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Message-ID: <0373ab35-d51a-83a8-e0fb-f8f5376bb983@suse.com> Date: Thu, 16 Mar 2023 17:02:07 +0100 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:102.0) Gecko/20100101 Thunderbird/102.8.0 Subject: Re: [PATCH v2] xen/console: Skip switching serial input to non existing domains Content-Language: en-US To: Michal Orzel Cc: Andrew Cooper , George Dunlap , Julien Grall , Stefano Stabellini , Wei Liu , xen-devel@lists.xenproject.org References: <20230316102635.6497-1-michal.orzel@amd.com> <2cfc0751-4360-5899-c93a-56ef952a6b2b@amd.com> From: Jan Beulich In-Reply-To: <2cfc0751-4360-5899-c93a-56ef952a6b2b@amd.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-ClientProxiedBy: FR3P281CA0182.DEUP281.PROD.OUTLOOK.COM (2603:10a6:d10:a4::17) To VE1PR04MB6560.eurprd04.prod.outlook.com (2603:10a6:803:122::25) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: VE1PR04MB6560:EE_|PAXPR04MB9642:EE_ X-MS-Office365-Filtering-Correlation-Id: 68d24b38-60cd-4c13-a91a-08db2637cc28 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0; 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Use proper caution when opening attachments, clicking links, or responding. >> >> >> On 16.03.2023 11:26, Michal Orzel wrote: >>> --- a/xen/drivers/char/console.c >>> +++ b/xen/drivers/char/console.c >>> @@ -490,7 +490,24 @@ static void switch_serial_input(void) >>> } >>> else >>> { >>> - console_rx++; >>> + unsigned int next_rx = console_rx + 1; >>> + >>> + /* Skip switching serial input to non existing domains */ >>> + while ( next_rx < max_init_domid + 1 ) >>> + { >>> + struct domain *d = rcu_lock_domain_by_id(next_rx - 1); >>> + >>> + if ( d ) >>> + { >>> + rcu_unlock_domain(d); >>> + break; >>> + } >>> + >>> + next_rx++; >>> + } >>> + >>> + console_rx = next_rx; >>> + >>> printk("*** Serial input to DOM%d", console_rx - 1); >>> } >> >> While at the first glance (when you sent it in reply to v1) it looked okay, >> I'm afraid it really isn't: Please consider what happens when the last of >> the DomU-s doesn't exist anymore. (You don't really check whether it still >> exists, because the range check comes ahead of the existence one.) In that >> case you want to move from second-to-last to Xen. I expect the entire >> if/else construct wants to be inside the loop. > I did this deliberately because I do not think the situation you describe is possible > (i.e. no domains at all - Xen still usable). With hardware domain in place, we can e.g. destroy the domain > which would invoke domain_kill() -> domain_destroy() that would free domain struct. > Without hwdom, the domain cannot kill/destroy itself. It can do the shutdown but it will not > destroy it (at least this is what I tested). So I do not think there can be a scenario where > there is not a single domain while Xen running and be usable. I didn't talk about "no domain left at all", but about the case where the domain with the highest domain ID is gone. Jan