分类： 嵌入式
2014-01-08 18:24:53
GSO用来扩展之前的TSO，目前已经并入upstream内核。TSO只能支持 tcp协议，而GSO可以支持tcpv4, tcpv6, udp等协议。在GSO之前，skb_shinfo(skb)有两个成员ufo_size, tso_size，分别表示udp fragmentation offloading支持的分片长度，以及tcp segmentation offloading支持的分段长度，现在都用skb_shinfo(skb)->gso_size代替。 skb_shinfo(skb)->ufo_segs, skb_shinfo(skb)->tso_segs也被替换成了skb_shinfo(skb)->gso_segs，表示分片的个数。
skb_shinfo(skb)->gso_type包括SKB_GSO_TCPv4, SKB_GSO_UDPv4，同时NETIF_F_XXX的标志也增加了相应的bit，标识设备是否支持TSO, GSO, e.g.
NETIF_F_TSO = SKB_GSO_TCPV4 << NETIF_F_GSO_SHIFT
NETIF_F_UFO = SKB_GSO_UDPV4 << NETIF_F_GSO_SHIFT
#define NETIF_F_GSO_SHIFT 16
dev_hard_start_xmit在调用设备驱动的发送函数之前，会通过netif_needs_gso判断是否需要软件做GSO，如果需要，那么会调用到dev_gso_segment
/**
* dev_gso_segment - Perform emulated hardware segmentation on skb.
* @skb: buffer to segment
*
* This function segments the given skb and stores the list of segments
* in skb->next.
*/
static int dev_gso_segment(struct sk_buff *skb)
{
struct net_device *dev = skb->dev;
struct sk_buff *segs;
int features = dev->features & ~(illegal_highdma(dev, skb) ?
NETIF_F_SG : 0);
segs = skb_gso_segment(skb, features);
/* Verifying header integrity only. */
if (!segs)
return 0;
if (IS_ERR(segs))
return PTR_ERR(segs);
skb->next = segs;
DEV_GSO_CB(skb)->destructor = skb->destructor;
skb->destructor = dev_gso_skb_destructor;
return 0;
}
分析skb_gso_segment之前，看下析构过程，此时skb经过分片之后已经是一个skb list，通过skb->next串在一起，此时把初始的skb->destructor函数存到skb->cb中，然后把 skb->destructor变更为dev_gso_skb_destructor
dev_gso_skb_destructor会把skb->next一个个通过kfree_skb释放掉，最后调用DEV_GSO_CB(skb)->destructor，即skb初始的析构函数做最后的清理
skb_gso_segment是通过软件方式模拟网卡分段的函数，
struct sk_buff *skb_gso_segment(struct sk_buff *skb, int features)
{
struct sk_buff *segs = ERR_PTR(-EPROTONOSUPPORT);
struct packet_type *ptype;
__be16 type = skb->protocol;
int err;
skb_reset_mac_header(skb);
skb->mac_len = skb->network_header - skb->mac_header;
__skb_pull(skb, skb->mac_len);
if (unlikely(skb->ip_summed != CHECKSUM_PARTIAL)) {
struct net_device *dev = skb->dev;
struct ethtool_drvinfo info = {};
if (dev && dev->ethtool_ops && dev->ethtool_ops->get_drvinfo)
dev->ethtool_ops->get_drvinfo(dev, &info);
WARN(1, "%s: caps=(0x%lx, 0x%lx) len=%d data_len=%d "
"ip_summed=%d",
info.driver, dev ? dev->features : 0L,
skb->sk ? skb->sk->sk_route_caps : 0L,
skb->len, skb->data_len, skb->ip_summed);
if (skb_header_cloned(skb) &&
(err = pskb_expand_head(skb, 0, 0, GFP_ATOMIC)))
return ERR_PTR(err);
如果skb header是clone，分离出来
}
如果skb->ip_summed 不是 CHECKSUM_PARTIAL，那么报个warning，因为GSO类型的skb其ip_summed一般都是CHECKSUM_PARTIAL
rcu_read_lock();
list_for_each_entry_rcu(ptype,
&ptype_base[ntohs(type) & PTYPE_HASH_MASK], list) {
if (ptype->type == type && !ptype->dev && ptype->gso_segment) {
if (unlikely(skb->ip_summed != CHECKSUM_PARTIAL)) {
err = ptype->gso_send_check(skb);
segs = ERR_PTR(err);
if (err || skb_gso_ok(skb, features))
break;
__skb_push(skb, (skb->data -
skb_network_header(skb)));
}
