linux-kernel - Re: bit fields && data tearing

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Message-ID: <540E6582.9020403@zytor.com>
Date:	Mon, 08 Sep 2014 19:27:14 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	James Bottomley <James.Bottomley@...senPartnership.com>,
	Chris Metcalf <cmetcalf@...era.com>
CC:	Peter Hurley <peter@...leysoftware.com>,
	paulmck@...ux.vnet.ibm.com,
	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	Jakub Jelinek <jakub@...hat.com>,
	Mikael Pettersson <mikpelinux@...il.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Richard Henderson <rth@...ddle.net>,
	Oleg Nesterov <oleg@...hat.com>,
	Miroslav Franc <mfranc@...hat.com>,
	Paul Mackerras <paulus@...ba.org>,
	linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org, Tony Luck <tony.luck@...el.com>,
	linux-ia64@...r.kernel.org
Subject: Re: bit fields && data tearing

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


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