From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from lists.xenproject.org (lists.xenproject.org [192.237.175.120]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 36C0AFB3CEB for ; Mon, 30 Mar 2026 09:58:08 +0000 (UTC) Received: from list by lists.xenproject.org with outflank-mailman.1267367.1556878 (Exim 4.92) (envelope-from ) id 1w79NP-0004lq-Ir; Mon, 30 Mar 2026 09:57:43 +0000 X-Outflank-Mailman: Message body and most headers restored to incoming version Received: by outflank-mailman (output) from mailman id 1267367.1556878; Mon, 30 Mar 2026 09:57:43 +0000 Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1w79NP-0004lj-FV; Mon, 30 Mar 2026 09:57:43 +0000 Received: by outflank-mailman (input) for mailman id 1267367; Mon, 30 Mar 2026 09:57:42 +0000 Received: from mx.expurgate.net ([195.190.135.10]) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1w79NO-0004ld-EN for xen-devel@lists.xenproject.org; Mon, 30 Mar 2026 09:57:42 +0000 Received: from mx.expurgate.net (helo=localhost) by mx.expurgate.net with esmtp id 1w79NM-007YUL-GL for xen-devel@lists.xenproject.org; Mon, 30 Mar 2026 11:57:40 +0200 Received: from [10.42.69.10] (helo=localhost) by localhost with ESMTP (eXpurgate MTA 0.9.1) (envelope-from ) id 69ca4907-2eae-0a2a0a5409dd-0a2a450a8194-48 for ; Mon, 30 Mar 2026 11:57:40 +0200 Received: from [40.107.200.46] (helo=CH5PR02CU005.outbound.protection.outlook.com) by tlsNG-4011c0.mxtls.expurgate.net with ESMTPS (eXpurgate 4.55.2) (envelope-from ) id 69ca4912-1772-0a2a450a0019-286bc82efab2-3 for ; Mon, 30 Mar 2026 11:57:40 +0200 Received: from CH7PR03MB7860.namprd03.prod.outlook.com (2603:10b6:610:24e::14) by SJ0PR03MB5760.namprd03.prod.outlook.com (2603:10b6:a03:2d3::8) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.9745.28; Mon, 30 Mar 2026 09:57:36 +0000 Received: from CH7PR03MB7860.namprd03.prod.outlook.com ([fe80::f5ba:35df:1c9f:b343]) by CH7PR03MB7860.namprd03.prod.outlook.com ([fe80::f5ba:35df:1c9f:b343%4]) with mapi id 15.20.9745.027; Mon, 30 Mar 2026 09:57:36 +0000 X-BeenThere: xen-devel@lists.xenproject.org List-Id: Xen developer discussion List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Errors-To: xen-devel-bounces@lists.xenproject.org Precedence: list Sender: "Xen-devel" Authentication-Results: eu.smtp.expurgate.cloud; dkim=pass header.s=selector1 header.d=citrix.com header.i="@citrix.com" header.h="From:Date:Subject:Message-ID:Content-Type:MIME-Version:X-MS-Exchange-SenderADCheck" ARC-Seal: i=1; a=rsa-sha256; s=arcselector10001; d=microsoft.com; cv=none; b=yvEtOwVBUlZu7A119Soj8xvd7Mf5lEy85A/vYife9dN96oZcmMk2R+pygR1B4HbQRi8NEdVd6GYlDZQGCKffKDbGFWhg1A4zP/cWMAz2AXz5xCbfJzjIjtBOVtiXOWFMXmC80ErA+4vrhWGh9doz+RcFhv88PfkVEYhLNtt5oviMIInnvd54cX2G5/0e2gDyfKzJWPY5sOJZHNKZX0AMPw+9ogjrlhtCJONBmkr42BGw/hnzKQh7XBBj5+FquhiijYdldP6+rUwVVFZAeJuLgJbgg2fgOoHW0WN/ZMoOnnaqUOdeuGfMB4/1deRr6qcxYv7mJxVo3yCcNqGXu1PNdA== ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=microsoft.com; s=arcselector10001; h=From:Date:Subject:Message-ID:Content-Type:MIME-Version:X-MS-Exchange-AntiSpam-MessageData-ChunkCount:X-MS-Exchange-AntiSpam-MessageData-0:X-MS-Exchange-AntiSpam-MessageData-1; bh=d/spZ7bUzzZXh1IwnaF8NBSOaFQlR5NAV/f+qjpJJuU=; b=J8hqWXHerI1qVUHMqO1h07TVlIInPxA7lXXcNCgrrboQFrgi9fyyRMfr7e5N5X3+4rOk7mNPbgFP/MavVr7om4cfhxkbko9zKIjIemQqT1leDrdWIaIsda6XXnrWdjgR62BzB1nEQQUil2cB0lH0cQNzLXy0qp4PB4uMv4EF6LirsVVxPv0qolO+MF+z+YO8m5oFZCWCTm7fkA5pd4aeBHPnNaHLp3YkL1/Hehg81nGRlMf8tchPYSDfH0n9Hk/F5H5o5aftcaUN3w7JyOm6gGKT/LEifjVvDkq9nYHtufeGjkBTdfsMxhGpRr0HxoMhRC4r55h1E1dvXpuNG24SFg== ARC-Authentication-Results: i=1; mx.microsoft.com 1; spf=pass smtp.mailfrom=citrix.com; dmarc=pass action=none header.from=citrix.com; dkim=pass header.d=citrix.com; arc=none DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=citrix.com; s=selector1; h=From:Date:Subject:Message-ID:Content-Type:MIME-Version:X-MS-Exchange-SenderADCheck; bh=d/spZ7bUzzZXh1IwnaF8NBSOaFQlR5NAV/f+qjpJJuU=; b=Sil3mGmQA+QrVIGKIJ4gQPrCniQeKd99QFGGFYqZX4G8MTjWtnREMW1TjPKTYWQvyuZGGe7WnfN+W8o+fugvru2ABDoEOYUAoLRJmywMZuSRIEwwYl/Pi299l46ofuhz5edekYQo0V2neOuoyd/DSfjpyJr5/1p6BQ3aNNUdpsQ= Authentication-Results: dkim=none (message not signed) header.d=none;dmarc=none action=none header.from=citrix.com; Date: Mon, 30 Mar 2026 11:57:32 +0200 From: Roger Pau =?utf-8?B?TW9ubsOp?= To: Jan Beulich Cc: Andrew Cooper , Anthony PERARD , Michal Orzel , Julien Grall , Stefano Stabellini , xen-devel@lists.xenproject.org Subject: Re: [PATCH v2 2/3] xen/uart: be more careful with changes to the PCI command register Message-ID: References: <20260327135420.7246-1-roger.pau@citrix.com> <20260327135420.7246-3-roger.pau@citrix.com> <93a09dbb-0a8c-4eeb-b544-c9409b9f85ce@suse.com> Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-ClientProxiedBy: MR1P264CA0051.FRAP264.PROD.OUTLOOK.COM (2603:10a6:501:3e::26) To CH7PR03MB7860.namprd03.prod.outlook.com (2603:10b6:610:24e::14) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: CH7PR03MB7860:EE_|SJ0PR03MB5760:EE_ X-MS-Office365-Filtering-Correlation-Id: 36ad50f9-c2fa-4190-5543-08de8e42c52b X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|1800799024|376014|366016|56012099003|22082099003|18002099003; X-Microsoft-Antispam-Message-Info: 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 X-Forefront-Antispam-Report: CIP:255.255.255.255;CTRY:;LANG:en;SCL:1;SRV:;IPV:NLI;SFV:NSPM;H:CH7PR03MB7860.namprd03.prod.outlook.com;PTR:;CAT:NONE;SFS:(13230040)(1800799024)(376014)(366016)(56012099003)(22082099003)(18002099003);DIR:OUT;SFP:1101; X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1 X-MS-Exchange-AntiSpam-MessageData-0: =?utf-8?B?VEtoV25FVWNXbFFBZXNrUGZBWEEzeVhpd29HSnlLS1FBSlhqcG40ZDhqdGVm?