From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932971AbaAaVdO (ORCPT ); Fri, 31 Jan 2014 16:33:14 -0500 Received: from mx1.redhat.com ([209.132.183.28]:34052 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932271AbaAaVdN (ORCPT ); Fri, 31 Jan 2014 16:33:13 -0500 Message-ID: <1391203964.6959.141.camel@bling.home> Subject: Re: Regression due to 0c44c2d0f459 x86: Use asm goto to implement better modify_and_test() functions From: Alex Williamson To: "H. Peter Anvin" Cc: Ingo Molnar , Peter Zijlstra , Jesse Brandeburg , Linux Kernel Mailing List Date: Fri, 31 Jan 2014 14:32:44 -0700 In-Reply-To: <529F5C02.2060706@zytor.com> References: <529E8719.4070202@zytor.com> <20131204085953.GC31778@gmail.com> <529F5C02.2060706@zytor.com> Content-Type: text/plain; charset="UTF-8" Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 2013-12-04 at 08:44 -0800, H. Peter Anvin wrote: > On 12/04/2013 12:59 AM, Ingo Molnar wrote: > > > > * H. Peter Anvin wrote: > > > >> Hi guys, > >> > >> 0c44c2d0f459 x86: Use asm goto to implement better modify_and_test() > >> functions > >> > >> causes a regression, because it incorrectly changed the constraints of > >> bitops. > >> > >> Specifically, the GEN_BINARY_RMWcc() hardcodes a constraint as "er", but > >> it needs to be "Ir" for the bitops themselves. "I" is correct (as > >> opposed to "J" even on 64 bits, because we only generate the 64-bit > >> version when we have a register operand. > >> > >> Unfortunately there isn't a way we can get gcc+gas to generate a version > >> with an offset pointer. > > > > Does the regression manifest itself in any actual breakage - if yes, > > how does it look like? (People experiencing similar symptoms will be > > helped by seeing a fix matching their problems.) > > > > It was discovered because it caused a build failure in a > not-yet-submitted driver patch. This happens when someone uses > test_and_set_bit() or another similar operation on a fixed bit index > above 255; the assembler throws an error at that point and the build fails. > > *HOWEVER*, for bit indicies in the range 32-255, the current code will > instead silently miscompile, as the CPU will truncate the argument to 5 > bits. I don't know if there are any such instances in the current > kernel, but it is entirely possible there is, with unknown but > potentially disastrous results. I'm seeing reports from folks using vfio for PCI device assignment through QEMU that hugepages aren't being released on VM shutdown and reverting 0c44c2d0f459 resolves the problem. I haven't confirmed this myself nor do I pretend to understand the changes this commit makes. Thanks, Alex