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

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Date:	Fri, 17 Jun 2011 11:30:32 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Petr Tesarik <ptesarik@...e.cz>
Cc:	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


* Petr Tesarik <ptesarik@...e.cz> wrote:

> This patch series enhances /dev/mem, so that read and write is 
> possible at any address. The patchset includes actual 
> implementation for x86.

This series lacks a description of why this is desired.

My strong opinion is that it's not desired at all: /dev/mem never 
worked beyond 4G addresses so by today it has become largely obsolete 
and is on the way out really.

I'm aware of these current /dev/mem uses:

 - Xorg maps below 4G non-RAM addresses and the video BIOS

 - It used to have some debugging role but these days kexec and kgdb 
   has largely taken over that role - partly due to the 4G limit.

 - there's some really horrible out-of-tree drivers that do mmap()s
   via /dev/mem, those should be fixed if they want to move beyond 
   4G: their char device should be mmap()able.

 - all distro kernel's i'm aware of use CONFIG_STRICT_DEVMEM=y, which
   restricts /dev/mem to non-RAM pages of physical memory.
   [ With the sad inclusion of the first 1MB, which Xorg needs. ]

Are you aware of any legitimate usecases?

Frankly, i dont think we ever *want* to 'fix' /dev/mem to support 
addresses beyond 4G and grow messy userspace (and kernelspace) that 
somehow relies on that. Thank goodness that we never supported it ...

Thanks,

	Ingo
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