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Message-ID: Date: Tue, 7 Mar 2023 11:12:00 +0100 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:102.0) Gecko/20100101 Thunderbird/102.8.0 Subject: Re: API/ABIs: Re: [RFC PATCH v2 0/2] Add a new acquire resource to query vcpu statistics Content-Language: en-US To: Matias Ezequiel Vara Larsen Cc: xen-devel@lists.xenproject.org, Matias Ezequiel Vara Larsen , =?UTF-8?Q?Roger_Pau_Monn=c3=a9?= , Wei Liu , George Dunlap , Julien Grall , Stefano Stabellini , Dario Faggioli , Anthony PERARD , Andrew Cooper References: <90a551bc-ffda-6db8-775b-11c100bf6f52@citrix.com> <20230306142315.GA745324@horizon> From: Jan Beulich In-Reply-To: <20230306142315.GA745324@horizon> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-ClientProxiedBy: FR3P281CA0058.DEUP281.PROD.OUTLOOK.COM (2603:10a6:d10:4b::19) To VE1PR04MB6560.eurprd04.prod.outlook.com (2603:10a6:803:122::25) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: VE1PR04MB6560:EE_|DU0PR04MB9300:EE_ X-MS-Office365-Filtering-Correlation-Id: 1902d995-41cf-42b8-2aa2-08db1ef465a5 X-LD-Processed: f7a17af6-1c5c-4a36-aa8b-f5be247aa4ba,ExtFwd X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0; 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I understand > this value shall be between the actual cacheline_size and PAGE_SIZE. A value > that could match most architectures could be 256 bytes. This isn't a concern anymore when offset and stride are stored in the header. It was a concern when trying to come up with a layout where these value were to be inferred (or known a priori). > - Xen shall use the "stats_active" field to indicate what it is producing. In > this field, reserved bits shall be 0. This shall allow us to agree on the > layout even when producer and consumer are compiled with different headers. I wonder how well such a bitfield is going to scale. It provides for only 32 (maybe 64) counters. Of course this may seem a lot right now, but you never know how quickly something like this can grow. Plus with ... > - In the vcpu_stats structure, new fields can only ever be appended. ... this rule the only ambiguity that could arise to consumers when no active flags existed would be that they can't tell "event never occurred" from "hypervisor doesn't know about this anymore". Jan