From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751015AbdKHIha (ORCPT ); Wed, 8 Nov 2017 03:37:30 -0500 Received: from mail-it0-f68.google.com ([209.85.214.68]:45838 "EHLO mail-it0-f68.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750735AbdKHIh2 (ORCPT ); Wed, 8 Nov 2017 03:37:28 -0500 X-Google-Smtp-Source: ABhQp+QDKNZsTTv9goajTEHCmkujb4KrLjnXKgUk/RcYfo8HY63w/bEL5S7qQb7g77zrJ3pKqV3Z0Q== MIME-Version: 1.0 In-Reply-To: <17a1c480-d0aa-b272-7ca3-5dd2cf2fd7dc@amd.com> References: <20171106201753.23059.86674.stgit@tlendack-t1.amdoffice.net> <17a1c480-d0aa-b272-7ca3-5dd2cf2fd7dc@amd.com> From: Tomeu Vizoso Date: Wed, 8 Nov 2017 09:37:06 +0100 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH] x86/boot: Fix boot failure when SMP MP-table is based at 0 To: Tom Lendacky Cc: "H. Peter Anvin" , x86@kernel.org, Peter Zijlstra , "linux-kernel@vger.kernel.org" , Ingo Molnar , Borislav Petkov , Thomas Gleixner Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 6 November 2017 at 23:01, Tom Lendacky wrote: > On 11/6/2017 3:41 PM, H. Peter Anvin wrote: >> >> On 11/06/17 12:17, Tom Lendacky wrote: >>> >>> When crosvm is used to boot a kernel as a VM, the SMP MP-table is found >>> at physical address 0x0. This causes mpf_base to be set to 0 and a >>> subsequent "if (!mpf_base)" check in default_get_smp_config() results in >>> the MP-table not being parsed. Further into the boot this results in an >>> oops when attempting a read_apic_id(). >>> >>> Add a boolean variable that is set to true when the MP-table is found. >>> Use this variable for testing if the MP-table was found so that even a >>> value of 0 for mpf_base will result in continued parsing of the MP-table. >>> >>> Reported-by: Tomeu Vizoso >>> Signed-off-by: Tom Lendacky >> >> >> Ahem... did anyone ever tell you that this is an epicly bad idea on your >> part? The low megabyte of physical memory has very special meaning on >> x86, and deviating from the standard use of this memory is a *very* >> dangerous thing to do, and imposing on the kernel a "fake null pointer" >> requirement that exists only for the convenience of your particular >> brokenness is not okay. >> >> -hpa > > > That was my initial thought... what was something doing down at the start > of memory. But when I looked at default_find_smp_config() it specifically > scans the bottom 1K for a an MP-table signature. I was hoping to get some > feedback as to whether this would really be an acceptable thing to do. So > I'm good with this patch being rejected, but the change I made in > > 5997efb96756 ("x86/boot: Use memremap() to map the MPF and MPC data") > > does break something that was working before. Do I understand correctly that the best we can do right now is reverting 5997efb96756? Thanks, Tomeu