Am?rico Wang p??e v Po 20. 06. 2011 v 10:42 +0800:

On Fri, Jun 17, 2011 at 5:55 PM, Petr Tesarik [off-list ref] wrote:

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Dne P? 17. ?ervna 2011 11:30:32 Ingo Molnar napsal(a):

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* Petr Tesarik [off-list ref] wrote:

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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,

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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.

(Sorry for the dealy, I was on vacation.)

/proc/kcore is in fact supported by the crash utility and it does work
with highmem to some extent. Unfortunately, it cannot handle all of it,
because it is ELF32 format, so there's no way to describe regions beyond
4GB.

Petr Tesarik