netdev - Re: [PATCH 1/3] etherdev: Avoid unnecessary byte swap in check for Ethertype

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Date:	Thu, 30 Apr 2015 16:03:49 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Alexander Duyck <alexander.h.duyck@...hat.com>
Cc:	netdev@...r.kernel.org, davem@...emloft.net
Subject: Re: [PATCH 1/3] etherdev: Avoid unnecessary byte swap in check for
 Ethertype

On Thu, 2015-04-30 at 14:53 -0700, Alexander Duyck wrote:
> This change takes advantage of the fact that ETH_P_802_3_MIN is aligned to
> 512 so as a result we can actually ignore the lower 8b when comparing the
> Ethertype to ETH_P_802_3_MIN.  This allows us to avoid a byte swap by simply
> masking the value and comparing it to the byte swapped value for
> ETH_P_802_3_MIN.
> 
> Signed-off-by: Alexander Duyck <alexander.h.duyck@...hat.com>
> ---
>  net/ethernet/eth.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/ethernet/eth.c b/net/ethernet/eth.c
> index f3bad41d725f..60069318d5d1 100644
> --- a/net/ethernet/eth.c
> +++ b/net/ethernet/eth.c
> @@ -178,7 +178,7 @@ __be16 eth_type_trans(struct sk_buff *skb, struct net_device *dev)
>  	if (unlikely(netdev_uses_dsa(dev)))
>  		return htons(ETH_P_XDSA);
>  
> -	if (likely(ntohs(eth->h_proto) >= ETH_P_802_3_MIN))
> +	if (likely((eth->h_proto & htons(0xFF00)) >= htons(ETH_P_802_3_MIN)))
>  		return eth->h_proto;

Then, a byte operation on x86 is shorter/faster than u16 one.

You also could use

if (likely(*(u8 *)&eth->h_proto >= (ETH_P_802_3_MIN>>8)))
	return eth->h_proto;

I would at least leave a comment here to explain the logic.



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