Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754051Ab1FTNYf (ORCPT ); Mon, 20 Jun 2011 09:24:35 -0400 Received: from mx4-phx2.redhat.com ([209.132.183.25]:60767 "EHLO mx4-phx2.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752891Ab1FTNYd convert rfc822-to-8bit (ORCPT ); Mon, 20 Jun 2011 09:24:33 -0400 Date: Mon, 20 Jun 2011 09:24:33 -0400 (EDT) From: Dave Anderson To: amwang@redhat.com Cc: Linux Kernel Mailing List Message-ID: <818132268.99935.1308576273046.JavaMail.root@zmail05.collab.prod.int.phx2.redhat.com> Subject: Re: [PATCH 00/10] Enhance /dev/mem to allow read/write of arbitrary physical addresses MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8BIT X-Originating-IP: [10.5.5.72] X-Mailer: Zimbra 6.0.9_GA_2686 (ZimbraWebClient - FF3.0 (Linux)/6.0.9_GA_2686) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2341 Lines: 54 > On Mon, Jun 20, 2011 at 10:42:47, Amerigo Wang wrote: >> On Fri, Jun 17, 2011 at 5:55 PM, Petr Tesarik wrote: >> Dne Pá 17. června 2011 11:30:32 Ingo Molnar napsal(a): >>> * Petr Tesarik 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/