linux-kernel - Re: [PATCH] Introduce compat_u64 and compat

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Message-ID: <20070616135655.GL8154@parisc-linux.org>
Date:	Sat, 16 Jun 2007 07:56:55 -0600
From:	Matthew Wilcox <matthew@....cx>
To:	David Woodhouse <dwmw2@...radead.org>
Cc:	Arnd Bergmann <arnd@...db.de>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Dave Airlie <airlied@...ux.ie>, linux-arch@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] Introduce compat_u64 and compat_s64 types

On Sat, Jun 16, 2007 at 12:34:11PM +0100, David Woodhouse wrote:
> You're right. My question was probably not relevant -- all these 64-bit
> architectures cope with misaligned loads anyway. If we ever have to deal
> with 32-bit compat on a 64-bit architecture which can't handle
> misalignment, I'm just going to hide under my desk and never come out.

... 32-bit compat on a 64-bit architecture where the 32-bit architecture
aligned 64-bit quantities to 32-bit boundaries ...

> > On x86_64, misaligned loads are always ok, so gcc never needs to
> > care about this, even attribute((packed)) does not cause byte access
> > here.
> 
> IA64 too, but it'll be handled there too -- either naturally or by
> fixups; it doesn't matter.

Yes.  iirc, McKinley and later handle misaligned loads within a cacheline
without interrupting.  Merced would interrupt on every misaligned load.
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