netdev - Re: [PATCH net-next v4 1/6] net: ethtool: allow symmetric-xor RSS hash for any flow type

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Date: Tue, 17 Oct 2023 11:37:52 -0700
From: Alexander Duyck <alexander.duyck@...il.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: Ahmed Zaki <ahmed.zaki@...el.com>, netdev@...r.kernel.org, 
	intel-wired-lan@...ts.osuosl.org, corbet@....net, jesse.brandeburg@...el.com, 
	anthony.l.nguyen@...el.com, davem@...emloft.net, edumazet@...gle.com, 
	pabeni@...hat.com, vladimir.oltean@....com, andrew@...n.ch, horms@...nel.org, 
	mkubecek@...e.cz, willemdebruijn.kernel@...il.com, linux-doc@...r.kernel.org, 
	Wojciech Drewek <wojciech.drewek@...el.com>
Subject: Re: [PATCH net-next v4 1/6] net: ethtool: allow symmetric-xor RSS
 hash for any flow type

On Mon, Oct 16, 2023 at 4:31 PM Jakub Kicinski <kuba@...nel.org> wrote:
>
> On Mon, 16 Oct 2023 15:55:21 -0700 Alexander Duyck wrote:
> > It would make more sense to just add it as a variant hash function of
> > toeplitz. If you did it right you could probably make the formatting
> > pretty, something like:
> > RSS hash function:
> >     toeplitz: on
> >         symmetric xor: on
> >     xor: off
> >     crc32: off
> >
> > It doesn't make sense to place it in the input flags and will just
> > cause quick congestion as things get added there. This is an algorithm
> > change so it makes more sense to place it there.
>
> Algo is also a bit confusing, it's more like key pre-processing?
> There's nothing toeplitz about xoring input fields. Works as well
> for CRC32.. or XOR.

I agree that the change to the algorithm doesn't necessarily have
anything to do with toeplitz, however it is still a change to the
algorithm by performing the extra XOR on the inputs prior to
processing. That is why I figured it might make sense to just add a
new hfunc value that would mean toeplitz w/ symmetric XOR.

> We can use one of the reserved fields of struct ethtool_rxfh to carry
> this extension. I think I asked for this at some point, but there's
> only so much repeated feedback one can send in a day :(

Why add an extra reserved field when this is just a variant on a hash
function? I view it as not being dissimilar to how we handle TSO or
tx-checksumming. It would make sense to me to just set something like
toeplitz-symmetric-xor to on in order to turn this on.