This implements partial checksum write support for tun/tap.

We use the virtio_net_hdr: it is an ABI already and designed to
encapsulate such metadata as GSO (in coming patches) and partial
checksums.

Performance before:
---
 drivers/net/tun.c      |   20 +++++++++++++++++++-
 include/linux/if_tun.h |    2 ++
 2 files changed, 21 insertions(+), 1 deletion(-)

diff -r 72be3d596d31 drivers/net/tun.c
--- a/drivers/net/tun.c	Wed Jan 09 15:57:40 2008 +1100
+++ b/drivers/net/tun.c	Wed Jan 09 16:56:41 2008 +1100
@@ -62,6 +62,7 @@
 #include <linux/if_ether.h>
 #include <linux/if_tun.h>
 #include <linux/crc32.h>
+#include <linux/virtio_net.h>
 #include <net/net_namespace.h>
 
 #include <asm/system.h>
@@ -242,6 +243,7 @@ static __inline__ ssize_t tun_get_user(s
 static __inline__ ssize_t tun_get_user(struct tun_struct *tun, struct iovec *iv, size_t count)
 {
 	struct tun_pi pi = { 0, __constant_htons(ETH_P_IP) };
+	struct virtio_net_hdr gso = { 0, VIRTIO_NET_HDR_GSO_NONE };
 	struct sk_buff *skb;
 	size_t len = count, align = 0;
 
@@ -252,10 +254,17 @@ static __inline__ ssize_t tun_get_user(s
 		if(memcpy_fromiovec((void *)&pi, iv, sizeof(pi)))
 			return -EFAULT;
 	}
+	if (tun->flags & TUN_GSO_HDR) {
+		if ((len -= sizeof(gso)) > count)
+			return -EINVAL;
 
+		if (memcpy_fromiovec((void *)&gso, iv, sizeof(gso)))
+			return -EFAULT;
+	}
 	if ((tun->flags & TUN_TYPE_MASK) == TUN_TAP_DEV)
 		align = NET_IP_ALIGN;
 
+	/* FIXME: Prevents GSO being used in anger. */
 	if (!(skb = alloc_skb(len + align, GFP_KERNEL))) {
 		tun->dev->stats.rx_dropped++;
 		return -ENOMEM;
@@ -280,7 +289,13 @@ static __inline__ ssize_t tun_get_user(s
 		break;
 	};
 
-	if (tun->flags & TUN_NOCHECKSUM)
+	if (gso.flags & (1 << VIRTIO_NET_F_NO_CSUM)) {
+		if (!skb_partial_csum_set(skb,gso.csum_start,gso.csum_offset)) {
+			tun->dev->stats.rx_dropped++;
+			kfree_skb(skb);
+			return -EINVAL;
+		}
+	} else if (tun->flags & TUN_NOCHECKSUM)
 		skb->ip_summed = CHECKSUM_UNNECESSARY;
 
 	netif_rx_ni(skb);
@@ -543,6 +558,9 @@ static int tun_set_iff(struct file *file
 
 	if (ifr->ifr_flags & IFF_ONE_QUEUE)
 		tun->flags |= TUN_ONE_QUEUE;
+
+	if (ifr->ifr_flags & IFF_GSO_HDR)
+		tun->flags |= TUN_GSO_HDR;
 
 	file->private_data = tun;
 	tun->attached = 1;
diff -r 72be3d596d31 include/linux/if_tun.h
--- a/include/linux/if_tun.h	Wed Jan 09 15:57:40 2008 +1100
+++ b/include/linux/if_tun.h	Wed Jan 09 16:56:41 2008 +1100
@@ -70,6 +70,7 @@ struct tun_struct {
 #define TUN_NO_PI	0x0040
 #define TUN_ONE_QUEUE	0x0080
 #define TUN_PERSIST 	0x0100	
+#define TUN_GSO_HDR 	0x0200	
 
 /* Ioctl defines */
 #define TUNSETNOCSUM  _IOW('T', 200, int) 
@@ -85,6 +86,7 @@ struct tun_struct {
 #define IFF_TAP		0x0002
 #define IFF_NO_PI	0x1000
 #define IFF_ONE_QUEUE	0x2000
+#define IFF_GSO_HDR	0x4000
 
 struct tun_pi {
 	unsigned short flags;