Prev: [PROPOSAL] NMI & register handling infrastructure
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From: Eric Dumazet on 11 May 2010 01:30
Le lundi 10 mai 2010 à 13:09 -0700, Sharat Masetty a écrit :
> Hello All,

Please dont use too long lines

>
> For my project I need 3 words of headroom in the skb in the network

> driver level, to add a custom header to the ethernet packet. I

> looked into the tcp code and figured out tcp uses sk->sk_prot->max_header

> for header allocation size. But I was not able to confirm that all other

> transport protocol use the same mechanism(?) For example in UDP/ICMP I was

> not able to figure out from the code where the allocation and header

> reservation happens(Any light here would be really helpful.)
>
> I have also looked at an API in skbuff skb_pad() which does what I want

> (add either headroom or tailroom), but I want to avoid that for performance

> reasons(skb_pad does kmalloc and memcpy). I want to figure out a good way

> (may be tune some parameters) to allocate extra 3 words for any skbuff

> independant of the transport protocol being used.

> Any light here would be very much appreciated.

LL_RESERVED_SPACE() is the magic you need.

#define LL_RESERVED_SPACE(dev) \
((((dev)->hard_header_len+(dev)->needed_headroom)&~(HH_DATA_MOD - 1)) + HH_DATA_MOD)

sendmsg() -> ip_append_data()
....
hh_len = LL_RESERVED_SPACE(rt->u.dst.dev);
fragheaderlen = sizeof(struct iphdr) + (opt ? opt->optlen : 0);
....

if (transhdrlen) {
skb = sock_alloc_send_skb(sk,
alloclen + hh_len + 15,
(flags & MSG_DONTWAIT), &err);
} else {
skb = NULL;
if (atomic_read(&sk->sk_wmem_alloc) <=
2 * sk->sk_sndbuf)
skb = sock_wmalloc(sk,
alloclen + hh_len + 15, 1,
sk->sk_allocation);


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