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Thu, 13 Oct 2022 07:23:18 +0000 Received: from VI1PR08MB3919.eurprd08.prod.outlook.com ([fe80::bbeb:a49e:d969:5d2]) by VI1PR08MB3919.eurprd08.prod.outlook.com ([fe80::bbeb:a49e:d969:5d2%6]) with mapi id 15.20.5709.021; Thu, 13 Oct 2022 07:23:18 +0000 Message-ID: Date: Thu, 13 Oct 2022 08:23:11 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.11.0 Subject: Re: [PATCH] [Arm] Remove dead FPA code Content-Language: en-US To: Pedro Alves , John Baldwin , gdb-patches@sourceware.org, David Spickett References: <20220920123012.189293-1-luis.machado@arm.com> <73479562-ab47-dfbf-aadc-7a2203c0f0e4@FreeBSD.org> <56653c70-593a-4b8d-ddf7-52f7dd0608f7@arm.com> <1946bc74-8270-23c4-9483-702b9dbc03de@FreeBSD.org> <3cb2d818-83e8-4e2d-5e1b-9b555d1a2217@arm.com> <1003e65d-50fe-8aed-4d98-d06833d27011@palves.net> From: Luis Machado In-Reply-To: <1003e65d-50fe-8aed-4d98-d06833d27011@palves.net> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-ClientProxiedBy: SN6PR16CA0067.namprd16.prod.outlook.com (2603:10b6:805:ca::44) To VI1PR08MB3919.eurprd08.prod.outlook.com (2603:10a6:803:c4::31) MIME-Version: 1.0 X-MS-TrafficTypeDiagnostic: VI1PR08MB3919:EE_|AS1PR08MB7401:EE_|VE1EUR03FT030:EE_|AM7PR08MB5430:EE_ X-MS-Office365-Filtering-Correlation-Id: 68531602-b92e-4893-557d-08daacebd2e6 x-checkrecipientrouted: true NoDisclaimer: true X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam-Untrusted: BCL:0; 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Ip=[63.35.35.123]; Helo=[64aa7808-outbound-1.mta.getcheckrecipient.com] X-MS-Exchange-CrossTenant-AuthSource: VE1EUR03FT030.eop-EUR03.prod.protection.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: AM7PR08MB5430 X-Spam-Status: No, score=-7.0 required=5.0 tests=BAYES_00, DKIM_SIGNED, DKIM_VALID, FORGED_SPF_HELO, KAM_DMARC_NONE, NICE_REPLY_A, RCVD_IN_MSPIKE_H2, SPF_HELO_PASS, SPF_NONE, TXREP, UNPARSEABLE_RELAY autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on server2.sourceware.org X-BeenThere: gdb-patches@sourceware.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Gdb-patches mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Thu, 13 Oct 2022 07:23:33 -0000 On 10/10/22 15:58, Pedro Alves wrote: > On 2022-10-05 9:26 a.m., Luis Machado wrote: > >> Yeah, that's what I was worried about. Register discoveries without XML are not great, and more recently debugging stubs have been >> exposing more system registers. Having to consider FPA (which was *removed* 10 years ago from GCC, but fell in disuse before then) is not >> acceptable at this point. >> >> If those debugging stubs want to skip XML, I think it would be reasonable for them to at least update the expected 'g' packet to contain just >> the basic registers, with CPSR as 16. >> >> That might need some coordination. I can coordinate this from the Linux Kernel's side, but I never dealt with the BSD kernels. > > I'm not seeing much point in this: > > #1 - If the remote side supplies XML, then the register number doesn't really matter, GDB auto-maps the numbers. > > #2 - If the remote side doesn't supply XML, then you're just setting up for a world of coordination pain for no benefit. > > In GDB, to keep things working, we just have to keep the FPA register number hole in place. That's hardly causing > any maintenance burden, IMO. We just have to have comments in place explaining why we have them. Based on a recent thread with the Arm Linux Kernel maintainer, he finds it reasonable to make kgdb return XML data and to keep a compatibility layer (the g guesses) going for a reasonable amount of time, after which we can retire the guesses and drop the code. It is not clear when we'll drop the compatibility layer, but at least we can get some traction towards making targets use XML and maybe add some deprecation warning to the g packet guesses. If we keep the code as-is, there is no incentive to update targets using old mechanisms. GDB talks about FPA as if it were something that existed not long ago, while in fact it was deprecated even before GDB started using XML descriptions. But the registers were kept that way and GDB was never updated (supposedly because it needed to be compatible with some other tool at the time), leading to other tools copying what GDB was exposing, even though they didn't support FPA at all. One such example is the Linux Kernel, which added these registers because GDB required them. So in this case GDB has the potential to influence other tools, and we should strive to keep things clean and updated as much as possible. This is an attempt at that.