netdev - Re: [PATCH 0/3] A few minor clean-ups to eth_type

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Message-ID: <20150430223510.GA13111@Alexeis-MBP.westell.com>
Date:	Thu, 30 Apr 2015 15:35:13 -0700
From:	Alexei Starovoitov <alexei.starovoitov@...il.com>
To:	Alexander Duyck <alexander.h.duyck@...hat.com>
Cc:	netdev@...r.kernel.org, davem@...emloft.net
Subject: Re: [PATCH 0/3] A few minor clean-ups to eth_type_trans

On Thu, Apr 30, 2015 at 02:53:42PM -0700, Alexander Duyck wrote:
> This series addresses a few minor issues I found in eth_type_trans that
> that allow us to gain back something like 3 or more cycles per packet.  
> 
> The first change is to drop the byte swap since it isn't necessary.  On x86
> we could just check the first byte and compare that against the upper 8
> bits of the Ethertype to determine if we are dealing with a size value or
> not.
> 
> The second makes it so that the value we read in to test for multicast can
> be used for the address comparison.  This allows us to avoid a second read
> of the destination address.
> 
> The final change is to avoid some unneeded instructions in computing the
> Ethernet header pointer.  When we start the call the Ethernet header is at
> skb->data, so we just use that rather than computing mac_header, and then
> adding that back to skb->head.

Great stuff! Excellent optimizations.

Only the comment 'ETH_P_802_3_MIN is aligned to 512' through me off.
It's divisible by 256 that matters.

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