netdev - Re: Endianness problem with u32 classifier hash masks

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Date:	Mon, 05 Nov 2007 08:43:32 -0500
From:	jamal <hadi@...erus.ca>
To:	Radu Rendec <radu.rendec@...s.ro>
Cc:	Jarek Poplawski <jarkao2@...pl>, netdev@...r.kernel.org
Subject: Re: Endianness problem with u32 classifier hash masks

On Mon, 2007-05-11 at 14:59 +0200, Radu Rendec wrote:
> Jarek, thanks for replying my message on the list and pointing it to the
> right direction.
> 
> Your example with "1" bits laying on exact nibble boundary is much
> easier to analyze than my original example. And your computation seems
> to be right: u32_hash_fold() would return 00.f0.00.0f (and would be cut
> off to 0f after applying the divisor mask).

Yes, that example i believe would work just fine today as is with no
changes.

[...]

> Jamal, I am aware that any computation on the fast path involves some
> performance loss. However, I don't see any speed gain with your patch,
> because you just moved the ntohl() call inside u32_hash_fold(). Since
> u32_hash_fold() is called unconditionally and the only call is that one
> in u32_classify(), htohl() will be called exactly the same number of
> times.
> 

Sorry, I didnt mean that to be a better approach - it suffers the same
problem as what you posted and infact is just your patch simplified
essentially (or my attempt at doing that).
A simpler version would be to do something in u32_change() only and
nowhere else. Thats what i alluded at using fshift. And even after
taking a shower i cant think of a simple way to achieve it.

> After almost a week of dealing with this, I still don't think it can be
> solved without byte re-ordering. If you guys think my patch is good, I
> would be more than glad to send it properly (change the comments as
> Jarek suggested and use git). Since I'm quite a newbie with git and
> haven't worked with kernel maintainers before, please be patient if it's
> not perfect at the first attempt :) What tree/branch should I make the
> patch against?
> 

Please try the patch i sent since it is simpler. It is your work more or
less - so you should get the credit as author. 
Use Davem's net-2.6 as basis. If it works send it to him and CC me and i
will just wsignoff/ack.
If it doesnt work, can you cutdown on the conversion in u32_change()
instead and send that?

cheers,
jamal

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