segs = ptype->gso_segment(skb, features);
break;
把skb->data指向network header，然后调用inet_gso_segment，四层的gso_segment会在inet_gso_segment中被调用
}
}
rcu_read_unlock();
__skb_push(skb, skb->data - skb_mac_header(skb));
把skb->data再次指向mac header
return segs;
}
static struct sk_buff *inet_gso_segment(struct sk_buff *skb, int features)
{
struct sk_buff *segs = ERR_PTR(-EINVAL);
struct iphdr *iph;
const struct net_protocol *ops;
int proto;
int ihl;
int id;
unsigned int offset = 0;
if (!(features & NETIF_F_V4_CSUM))
features &= ~NETIF_F_SG;
如果设备不支持NETIF_F_V4_CSUM，那么就当设备不支持SG
if (unlikely(skb_shinfo(skb)->gso_type &
~(SKB_GSO_TCPV4 |
SKB_GSO_UDP |
SKB_GSO_DODGY |
SKB_GSO_TCP_ECN |
0)))
goto out;
gso_type不合法，直接返错
if (unlikely(!pskb_may_pull(skb, sizeof(*iph))))
goto out;
20字节ip头部无法获得，返错
iph = ip_hdr(skb);
ihl = iph->ihl * 4;
if (ihl < sizeof(*iph))
goto out;
if (unlikely(!pskb_may_pull(skb, ihl)))
goto out;
实际ip头部无法获得，返错
__skb_pull(skb, ihl);
skb_reset_transport_header(skb);
iph = ip_hdr(skb);
OK，现在拿到ip头部了
id = ntohs(iph->id);
ip包的id
proto = iph->protocol & (MAX_INET_PROTOS - 1);
segs = ERR_PTR(-EPROTONOSUPPORT);
rcu_read_lock();
ops = rcu_dereference(inet_protos[proto]);
if (likely(ops && ops->gso_segment))
segs = ops->gso_segment(skb, features);
如果是tcp，那么调用tcp_tso_segment，如果是udp，那么调用udp4_ufo_fragment
rcu_read_unlock();
if (!segs || IS_ERR(segs))
goto out;
skb = segs;
do {
iph = ip_hdr(skb);
if (proto == IPPROTO_UDP) {
iph->id = htons(id);
iph->frag_off = htons(offset >> 3);
if (skb->next != NULL)
iph->frag_off |= htons(IP_MF);
offset += (skb->len - skb->mac_len - iph->ihl * 4);
} else
iph->id = htons(id++);
iph->tot_len = htons(skb->len - skb->mac_len);
iph->check = 0;
iph->check = ip_fast_csum(skb_network_header(skb), iph->ihl);
} while ((skb = skb->next));
对每一个skb segment，填充ip包头，计算ip checksum。如果是tcp segmentation，那么ip头的id递增。如果是udp fragmentation，那么ip头的id不变，每次计算增加的offset，等于是在做ip分片
out:
return segs;
}
下面来看TCP协议的分段函数tcp_tso_segment
struct sk_buff *tcp_tso_segment(struct sk_buff *skb, int features)
{
struct sk_buff *segs = ERR_PTR(-EINVAL);
struct tcphdr *th;
unsigned thlen;
unsigned int seq;
__be32 delta;
unsigned int oldlen;
unsigned int mss;
if (!pskb_may_pull(skb, sizeof(*th)))
goto out;
th = tcp_hdr(skb);
thlen = th->doff * 4;
if (thlen < sizeof(*th))
goto out;
if (!pskb_may_pull(skb, thlen))
goto out;
oldlen = (u16)~skb->len;
__skb_pull(skb, thlen);
把tcp header移到skb header里，把skb->len存到oldlen中，此时skb->len就只有tcp payload的长度
mss = skb_shinfo(skb)->gso_size;
if (unlikely(skb->len <= mss))
goto out;
if (skb_gso_ok(skb, features | NETIF_F_GSO_ROBUST)) {
/* Packet is from an untrusted source, reset gso_segs. */
int type = skb_shinfo(skb)->gso_type;
if (unlikely(type &
~(SKB_GSO_TCPV4 |
SKB_GSO_DODGY |
SKB_GSO_TCP_ECN |
SKB_GSO_TCPV6 |
0) ||
!(type & (SKB_GSO_TCPV4 | SKB_GSO_TCPV6))))
goto out;
skb_shinfo(skb)->gso_segs = DIV_ROUND_UP(skb->len, mss);
重新计算skb_shinfo(skb)->gso_segs的个数，基于skb->len和mss值
