = 0;

				copy_off = 0;

			}



		if (state->copy_off + bytes > MAX_BUFFER_OFFSET)

			bytes = MAX_BUFFER_OFFSET - state->copy_off;

			if (copy_off + bytes > MAX_BUFFER_OFFSET)

				bytes = MAX_BUFFER_OFFSET - copy_off;



		state->copy_off += bytes;

			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

 * xenvif_gop_frag_copy.

 */

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;



	state.head = true;

	state.copy_off = 0;



	/* Slot for the first (partial) page of data. */

	count = 1;



	/* 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;



		if (data + size > skb_tail_pointer(skb))

			size = skb_tail_pointer(skb) - data;



		count += xenvif_count_frag_slots(vif, offset, size, &state);



		data += size;

	}



	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;



		count += xenvif_count_frag_slots(vif, offset, size, &state);

	}

	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;

	}