From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755086AbXEDLqd (ORCPT ); Fri, 4 May 2007 07:46:33 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1755097AbXEDLqc (ORCPT ); Fri, 4 May 2007 07:46:32 -0400 Received: from ebiederm.dsl.xmission.com ([166.70.28.69]:53033 "EHLO ebiederm.dsl.xmission.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755086AbXEDLqb (ORCPT ); Fri, 4 May 2007 07:46:31 -0400 From: ebiederm@xmission.com (Eric W. Biederman) To: Jeremy Fitzhardinge Cc: Rusty Russell , "H. Peter Anvin" , Andi Kleen , Linux Kernel Mailing List Subject: Re: [PATCH] i386: always clear bss References: <463AED07.1000505@goop.org> Date: Fri, 04 May 2007 05:46:01 -0600 In-Reply-To: <463AED07.1000505@goop.org> (Jeremy Fitzhardinge's message of "Fri, 04 May 2007 01:21:27 -0700") Message-ID: User-Agent: Gnus/5.110006 (No Gnus v0.6) Emacs/21.4 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Jeremy Fitzhardinge writes: > When the paravirt dispatcher gets run immediately on entry to > startup_32, the bss isn't cleared. This happens to work if the > hypervisor's domain builder loaded the complete kernel image and > cleared the bss for us, but this may not always be true (for example, > if we're running out of a decompressed bzImage). > > Change head.S so that it unconditionally clears the bss before doing > the paravirt dispatch or continuing on to normal native boot. > > There are a couple of points to note: > - We can't, in general, load the segment registers before paravirt > dispatch, because we could be running with a non-standard gdt and > segment selectors. In practice though, all code which ends up > jumping into startup_32 will have already set the segment registers > up to sane values, so we don't need to do it again. > - Paging may or may not be enabled, and if enabled we may or may not > be mapped to the proper kernel virtual address. To deal with this, > we compare the kernel's linked address with where we're actually > running, and use that to offset the bss pointer. NAK. Skipping the segment register load is likely fine. Supporting V!=P at startup_32 is not. Assuming that we have a stack at startup_32 is not. If you want to figure out where the kernel is loaded you can do (from arch/i386/boot/head.S) > > /* Calculate the delta between where we were compiled to run > * at and where we were actually loaded at. This can only be done > * with a short local call on x86. Nothing else will tell us what > * address we are running at. The reserved chunk of the real-mode > * data at 0x34-0x3f are used as the stack for this calculation. > * Only 4 bytes are needed. > */ > leal 0x40(%esi), %esp > call 1f > 1: popl %ebp > subl $1b, %ebp > Eric