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

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Date:	Mon, 20 Jun 2011 10:42:47 +0800
From:	Américo Wang <xiyou.wangcong@...il.com>
To:	Petr Tesarik <ptesarik@...e.cz>
Cc:	Ingo Molnar <mingo@...e.hu>,
	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, Jun 17, 2011 at 5:55 PM, Petr Tesarik <ptesarik@...e.cz> wrote:
> Dne Pá 17. června 2011 11:30:32 Ingo Molnar napsal(a):
>> * 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.
>
> Hi Ingo,
>
>> 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.
>
> It is still used as a "memory source" by Dave Anderson's crash utility for
> live examination of a running system. Redhat has "overcome" the /dev/mem
> deficiencies by writing an out-of-tree re-implementation of /dev/mem, which
> uses /dev/crash instead. As it is an "unnecessary duplication of an existing
> driver", this method was rejected by the project manager here at SUSE.
>
> The suggested alternative was to enhance (or fix) the existing driver. Without
> this patch series there is no way to access high memory. In conjunction with
> CONFIG_HIGHPTE, it makes the crash utility near to useless on anything with
> high memory, because crash can no longer translate virtual to physical
> addresses.
>

How about /proc/kcore? AFAIK, it can access highmem, but Dave didn't consider
it for some reason.
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