segs = NULL;
goto out;
}
segs = skb_segment(skb, features);
if (IS_ERR(segs))
goto out;
skb_segment是真正的分段实现，后面再分析
delta = htonl(oldlen + (thlen + mss));
skb = segs;
th = tcp_hdr(skb);
seq = ntohl(th->seq);
do {
th->fin = th->psh = 0;
th->check = ~csum_fold((__force __wsum)((__force u32)th->check +
(__force u32)delta));
if (skb->ip_summed != CHECKSUM_PARTIAL)
th->check =
csum_fold(csum_partial(skb_transport_header(skb),
thlen, skb->csum));
对每个分段都要计算tcp checksum
seq += mss;
skb = skb->next;
th = tcp_hdr(skb);
th->seq = htonl(seq);
对每个分段重新计算sequence值
th->cwr = 0;
} while (skb->next);
delta = htonl(oldlen + (skb->tail - skb->transport_header) +
skb->data_len);
th->check = ~csum_fold((__force __wsum)((__force u32)th->check +
(__force u32)delta));
if (skb->ip_summed != CHECKSUM_PARTIAL)
th->check = csum_fold(csum_partial(skb_transport_header(skb),
thlen, skb->csum));
out:
return segs;
}
UDP协议的分片函数是udp4_ufo_fragment
struct sk_buff *udp4_ufo_fragment(struct sk_buff *skb, int features)
{
struct sk_buff *segs = ERR_PTR(-EINVAL);
unsigned int mss;
int offset;
__wsum csum;
mss = skb_shinfo(skb)->gso_size;
if (unlikely(skb->len <= mss))
goto out;
if (skb_gso_ok(skb, features | NETIF_F_GSO_ROBUST)) {
/* Packet is from an untrusted source, reset gso_segs. */
int type = skb_shinfo(skb)->gso_type;
if (unlikely(type & ~(SKB_GSO_UDP | SKB_GSO_DODGY) ||
!(type & (SKB_GSO_UDP))))
goto out;
skb_shinfo(skb)->gso_segs = DIV_ROUND_UP(skb->len, mss);
segs = NULL;
goto out;
}
/* Do software UFO. Complete and fill in the UDP checksum as HW cannot
* do checksum of UDP packets sent as multiple IP fragments.
*/
offset = skb->csum_start - skb_headroom(skb);
csum = skb_checksum(skb, offset, skb->len - offset, 0);
offset += skb->csum_offset;
*(__sum16 *)(skb->data + offset) = csum_fold(csum);
skb->ip_summed = CHECKSUM_NONE;
计算udp的checksum
/* Fragment the skb. IP headers of the fragments are updated in
* inet_gso_segment()
*/
segs = skb_segment(skb, features);
out:
return segs;
}
udp的分段其实和ip的分片没什么区别，只是多一个计算checksum的步骤
最后来分析下skb_segment
struct sk_buff *skb_segment(struct sk_buff *skb, int features)
{
struct sk_buff *segs = NULL;
struct sk_buff *tail = NULL;
struct sk_buff *fskb = skb_shinfo(skb)->frag_list;
unsigned int mss = skb_shinfo(skb)->gso_size;
unsigned int doffset = skb->data - skb_mac_header(skb);
unsigned int offset = doffset;
unsigned int headroom;
unsigned int len;
int sg = features & NETIF_F_SG;
int nfrags = skb_shinfo(skb)->nr_frags;
int err = -ENOMEM;
int i = 0;
int pos;
__skb_push(skb, doffset);
headroom = skb_headroom(skb);
pos = skb_headlen(skb);
skb->data指向mac header，计算headroom，skb_headlen长度
do {
struct sk_buff *nskb;
skb_frag_t *frag;
int hsize;
int size;
len = skb->len - offset;
if (len > mss)
len = mss;
len为skb->len减去直到offset的部分。开始时，offset只是mac header + ip header + tcp
header的长度，len即tcp payload的长度。随着segment增加,
offset每次都增加mss长度。因此len的定义是每个segment的payload长度（最后一个segment的payload可能小于一个
