netdev - Re: [PATCH 1/2] net: Add function to get SW rxhash

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Message-ID: <1380042180.3165.102.camel@edumazet-glaptop>
Date:	Tue, 24 Sep 2013 10:03:00 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Tom Herbert <therbert@...gle.com>
Cc:	davem@...emloft.net, netdev@...r.kernel.org
Subject: Re: [PATCH 1/2] net: Add function to get SW rxhash

On Tue, 2013-09-24 at 09:20 -0700, Tom Herbert wrote:
> Some uses of skb_get_rxhash expect that the function will return
> a consistent value whether it is called on RX or TX paths. On RX
> path, we will use the rxhash if provided by the NIC, so this
> would not normally be the same result computed in TX path which is
> a software calculation.
> 
> This patch adds skb_get_sw_rxhash to explicitly request a hash
> calculated by the stack, disregarding the hash provided by NIC.
> 
> Signed-off-by: Tom Herbert <therbert@...gle.com>
> ---

1) Adding another skb field for this corner case, is it worth it ?
   Computing rxhash in software should be real fast for the
   cases where we really care of symmetry.

  ie ignore skb->rxhash and use __skb_get_rxhash()

2) Not copying sw_rxhash in __copy_skb_header()

   -> value will be undefined after a clone()




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