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Date: Fri, 12 Apr 2013 15:17:39 -0700
From: Dave Hansen <dave@sr71.net>
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Subject: Re: [PATCH 5/5] kexec: X86: Pass memory ranges via e820 table instead
 of memmap= boot parameter
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To: "H. Peter Anvin" <hpa@zytor.com>
Cc: "kexec@lists.infradead.org" <kexec@lists.infradead.org>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, Cliff Wickman <cpw@sgi.com>, Simon Horman <horms@verge.net.au>, "Eric W. Biederman" <ebiederm@xmission.com>, Yinghai Lu <yinghai@kernel.org>, Thomas Renninger <trenn@suse.de>, Vivek Goyal <vgoyal@redhat.com>

On 04/12/2013 07:56 AM, H. Peter Anvin wrote:
> On 04/12/2013 07:31 AM, Vivek Goyal wrote:
>>> I also have to admit that I don't see the difference between /dev/mem
>>> and /dev/oldmem, as the former allows access to memory ranges outside
>>> the ones used by the current kernel, which is what the oldmem device
>>> seems to be intended to od.

It varies from arch to arch of course.

But, /dev/mem has restrictions on it, like CONFIG_STRICT_DEVMEM or the
ARCH_HAS_VALID_PHYS_ADDR_RANGE.  There's a lot of stuff that depends on
it, *and* folks have tried to fix it up so that it's not _as_ blatant of
a way to completely screw your system.

/dev/mem also tries to be nice to arches that have restrictions like:

>                         /*
>                          * On ia64 if a page has been mapped somewhere as
>                          * uncached, then it must also be accessed uncached
>                          * by the kernel or data corruption may occur
>                          */

I think /dev/oldmem isn't so nice and could actually cause some real
problems if used on ia64 where the cached/uncached accesses are mixed.

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