From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-3.9 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 7F21DC55179 for ; Wed, 28 Oct 2020 22:11:35 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id DED262473C for ; Wed, 28 Oct 2020 22:11:34 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=alien8.de header.i=@alien8.de header.b="KM2UZ92z" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730688AbgJ1WLd (ORCPT ); Wed, 28 Oct 2020 18:11:33 -0400 Received: from mail.skyhub.de ([5.9.137.197]:34574 "EHLO mail.skyhub.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730665AbgJ1WLX (ORCPT ); Wed, 28 Oct 2020 18:11:23 -0400 Received: from zn.tnic (p200300ec2f0aab00f97e6c5296953a73.dip0.t-ipconnect.de [IPv6:2003:ec:2f0a:ab00:f97e:6c52:9695:3a73]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.skyhub.de (SuperMail on ZX Spectrum 128k) with ESMTPSA id 561D51EC03C1; Wed, 28 Oct 2020 20:44:04 +0100 (CET) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=alien8.de; s=dkim; t=1603914244; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:in-reply-to:in-reply-to: references:references; bh=kmhsCqnOlHi4ixOVZ87EzyZjkxp8ouvhlUNx8nUfMNI=; b=KM2UZ92zuUSHGj0wXw1SSR97tAhO8TD14jN4ALj3QsH/FMGj68Squ6Qf/ED6JOmNn6X9DD qbnNPmqy4PYEyoPfu/cEofZhVuWfBHQgqqvAlBjMS6HSt4y+/u0FHVoNH1J7GQILHaVxIN BLJNlTRd/dw9XJ2nn/YnsfriM9zBOfc= Date: Wed, 28 Oct 2020 20:43:55 +0100 From: Borislav Petkov To: Arvind Sankar Cc: x86@kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] x86/build: Fix vmlinux size check on 64-bit Message-ID: <20201028194355.GB27112@zn.tnic> References: <20201005151539.2214095-1-nivedita@alum.mit.edu> <20201027200803.GL15580@zn.tnic> <20201027211422.GC1833548@rani.riverdale.lan> <20201028133909.GA27112@zn.tnic> <20201028164551.GA1989568@rani.riverdale.lan> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20201028164551.GA1989568@rani.riverdale.lan> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Oct 28, 2020 at 12:45:51PM -0400, Arvind Sankar wrote: > You don't want to try to run the kernel from physical address 0 in any > case. The default is set to 16MiB to avoid low memory, historically to > avoid the 24-bit ISA DMA range. Sure, that's why I wrote: "... so I guess this should be a range > 0 specification but I guess not important." So how about a sentence or two alluding to that fact in the help text of that option? > This doesn't matter for the 64-bit kernel, which can be run from any > physical address independent of the RELOCATABLE/PHYSICAL_START settings. > It only matters on 32-bit, where VA and PA are tied together by > VA == __PAGE_OFFSET + PA You mean the kernel text mapping I assume because we do #define __va(x) ((void *)((unsigned long)(x)+PAGE_OFFSET)) on 64-bit too but that's the direct mapping of all physical memory. > On 64-bit, the kernel's location in VA space and physical space can be > independently moved around, so a kernel that starts at 16MiB in VA space > can be loaded anywhere above 16MiB in physical space. Right. > KERNEL_IMAGE_SIZE is _not_ the size of the kernel image, the name is > misleading. So that needs fixing too, I guess. > It is the maximum VA that the kernel can occupy, it is used > to prepopulate the PMD-level pagetable for initial boot (level2_kernel_pgt) > and is also used to define MODULES_VADDR, so it _is_ talking about > mappings. If you have a 30MiB kernel that is placed at a starting VA of > 510MiB when KERNEL_IMAGE_SIZE is 512MiB, it won't boot. ... because not the whole kernel will be mapped, sure. There's a comment above KERNEL_IMAGE_SIZE which could use some of that explanation. > Increasing vmlinux size can trigger the problem by pushing _end > beyond KERNEL_IMAGE_SIZE, but the problem occurs once _end - > __START_KERNEL_map exceeds KERNEL_IMAGE_SIZE, not when _end - _text > exceeds it, hence this patch. Understood - in both cases, once _end goes beyond the 512MiB end of the PMD mapping, we've lost. Please add that part to the commit message too because we will forget. Thx. -- Regards/Gruss, Boris. https://people.kernel.org/tglx/notes-about-netiquette