From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752541AbcEJOFV (ORCPT ); Tue, 10 May 2016 10:05:21 -0400 Received: from mail-wm0-f43.google.com ([74.125.82.43]:33755 "EHLO mail-wm0-f43.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751566AbcEJOFT (ORCPT ); Tue, 10 May 2016 10:05:19 -0400 Subject: Re: [PATCH] arm64: kgdb: Match pstate size with gdbserver protocol To: Will Deacon References: <1462815566-15086-1-git-send-email-daniel.thompson@linaro.org> <20160510102838.GF22813@arm.com> Cc: Catalin Marinas , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, patches@linaro.org, linaro-kernel@lists.linaro.org, Jason Wessel , Dave P Martin , Mark Rutland , Vijaya Kumar K From: Daniel Thompson Message-ID: Date: Tue, 10 May 2016 15:05:15 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.0 MIME-Version: 1.0 In-Reply-To: <20160510102838.GF22813@arm.com> Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 10/05/16 11:28, Will Deacon wrote: > Hi Daniel, > > So does kgdb do something useful after this patch? On this occasion I was debugging it on behalf of another developer rather than trying to use it myself to debug something so I didn't do much additional testing beyond sanity tested the contents of the register set. Naturally it will be *totally* awesome if I get the pseudo-NMI stuff working perfectly. ;-) > On Mon, May 09, 2016 at 06:39:26PM +0100, Daniel Thompson wrote: >> Current versions of gdb do not interoperate cleanly with kgdb on arm64 >> systems because gdb and kgdb do not use the same register description. >> This patch modifies kgdb to work with recent releases of gdb (>= 7.8.1). >> >> Compatibility with gdb (after the patch is applied) is as follows: >> >> gdb-7.6 and earlier Ok >> gdb-7.7 series Works if user provides custom target description >> gdb-7.8(.0) Works if user provides custom target description >> gdb-7.8.1 and later Ok >> >> When commit 44679a4f142b ("arm64: KGDB: Add step debugging support") was >> introduced it was paired with a gdb patch that made an incompatible >> change to the gdbserver protocol. This patch was eventually merged into >> the gdb sources: >> https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=commit;h=a4d9ba85ec5597a6a556afe26b712e878374b9dd >> >> The change to the protocol was mostly made to simplify big-endian support >> inside the kernel gdb stub. Unfortunately the gdb project released >> gdb-7.7.x and gdb-7.8.0 before the protocol incompatibility was identified >> and reversed: >> https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=commit;h=bdc144174bcb11e808b4e73089b850cf9620a7ee >> >> This leaves us in a position where kgdb still uses the no-longer-used >> protocol; gdb-7.8.1, which restored the original behaviour,If was >> released on 2014-10-29. >> >> I don't believe it is possible to detect/correct the protocol >> incompatiblity which means the kernel must take a view about which >> version of the gdb remote protocol is "correct". This patch takes the >> view that the original/current version of the protocol is correct >> and that version found in gdb-7.7.x and gdb-7.8.0 is anomalous. > > Urgh, this is filthy! Still, without a time machine, I guess there's > little we can do about it. Can I ask you to respin the patch but with > the rationale as a comment in the header file, and a pointer to the > comment from the C code too, please? > > The code looks incorrect after this change, so we should justify how > we've ended up in this state and not everybody looks at the git log > for that rationale. Will do. Daniel.