= =?utf-8?B?aXBybE5KR0RaYXg4YWQvaEFGTWJWN2hHSDAzbjlHV1VMWm9KQzc0Qk5tY3pM?= =?utf-8?B?L0c1cEFSNDVtcFBWMzlsRm1UeURCZ1RDSWVVdVZKYURPUVZmRnFwblRKbnlK?= =?utf-8?B?VlY1VnFNWXNzWjJ5TnAvL1ZIdnZZNEpZNWJMcDBDSTRrU1E2VElwUDVGSFRr?= =?utf-8?B?ZmEyQzQzQ24rMFBiMUFQOXZHS1VFR282QUtQZzhjRlRoLzdPN0ZTZjRpYzZn?= =?utf-8?B?aXRpRVR3eDYyeFl2ZCtlSFZqRVFCN2h6ZHh2czRNRnVEOTdUTlRjd0FwdVdK?= =?utf-8?B?ajc4NlJqT0ZmMitqZ1BtRkZGY3FCTldqcmhzV3B5aUl1bEFIc29udTFBNVhr?= =?utf-8?B?QXFEZm5NcG5GRG1YbkRISG1WdGRGOU9ld3BQWERwcjVTRnF4Sk5sdDJKVWo4?= =?utf-8?B?d3YydWFkY1dVdG9Lb1NZOXE4aE84RjRtYlB4MS9ncW55aWlyV3M3blcvTGF2?= =?utf-8?B?WEV2VE0vdEEwejZTbHRDMk9pRzVVZFhLQUJvU3FOa0N6ZmJHNlZMQzV4eEVY?= =?utf-8?B?UFhpR2R0YldQV0pUN08wVUlvQW9DOWU2TWFkMmgrZ2JNYWxnYkdnNE1RNHdX?= =?utf-8?B?QW02VjlMeXF3a3JBcFVmZjY3VkhjK2l3bUtjUVh0aUY4a0RzcExXWUNUb1ZQ?= =?utf-8?B?YW8wZnJJaEhBNDYrNzdRbmovbExYeGlnSVJNVitlQTlqbzRCYmFXVTFUa0pJ?= =?utf-8?B?d3g3UjlQTWtzNlF3UEhpZmpDMTg3QVlkV0k5aEtycHFCL1Awb3dYdC9IU1Fs?= =?utf-8?B?WUI0MjJIMGNDdThxSUI0NTFJS2NweFQwVktkMzJPRkM3amhEZFdnZFRWNUox?= =?utf-8?B?eGxXdVZTYW1DWmtVVVBJWk1pZm4vejVGRFNvWXNlYjhEZUlWMjZ5NFJGa3l3?= =?utf-8?B?eXZqZmVlR2J4em5JS1lJZHNHS0lTalZTRUNDbXhEREJ1aS9DdkEzeE1YMndz?= =?utf-8?B?NUJ6TEx6QkRaWVRlV0ZhRHh3S016eFZjeWNlM2tIQkV2TEJHU1B1K29mSUd6?= =?utf-8?B?d09hUVBvN2JBbzU1V1dOZklPSWgzOEk0eGRVWis2Qk1RakhpMWVZM1lmMUxj?= =?utf-8?B?M3ZkZ1RaRDBVR3JYbHFzNmpHOVJEQWJVTXM5V1NzY3ZUVFpZK1QzR3lmMkZG?= =?utf-8?B?dHdqTHBhUkQ2QUJjaWdGM0JxTVJBUUw3NnlUOHhFZmhISXlhdm1kSGg0THZv?= =?utf-8?B?T2F2L2hqZTgrT1lGSWcwYkdlTyt6N05YMm1YaGdHSVdmL3FOUlVFd0RHRkhr?= =?utf-8?B?c0RBWUo0a0NoVWFHUmthZFFraDZuUjFYbGNSK0wvWlBBRW8yUDJGQ1c4Tmwz?= =?utf-8?B?UEdsZXpsN0h1YkxPc0pEWUpjZzA2UGdoYnNkRENoR1BUOXZDenJ2WUJpVzdy?= =?utf-8?B?Kzljb0Y5RmRXT212OGE0SGhOVHg1UG1WN0h1MXJTT2xORmtoUGtRdUVsU2FT?= =?utf-8?B?YURYS3R0enJyWUVqWlhKb2lON2dpVUJycldkY2JNcTk4dUhMZXRjQTFrVUg3?= =?utf-8?B?dGZXNzQ1Z29NU2s1Q29MaGhNMW0wOVhrY1k1b2xydUJXWXg3bStMY2ZVOHlE?= =?utf-8?B?Q1VUU081Y0tjM2R1YjkrQzl1SlNpYnM1TmlXUTdXOG1iRlBRZmVaTFA0Q2M3?= =?utf-8?B?MGZhTWw2eFY0WVpJZ3cybTk4cUk0MmM3YkhFVzNDK3BJNzBmMlRUa0JYRGtC?= =?utf-8?B?dUp4SGVyYVAwKzJFMncyQXZpTVRFaUp4U091VjZJZE5NSmVmeWVqUDBmWTZw?= =?utf-8?B?ZnRSKzYwQVhlajBQVFVXUWtnUURYVUc0SkpOMENLdUtZUTMyaWNnWE9KUnYz?= =?utf-8?B?YitBcm1PR0NUSmRGMUpOUGV3VWdNdUZKUFcxeWJmWlF3UEUzeVc0bnFrMDEw?= =?utf-8?B?R3E2V29rdmg5ejlsK25SeGl4Wk9QSHE3b1dXWW9PS2d6SWJ1TWZxaEc5cUFS?= =?utf-8?B?bVNCN1Bxd3lTTS9SZjFyT1pOMGhnR0RyRHlMUjVxWlBxclh6d2U3TVkrblhM?= =?utf-8?B?QWZQakZpWXFBVlFKcVZEY0V6NmVQTVQvSWUzaTUvdER6cGlRVTQ3NnhQaUFa?= =?utf-8?B?Q2V5R0lwWkpoaEdWdTVHVTdPR3Q3YW0xK0hjeXhDb1cyTFBTQnRJbnczQksx?= =?utf-8?B?Q0xoRDEzdmNIT0FRcU5PdUN5RFc4S284cndtdDRpcy8zSXhVb043WEVMSXpW?= =?utf-8?B?dmhGSzVTV0dOWTJzeHFFNW51TkJjZ0tnOGpWbWliYVI5a25sQmpFR3RHVkw5?= =?utf-8?B?SDlKRk8wdVV2Y1FzMkhSMEFkNThHR0lRRmtzQitqb0Vkcm9pRHhiUT09?