[3.16.y-ckt stable] Patch "xen-netfront: transmit fully GSO-sized packets" has been added to staging queue

[Luis Henriques]

This is a note to let you know that I have just added a patch titled

    xen-netfront: transmit fully GSO-sized packets

to the linux-3.16.y-queue branch of the 3.16.y-ckt extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.16.y-queue

This patch is scheduled to be released in version 3.16.7-ckt10.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.16.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

------

>From 53d556c7964897655f24b89177eb492688e54fe3 Mon Sep 17 00:00:00 2001
From: Jonathan Davies <jonathan.davies at citrix.com>
Date: Tue, 31 Mar 2015 11:05:15 +0100
Subject: xen-netfront: transmit fully GSO-sized packets

commit 0c36820e2ab7d943ab1188230fdf2149826d33c0 upstream.

xen-netfront limits transmitted skbs to be at most 44 segments in size. However,
GSO permits up to 65536 bytes, which means a maximum of 45 segments of 1448
bytes each. This slight reduction in the size of packets means a slight loss in
efficiency.

Since c/s 9ecd1a75d, xen-netfront sets gso_max_size to
    XEN_NETIF_MAX_TX_SIZE - MAX_TCP_HEADER,
where XEN_NETIF_MAX_TX_SIZE is 65535 bytes.

The calculation used by tcp_tso_autosize (and also tcp_xmit_size_goal since c/s
6c09fa09d) in determining when to split an skb into two is
    sk->sk_gso_max_size - 1 - MAX_TCP_HEADER.

So the maximum permitted size of an skb is calculated to be
    (XEN_NETIF_MAX_TX_SIZE - MAX_TCP_HEADER) - 1 - MAX_TCP_HEADER.

Intuitively, this looks like the wrong formula -- we don't need two TCP headers.
Instead, there is no need to deviate from the default gso_max_size of 65536 as
this already accommodates the size of the header.

Currently, the largest skb transmitted by netfront is 63712 bytes (44 segments
of 1448 bytes each), as observed via tcpdump. This patch makes netfront send
skbs of up to 65160 bytes (45 segments of 1448 bytes each).

Similarly, the maximum allowable mtu does not need to subtract MAX_TCP_HEADER as
it relates to the size of the whole packet, including the header.

Fixes: 9ecd1a75d977 ("xen-netfront: reduce gso_max_size to account for max TCP header")
Signed-off-by: Jonathan Davies <jonathan.davies at citrix.com>
Signed-off-by: David S. Miller <davem at davemloft.net>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 drivers/net/xen-netfront.c | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/drivers/net/xen-netfront.c b/drivers/net/xen-netfront.c
index fc49005ddb5d..0838a2db5570 100644
--- a/drivers/net/xen-netfront.c
+++ b/drivers/net/xen-netfront.c
@@ -1098,8 +1098,7 @@ err:

 static int xennet_change_mtu(struct net_device *dev, int mtu)
 {
-	int max = xennet_can_sg(dev) ?
-		XEN_NETIF_MAX_TX_SIZE - MAX_TCP_HEADER : ETH_DATA_LEN;
+	int max = xennet_can_sg(dev) ? XEN_NETIF_MAX_TX_SIZE : ETH_DATA_LEN;

 	if (mtu > max)
 		return -EINVAL;
@@ -1370,8 +1369,6 @@ static struct net_device *xennet_create_dev(struct xenbus_device *dev)
 	netdev->ethtool_ops = &xennet_ethtool_ops;
 	SET_NETDEV_DEV(netdev, &dev->dev);

-	netif_set_gso_max_size(netdev, XEN_NETIF_MAX_TX_SIZE - MAX_TCP_HEADER);
-
 	np->netdev = netdev;

 	netif_carrier_off(netdev);