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: <20090807095426.GI8725@shareable.org>
Date:	Fri, 7 Aug 2009 10:54:27 +0100
From:	Jamie Lokier <jamie@...reable.org>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Laurent Pinchart <laurent.pinchart@...asonboard.com>,
	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 ?")

Russell King - ARM Linux wrote:
> 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.

1. Does the architecture not prevent speculative instruction
prefetches from crossing a page boundary?  It would be handy under the
circumstances.

2. Is NX available on all the CPUs with speculative prefetching
behaviour?  If it is, just use that for device mappings?

-- Jamie
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