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Message-ID: Date: Mon, 13 Jun 2022 11:18:49 +0200 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:91.0) Gecko/20100101 Thunderbird/91.10.0 Subject: Re: [PATCH] xen/console: do not drop serial output from the hardware domain Content-Language: en-US To: =?UTF-8?Q?Roger_Pau_Monn=c3=a9?= Cc: Andrew Cooper , George Dunlap , Julien Grall , Stefano Stabellini , Wei Liu , xen-devel@lists.xenproject.org References: <20220610150651.29933-1-roger.pau@citrix.com> <3a462021-1802-4764-3547-6d0a02cd092f@suse.com> <3d0d74d8-55a9-cdb6-0c5e-616ddd47bbc0@suse.com> From: Jan Beulich In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-ClientProxiedBy: AS9PR07CA0020.eurprd07.prod.outlook.com (2603:10a6:20b:46c::21) To VE1PR04MB6560.eurprd04.prod.outlook.com (2603:10a6:803:122::25) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-Office365-Filtering-Correlation-Id: a9e08681-ff9a-475d-c9b6-08da4d1db914 X-MS-TrafficTypeDiagnostic: PA4PR04MB7824:EE_ X-Microsoft-Antispam-PRVS: X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0; 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And with this >>>> aiming at boot output only, wouldn't you want to stop the overriding >>>> once boot has completed (of which, if I'm not mistaken, we don't >>>> really have any signal coming from Dom0)? And even during boot I'm >>>> not convinced we'd want to let through everything, but perhaps just >>>> Dom0's kernel messages? >>> >>> I normally use sync_console on all the boxes I'm doing dev work, so >>> this request is something that come up internally. >>> >>> Didn't realize Xen output wasn't forced, since we already have rate >>> limiting based on log levels I was assuming that non-ratelimited >>> messages wouldn't be dropped. But yes, I agree that Xen (non-guest >>> triggered) output shouldn't be rate limited either. >> >> Which would raise the question of why we have log levels for non-guest >> messages. > > Hm, maybe I'm confused, but I don't see a direct relation between log > levels and rate limiting. If I set log level to WARNING I would > expect to not loose _any_ non-guest log messages with level WARNING or > above. It's still useful to have log levels for non-guest messages, > since user might want to filter out DEBUG non-guest messages for > example. It was me who was confused, because of the two log-everything variants we have (console and serial). You're right that your change is unrelated to log levels. However, when there are e.g. many warnings or when an admin has lowered the log level, what you (would) do is effectively force sync_console mode transiently (for a subset of messages, but that's secondary, especially because the "forced" output would still be waiting for earlier output to make it out). We strongly advise against use of sync_console in production environments, so I'm afraid I have trouble seeing how using this mode transiently can be safe. This is quite different from forcing all output to appear when e.g. we're about to crash Xen. Jan