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Date: Mon, 08 Sep 2014 19:27:14 -0700
From: "H. Peter Anvin" <hpa@zytor.com>
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To: James Bottomley <James.Bottomley@HansenPartnership.com>,
        Chris Metcalf <cmetcalf@tilera.com>
CC: Peter Hurley <peter@hurleysoftware.com>, paulmck@linux.vnet.ibm.com,
        One Thousand Gnomes <gnomes@lxorguk.ukuu.org.uk>,
        Jakub Jelinek <jakub@redhat.com>,
        Mikael Pettersson <mikpelinux@gmail.com>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Richard Henderson <rth@twiddle.net>, Oleg Nesterov <oleg@redhat.com>,
        Miroslav Franc <mfranc@redhat.com>, Paul Mackerras <paulus@samba.org>,
        linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org,
        linux-arch@vger.kernel.org, Tony Luck <tony.luck@intel.com>,
        linux-ia64@vger.kernel.org
Subject: Re: bit fields && data tearing
References: <21512.10628.412205.873477@gargle.gargle.HOWL>			 <20140904090952.GW17454@tucnak.redhat.com>			 <540859EC.5000407@hurleysoftware.com>			 <20140904175044.4697aee4@alan.etchedpixels.co.uk>			 <5408C0AB.6050801@hurleysoftware.com>			 <20140905001751.GL5001@linux.vnet.ibm.com>			 <1409883098.5078.14.camel@jarvis.lan>		 <5409243C.4080704@hurleysoftware.com>			 <20140905040645.GO5001@linux.vnet.ibm.com>			 <1410066442.12512.13.camel@jarvis.lan>			 <20140907162146.GK5001@linux.vnet.ibm.com>		 <1410116687.2027.19.camel@jarvis.lan>	 <540CC305.8010407@hurleysoftware.com> <1410155407.2027.29.camel@jarvis.lan>	 <540E1587.3060108@tilera.com> <1410216228.2027.88.camel@jarvis.lan>
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On 09/08/2014 03:43 PM, James Bottomley wrote:
> 
> This was years ago (possibly decades).  We had to implement in-kernel
> unaligned traps for the networking layer because it could access short
> and int fields that weren't of the correct alignment when processing
> packets.  It that's all corrected now, we wouldn't really notice (except
> a bit of a speed up since an unaligned trap effectively places the
> broken out instructions into the bit stream).
> 
> James
> 

Well, ARM doesn't trap, it just silently gives garbage on unaligned
memory references.

	-hpa