linux-kernel - Re: [PATCH 00/10] Enhance /dev/mem to allow read/write of arbitrary physical addresses

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Date:	Fri, 1 Jul 2011 09:47:05 -0400
From:	Christoph Hellwig <hch@...radead.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Petr Tesarik <ptesarik@...e.cz>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Fenghua Yu <fenghua.yu@...el.com>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
	Paul Mundt <lethal@...ux-sh.org>,
	Russell King <linux@....linux.org.uk>,
	Thomas Gleixner <tglx@...utronix.de>,
	Tony Luck <tony.luck@...el.com>, x86@...nel.org,
	linux-arm-kernel@...ts.infradead.org, linux-ia64@...r.kernel.org,
	linux-sh@...r.kernel.org, linux-kernel@...r.kernel.org,
	Arjan van de Ven <arjan@...radead.org>,
	Dave Jones <davej@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 00/10] Enhance /dev/mem to allow read/write of arbitrary
 physical addresses

On Fri, Jul 01, 2011 at 02:58:02PM +0200, Ingo Molnar wrote:
> So what was not mentioned in your series, what is *your* motivation 
> and your usecase? Enabling closed-source userspace drivers? Enabling 
> the crash utility?

He stated it pretty clearly in the thread, it's the crash utility.

> 
> If the former then shame on you, if the latter then how do you 
> explain that distros appear to disable the RAM aspect of /dev/mem:
> 
>  $ grep DEVMEM $(rpm -ql kernel-2.6.38-0.rc7.git2.3.fc16.x86_64 | grep config-2.6 )
>  CONFIG_STRICT_DEVMEM=y
> 
> So the crash utility use-case does not work on unpatched, default 
> kernels, right?

Not if you have highmem.  That's why Redhat or Fedora to quote your
example patch in the /dev/crash driver, which totally defeats the
CONFIG_STRICT_DEVMEM setting.  But apparently it's good enough that no
one either noticed or at least doesn't care.

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