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 09:24:33 -0400 (EDT)
From:	Dave Anderson <anderson@...hat.com>
To:	amwang@...hat.com
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 00/10] Enhance /dev/mem to allow read/write of arbitrary
 physical addresses

> On Mon, Jun 20, 2011 at 10:42:47, Amerigo Wang <amwang@...hat.com> wrote:
>> On Fri, Jun 17, 2011 at 5:55 PM, Petr Tesarik <ptesarik@...e.cz> wrote:
>> Dne PÃ¡ 17. Äervna 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.

I don't know what you mean by I "didn't consider it", because 
the crash utility does support using /proc/kcore as an alternative
live memory source.

The problem is that /proc/kcore can only access highmem if it
is mapped as kernel virtual address.  So it cannot read 32-bit 
highmem PTE's, user-space memory, etc.

Dave

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