linux-kernel - Re: How to efficiently handle DMA and cache on ARMv7 ? (was "Is get_user_pages() enough to prevent pages from being swapped out ?")

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Message-ID: <20090807081041.GB18343@n2100.arm.linux.org.uk>
Date:	Fri, 7 Aug 2009 09:10:41 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Laurent Pinchart <laurent.pinchart@...asonboard.com>
Cc:	David Xiao <dxiao@...adcom.com>, Ben Dooks <ben-linux@...ff.org>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Robin Holt <holt@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	v4l2_linux <linux-media@...r.kernel.org>,
	"linux-arm-kernel@...ts.arm.linux.org.uk" 
	<linux-arm-kernel@...ts.arm.linux.org.uk>
Subject: Re: How to efficiently handle DMA and cache on ARMv7 ? (was "Is
	get_user_pages() enough to prevent pages from being swapped out ?")

On Fri, Aug 07, 2009 at 09:58:30AM +0200, Laurent Pinchart wrote:
> Sorry about this, but I'm not sure to understand the speculative prefetching 
> cache issue completely.

The general case with speculative prefetching is that if memory is
accessible, it can be prefetched.

In other words, if we mapped devices without NX (non-exec) set, the
CPU can prefetch instructions from devices, causing random read
accesses.  Yes, I know it sounds crazy, but that's what I'm told
_can_ happen.
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