From mboxrd@z Thu Jan 1 00:00:00 1970 Return-path: Received: from metis.ext.pengutronix.de ([2001:6f8:1178:4:290:27ff:fe1d:cc33]) by bombadil.infradead.org with esmtps (Exim 4.76 #1 (Red Hat Linux)) id 1SMCcx-0003U5-L0 for barebox@lists.infradead.org; Mon, 23 Apr 2012 06:20:02 +0000 Date: Mon, 23 Apr 2012 08:19:53 +0200 From: Sascha Hauer Message-ID: <20120423061953.GV3852@pengutronix.de> References: MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: barebox-bounces@lists.infradead.org Errors-To: barebox-bounces+u.kleine-koenig=pengutronix.de@lists.infradead.org Subject: Re: ARM page 0 set for page fault To: Krzysztof Halasa Cc: barebox@lists.infradead.org Hi Krzysztof, On Sun, Apr 22, 2012 at 08:38:50PM +0200, Krzysztof Halasa wrote: > Hi, > > I'm trying to port my old IXP4xx (Intel XScale ARM big-endian mostly) > patches to the current Barebox. Noticed the page #0 (virtual = physical > addresses 0 - 0x1FFF, or the first 4 KiB of RAM) is not present: > > commit abcf935e > Author: Sascha Hauer > Date: Sun Aug 7 19:00:56 2011 +0200 > > ARM mmu: use high vectors if possible > > Using high vectors allows us to map a faulting zero page to > catch NULL pointer dereferences. > > Unfortunately this causes "md" (memory dump) etc. to fail (also > executing Linux kernel fails since the tags are located at 0x100 - > should I relocate them?). > > Should I do it differently, e.g. access RAM through an alias (IXP4xx has > RAM aliased all over the place, one could also use MMU tables for this) > so that the page fault only occurs on internal, not user-visible > accesses (= users should not be able to access the first virtual page, > even it they think the do)? I would prefer the following order of precedence: - Locate ATAGS to somewhere else. The kernel takes a pointer to ATAGs anyway, so this should work. - Map SDRAM at 0x0 to some other place using the MMU. When the kernel then starts with MMU off the ATAGs are where the Kernel expects them to be. - At an #ifdef around the code in mmu.c to make the zero page non faulting. Having the zero page faulting is a very useful feature though, so I would only go this way if nothing else helps. Sascha -- Pengutronix e.K. | | Industrial Linux Solutions | http://www.pengutronix.de/ | Peiner Str. 6-8, 31137 Hildesheim, Germany | Phone: +49-5121-206917-0 | Amtsgericht Hildesheim, HRA 2686 | Fax: +49-5121-206917-5555 | _______________________________________________ barebox mailing list barebox@lists.infradead.org http://lists.infradead.org/mailman/listinfo/barebox