= X-OriginatorOrg: citrix.com X-MS-Exchange-CrossTenant-Network-Message-Id: 36ad50f9-c2fa-4190-5543-08de8e42c52b X-MS-Exchange-CrossTenant-AuthSource: CH7PR03MB7860.namprd03.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-OriginalArrivalTime: 30 Mar 2026 09:57:36.2720 (UTC) X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted X-MS-Exchange-CrossTenant-Id: 335836de-42ef-43a2-b145-348c2ee9ca5b X-MS-Exchange-CrossTenant-MailboxType: HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: uDsOHn8XwYY915HP3G5rMOShITxgo8EcwvxRhB5I8DMYKZc2W26KUQqjptAZkde+SbOoxI3C49LZu/0L811aoQ== X-MS-Exchange-Transport-CrossTenantHeadersStamped: SJ0PR03MB5760 X-purgate-ID: tlsNG-4011c0/1774864660-BFC9C900-41A61AA8/0/0 X-purgate-type: clean X-purgate-size: 2142 On Mon, Mar 30, 2026 at 11:09:10AM +0200, Jan Beulich wrote: > On 30.03.2026 11:06, Roger Pau Monné wrote: > > On Mon, Mar 30, 2026 at 10:00:05AM +0200, Jan Beulich wrote: > >> On 27.03.2026 14:54, Roger Pau Monne wrote: > >>> Read the existing PCI command register and only add the required bits to > >>> it, as to avoid clearing bits that might be possibly set by the firmware > >>> already, which might put the device into a non-working state. > >>> > >>> Fixes: f2ff5d6628b3 ("ns16550: enable PCI serial card usage") > >>> Signed-off-by: Roger Pau Monné > >> > >> Reviewed-by: Jan Beulich > >> > >> I would have preferred if the description mentioned the particular case, > >> turning this more into a workaround than an apparent bugfix. > > > > It turns out that the console does seem to work fine, even with memory > > decoding disabled on the device (as expected). I've updated the > > firmware in the meantime, so I'm unsure whether that update has > > changed the behavior of the device, or it simply was some other > > instability that was causing the issue in the past. This SOL AMT > > device is not reliable at all I'm afraid. > > > >> As mentioned, > >> us driving the device generally means we're free to do whatever we want to > >> the command register, as long as resulting device state is consistent > >> overall (or else we may indeed have a non-working device). Having to keep > >> memory decoding enabled in order for I/O ports to function is pretty > >> clearly a bug in the device, and hence us "violating" that requirement > >> isn't really o bug of ours. > > > > I think given the fragility of some of those SOL devices it's best to > > limit the number of bits Xen changes, as to having a bigger chances of > > getting output working. > > That's okay(ish); I merely would wish the patch description was less > suggesting that Xen was actually buggy. What about if I change the title to: xen/uart: avoid clearing PCI command register bits set by the firmware I think that's clearer and less blameful? Thanks, Roger.