netdev - Re: Endianness problem with u32 classifier hash masks

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Date:	Tue, 06 Nov 2007 08:34:31 -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 Tue, 2007-06-11 at 10:09 +0200, Radu Rendec wrote:

> Yup, you're right. Bitwise anding is the same regardless of the byte
> ordering of the operands. As long as you don't have one operand in host
> order and the other in net order, it's ok.

Ok

> However, Jarek's computations with his mask and your patch seemed
> correct to me yesterday. And I think I know the answer: data must be
> changed to host order _before_ shifting. I mean something like this:
> 
> static __inline__ unsigned u32_hash_fold(u32 key, struct tc_u32_sel
> *sel, u8 fshift)
> {
>        unsigned h = ntohl(key & sel->hmask) >> fshift;
>        return h;
> }

Even better than what i suggested ;->

> > > On paper i get the same result with the new or old scheme for the bucket selection.
> > > As i stated on the patch - i never did test the theory.
> 
> Well, neither did I (about what I stated above). But still I think,
> Jarek was right yesterday and I can't figure out how it worked for you
> on paper. How about this new version?

Looks good - we can think of optimizing later.

> Well, I think it's pretty clear now: I'll try my version of Jamal's
> patch :) 

Which derived from your original patch using little effort in comparison
to yours. All the hardwork is yours.
You did quiet an impressive debug work. Please give yourself a little
pat on the back for me.

> But not right now, because I also have to show up at work.

I empathize. 
Please send two patches instead of one. One for this and the next for
the ffs conversion (please run some simple tests in both cases).

Jarek,
Heres a few more derivations of Canada for you:

Legend has it that Canada's name is derived from  
"settlement" in Iroquoian (One the First Nations in present day Canada).
I think it was pronounced "Kanata"
An alternative legend says the early Spanish called it acánada  meaning
"nothing here"

I tend to believe the Iroquoian version since to this day Canada
continues to serve as a new settlement for many people. And the Spanish
were totaly wrong - there is something here ;-> At least Tim Hortons
coffee.

cheers,
jamal

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