mss长度）
hsize = skb_headlen(skb) - offset;
hsize为skb header减去offset后的大小，如果hsize小于0，那么说明payload在skb的frags, frag_list中。随着offset一直增长，必定会有hsize一直<0的情况开始出现，除非skb是一个完全linearize化的skb
if (hsize < 0)
hsize = 0;
这种情况说明skb_headlen没有tcp payload的部分，需要pull数据过来
if (hsize > len || !sg)
hsize = len;
如果不支持sg同时hsize大于len，那么hsize就为len，此时说明segment的payload还在skb header中
if (!hsize && i >= nfrags) {
BUG_ON(fskb->len != len);
pos += len;
nskb = skb_clone(fskb, GFP_ATOMIC);
fskb = fskb->next;
if (unlikely(!nskb))
goto err;
hsize = skb_end_pointer(nskb) - nskb->head;
if (skb_cow_head(nskb, doffset + headroom)) {
kfree_skb(nskb);
goto err;
}
nskb->truesize += skb_end_pointer(nskb) - nskb->head -
hsize;
skb_release_head_state(nskb);
__skb_push(nskb, doffset);
} else {
nskb = alloc_skb(hsize + doffset + headroom,
GFP_ATOMIC);
if (unlikely(!nskb))
goto err;
skb_reserve(nskb, headroom);
__skb_put(nskb, doffset);
alloc新的skb，skb->data到skb->head之间保留headroom，skb->tail到skb->data之间保留mac header + ip header + tcp header + hsize的长度
}
if (segs)
tail->next = nskb;
else
segs = nskb;
tail = nskb;
__copy_skb_header(nskb, skb);
nskb->mac_len = skb->mac_len;
把老skb的skb_buff内容拷贝到新skb中
/* nskb and skb might have different headroom */
if (nskb->ip_summed == CHECKSUM_PARTIAL)
nskb->csum_start += skb_headroom(nskb) - headroom;
修正下checksum计算的位置
skb_reset_mac_header(nskb);
skb_set_network_header(nskb, skb->mac_len);
nskb->transport_header = (nskb->network_header +
skb_network_header_len(skb));
skb_copy_from_linear_data(skb, nskb->data, doffset);
把skb->data开始doffset长度的内容拷贝到nskb->data中，即把mac header , ip header, tcp header都复制过去
if (fskb != skb_shinfo(skb)->frag_list)
continue;
if (!sg) {
nskb->ip_summed = CHECKSUM_NONE;
nskb->csum = skb_copy_and_csum_bits(skb, offset,
skb_put(nskb, len),
len, 0);
continue;
}
frag = skb_shinfo(nskb)->frags;
skb_copy_from_linear_data_offset(skb, offset,
skb_put(nskb, hsize), hsize);
如果hsize不为0，那么拷贝hsize的内容到nskb header中
while (pos < offset + len && i < nfrags) {
offset + len长度超过了pos，即超过了nskb header，这时需要用到frag
*frag = skb_shinfo(skb)->frags[i];
get_page(frag->page);
size = frag->size;
if (pos < offset) {
frag->page_offset += offset - pos;
frag->size -= offset - pos;
}
skb_shinfo(nskb)->nr_frags++;
if (pos + size <= offset + len) {
i++;
pos += size;
} else {
frag->size -= pos + size - (offset + len);
goto skip_fraglist;
}
frag++;
}
如果skb header空间不够，那么通过frag，把一个mss的内容拷贝到nskb的frag中
if (pos < offset + len) {
struct sk_buff *fskb2 = fskb;
BUG_ON(pos + fskb->len != offset + len);
pos += fskb->len;
fskb = fskb->next;
if (fskb2->next) {
fskb2 = skb_clone(fskb2, GFP_ATOMIC);
if (!fskb2)
goto err;
} else
skb_get(fskb2);
SKB_FRAG_ASSERT(nskb);
skb_shinfo(nskb)->frag_list = fskb2;
}
如果frag都用完还是无法满足mss的大小，那么就要用到frag_list，这段代码跳过去了，因为基本永远不会走到这里
skip_fraglist:
nskb->data_len = len - hsize;
nskb->len += nskb->data_len;
nskb->truesize += nskb->data_len;
} while ((offset += len) < skb->len);
