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Message-ID: Date: Wed, 26 Oct 2022 13:48:27 +0200 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:102.0) Gecko/20100101 Thunderbird/102.4.0 Subject: Re: [PATCH] xen/x86: Make XEN_DOMCTL_get_vcpu_msrs more configurable Content-Language: en-US To: Tamas K Lengyel Cc: Tamas K Lengyel , Xen-devel , Wei Liu , Anthony Perard , Juergen Gross , Roger Pau Monne , Jun Nakajima , Kevin Tian , Andrew Cooper References: <854cdedcdd2bfff08ea45a3c13367c610d710aaf.1666630317.git.tamas.lengyel@intel.com> <0c7ca888-3142-1719-13c2-4f006ecdf683@citrix.com> From: Jan Beulich In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-ClientProxiedBy: FR3P281CA0034.DEUP281.PROD.OUTLOOK.COM (2603:10a6:d10:1c::20) To VE1PR04MB6560.eurprd04.prod.outlook.com (2603:10a6:803:122::25) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: VE1PR04MB6560:EE_|AM8PR04MB7476:EE_ X-MS-Office365-Filtering-Correlation-Id: 719579dd-7c41-454f-ce0f-08dab747ffa0 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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In our use-case gathering the vPMU MSRs by an >>> external privileged tool is necessary, thus we extend the domctl to >> allow for >>> querying for any guest MSRs. To remain compatible with the existing >> setup if >>> no specific MSR is requested via the domctl the default list is returned. >>> >>> Signed-off-by: Tamas K Lengyel >> >> Naming aside, XEN_DOMCTL_{get,set}_vcpu_msrs is supposed to be "get me >> all MSRs needed to migrate a vCPU". (I do intend to retire the >> hypercall as part of fixing the Xen side of migration, but that's ages >> away) >> >> It seems like what you want is something more like >> XEN_DOMCTL_{rd,wr}msr_list (convenient timing, given the recent ISE >> update). I think those would be better as a separate pair of >> hypercalls, rather than trying to repurpose an existing hypercall. >> >> >> As for actually getting the values, please fix up guest_{rd,wr}msr() to >> access the perf MSRs safely. I know the vpmu MSR handling is in a >> tragic state, but this new get_msr subop is making the problem even more >> tangled. >> > > Adding a separate hypercall is fine. Which will then also avoid altering the behavior of the existing hypercall: You can't just assume e.g. input fields to be zeroed (or otherwise suitably set) by existing callers, when those were previously output only. Jan