Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S965112AbbEECHu (ORCPT ); Mon, 4 May 2015 22:07:50 -0400 Received: from shadbolt.e.decadent.org.uk ([88.96.1.126]:50972 "EHLO shadbolt.e.decadent.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752207AbbEEBV7 (ORCPT ); Mon, 4 May 2015 21:21:59 -0400 Content-Type: text/plain; charset="UTF-8" Content-Disposition: inline Content-Transfer-Encoding: 8bit MIME-Version: 1.0 From: Ben Hutchings To: linux-kernel@vger.kernel.org, stable@vger.kernel.org CC: akpm@linux-foundation.org, "Jonathan Davies" , "David S. Miller" Date: Tue, 05 May 2015 02:16:39 +0100 Message-ID: X-Mailer: LinuxStableQueue (scripts by bwh) Subject: [PATCH 3.2 171/221] xen-netfront: transmit fully GSO-sized packets In-Reply-To: X-SA-Exim-Connect-IP: 192.168.4.249 X-SA-Exim-Mail-From: ben@decadent.org.uk X-SA-Exim-Scanned: No (on shadbolt.decadent.org.uk); SAEximRunCond expanded to false Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2722 Lines: 70 3.2.69-rc1 review patch. If anyone has any objections, please let me know. ------------------ From: Jonathan Davies 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 Signed-off-by: David S. Miller Signed-off-by: Ben Hutchings --- drivers/net/xen-netfront.c | 5 +---- 1 file changed, 1 insertion(+), 4 deletions(-) --- a/drivers/net/xen-netfront.c +++ b/drivers/net/xen-netfront.c @@ -1053,8 +1053,7 @@ 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; @@ -1329,8 +1328,6 @@ static struct net_device * __devinit xen SET_ETHTOOL_OPS(netdev, &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); -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/