From mboxrd@z Thu Jan 1 00:00:00 1970 From: Ingo Molnar Date: Fri, 17 Jun 2011 09:30:32 +0000 Subject: Re: [PATCH 00/10] Enhance /dev/mem to allow read/write of arbitrary Message-Id: <20110617093032.GA19235@elte.hu> List-Id: References: <201106171038.25988.ptesarik@suse.cz> In-Reply-To: <201106171038.25988.ptesarik@suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: linux-arm-kernel@lists.infradead.org * 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. 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. - there's some really horrible out-of-tree drivers that do mmap()s via /dev/mem, those should be fixed if they want to move beyond 4G: their char device should be mmap()able. - all distro kernel's i'm aware of use CONFIG_STRICT_DEVMEM=y, which restricts /dev/mem to non-RAM pages of physical memory. [ With the sad inclusion of the first 1MB, which Xorg needs. ] Are you aware of any legitimate usecases? Frankly, i dont think we ever *want* to 'fix' /dev/mem to support addresses beyond 4G and grow messy userspace (and kernelspace) that somehow relies on that. Thank goodness that we never supported it ... Thanks, Ingo