From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:60524) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1ZEglr-0002vN-By for qemu-devel@nongnu.org; Mon, 13 Jul 2015 12:40:00 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1ZEglm-0001bP-94 for qemu-devel@nongnu.org; Mon, 13 Jul 2015 12:39:59 -0400 Received: from cantor2.suse.de ([195.135.220.15]:49605 helo=mx2.suse.de) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1ZEglm-0001aW-0J for qemu-devel@nongnu.org; Mon, 13 Jul 2015 12:39:54 -0400 Message-ID: <55A3E9D8.3050202@suse.de> Date: Mon, 13 Jul 2015 18:39:52 +0200 From: Alexander Graf MIME-Version: 1.0 References: <1436802997-170244-1-git-send-email-agraf@suse.de> <55A3E559.7060704@suse.de> In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH 2.4] hw/arm/boot: Increase fdt alignment List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Peter Maydell Cc: QEMU Developers On 07/13/15 18:27, Peter Maydell wrote: > On 13 July 2015 at 17:20, Alexander Graf wrote: >> On 07/13/15 18:13, Peter Maydell wrote: >>> On 13 July 2015 at 16:56, Alexander Graf wrote: >>>> The Linux kernel on aarch64 creates a page table entry at early bootup >>>> that spans the 2MB range on memory spanning the fdt start address: >>>> >>>> [ ALIGN_DOWN(fdt, 2MB) ... ALIGN_DOWN(fdt, 2MB) + 2MB ] >>>> >>>> This means that when our current 4k alignment happens to fall at the end >>>> of the aligned region, Linux tries to access memory that is not mapped. >>>> >>>> The easy fix is to instead increase the alignment to 2MB, making Linux's >>>> logic always succeed. >>> This is a kernel bug and should be fixed there. The booting >>> protocol document is pretty clear: >>> https://www.kernel.org/doc/Documentation/arm64/booting.txt >>> >>> "The device tree blob (dtb) must be placed on an 8-byte boundary" >> >> While I tend to agree, we can't fix old / released kernels. So maybe we >> should just fix it in both? > Maybe. The trouble with just bumping up the alignment though is that > not all boards have so much RAM that they can trivially waste another > meg or two on alignment padding without noticing it. AArch64 boards > are probably OK but some of the old 32 bit boards are not anywhere > near as well supplied with RAM. Picking dtb alignment based on 32/64 > bits might be safer. Ugh, we don't know the size yet at this point. And calling load_fdt multiple times feels like a can of worms I don't want to open. So yeah, I'll just make it depend on the RAM size - if there are >= 128MB RAM in our VM we align on 2MB. Alex