From mboxrd@z Thu Jan 1 00:00:00 1970 From: Eric Dumazet Subject: Re: [PATCH 1/3] etherdev: Avoid unnecessary byte swap in check for Ethertype Date: Thu, 30 Apr 2015 16:03:49 -0700 Message-ID: <1430435029.3711.106.camel@edumazet-glaptop2.roam.corp.google.com> References: <20150430214917.1798.49769.stgit@ahduyck-vm-fedora22> <20150430215348.1798.15509.stgit@ahduyck-vm-fedora22> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit Cc: netdev@vger.kernel.org, davem@davemloft.net To: Alexander Duyck Return-path: Received: from mail-ie0-f169.google.com ([209.85.223.169]:35565 "EHLO mail-ie0-f169.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750831AbbD3XDv (ORCPT ); Thu, 30 Apr 2015 19:03:51 -0400 Received: by iejt8 with SMTP id t8so79509250iej.2 for ; Thu, 30 Apr 2015 16:03:50 -0700 (PDT) In-Reply-To: <20150430215348.1798.15509.stgit@ahduyck-vm-fedora22> Sender: netdev-owner@vger.kernel.org List-ID: 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 > --- > 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 *)ð->h_proto >= (ETH_P_802_3_MIN>>8))) return eth->h_proto; I would at least leave a comment here to explain the logic.