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From: Dominique Martinet <>
To: Dave Watson <>
Cc: Doron Roberts-Kedes <>,
	Tom Herbert <>,
	"David S. Miller" <>,,
Subject: Re: [PATCH] strparser: remove any offset before parsing messages
Date: Thu, 23 Aug 2018 03:04:42 +0200	[thread overview]
Message-ID: <20180823010442.GA6244@nautica> (raw)
In-Reply-To: <>

Dave Watson wrote on Wed, Aug 22, 2018:
> > I've tried measuring that overhead as well by writing a more complex bpf
> > program that would fetch the offset in the skb but for some reason I'm
> > reading a 0 offset when it's not zero... well, not like there's much
> > choice for this at this point anyway; I don't think we'll do this
> > without pull, I'll put that on background.
> For what it is worth we checked the offset in bpf, something
> along the lines of

Oh, thanks!

> 	  > struct kcm_rx_msg {   int full_len;  int offset;};
> static inline struct kcm_rx_msg *kcm_rx_msg(struct __sk_buff *skb)
>       { return (struct kcm_rx_msg *)skb->cb;}
> int decode_framing(struct __sk_buff *skb)
> { return load_word(skb, kcm_rx_msg(skb)->offset);}

So you're taking directly the address at skb->cb but the linux code has
this function:
static inline struct strp_msg *strp_msg(struct sk_buff *skb)
        return (struct strp_msg *)((void *)skb->cb +
                offsetof(struct qdisc_skb_cb, data));
and is another 8 bytes in, that would explain I had
different results (and now I'm trying your snippet it does work), but
I'll have to admit I fail to understand this....

Ok, so 'cb' in __sk_buff is 48 bytes in but 'cb' in sk_buff is 40 bytes
in -- I might just start getting annoyed over this, is there a reason
for the different offset?!

> Although it did puzzle me for a while figuring that out when I ran in
> to it.

Well, at least it means some people were aware of the problem and worked
around it in their own way -- what do you think of pulling instead?
I mean, we could just document that "really well" and provide the
get-offset function in some header that would be made include-able from
bpf.. But right now this isn't really the case.

FWIW now I have this version I also don't notice any performance change
with the pull on my example, it actually looks like the bpf load_word is
slightly slower than pull to access data that is not in the head, but
the noise level is pretty bad.


      reply	other threads:[~2018-08-23  1:05 UTC|newest]

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-08-09 22:40 [PATCH v0] strparser: remove any offset before parsing messages Dominique Martinet
2018-08-21 12:51 ` [PATCH] " Dominique Martinet
2018-08-21 14:53   ` Doron Roberts-Kedes
2018-08-21 19:36     ` Dominique Martinet
2018-08-21 21:15       ` Doron Roberts-Kedes
2018-08-21 22:51         ` Dominique Martinet
2018-08-21 23:35           ` Doron Roberts-Kedes
2018-08-22  0:46             ` Dominique Martinet
2018-08-22  2:33               ` Doron Roberts-Kedes
2018-08-22  5:47                 ` Dominique Martinet
2018-08-22 18:38                   ` Dave Watson
2018-08-23  1:04                     ` Dominique Martinet [this message]

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