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Message-ID: Date: Tue, 21 Feb 2023 09:48:10 +0100 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:102.0) Gecko/20100101 Thunderbird/102.8.0 Subject: Re: [RFC PATCH v2 1/2] xen/memory : Add a stats_table resource type Content-Language: en-US To: Matias Ezequiel Vara Larsen Cc: Matias Ezequiel Vara Larsen , Andrew Cooper , =?UTF-8?Q?Roger_Pau_Monn=c3=a9?= , Wei Liu , George Dunlap , Julien Grall , Stefano Stabellini , Dario Faggioli , xen-devel@lists.xenproject.org References: <20230216150734.GB2137216@horizon> <20230220165144.GA2653336@horizon> From: Jan Beulich In-Reply-To: <20230220165144.GA2653336@horizon> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-ClientProxiedBy: FR0P281CA0118.DEUP281.PROD.OUTLOOK.COM (2603:10a6:d10:a8::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_|DBBPR04MB7499:EE_ X-MS-Office365-Filtering-Correlation-Id: 4cba2ac6-6928-411d-aa5e-08db13e85b08 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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>>>>> + >>>>> + vcpustats_va = (shared_vcpustatspage_t*)d->vcpustats_page.va; >>>>> + if ( vcpustats_va ) >>>>> + { >>>>> + vcpustats_va->vcpu_info[v->vcpu_id].version = >>>>> + version_update_begin(vcpustats_va->vcpu_info[v->vcpu_id].version); >>>>> + smp_wmb(); >>>>> + memcpy(&vcpustats_va->vcpu_info[v->vcpu_id].runstate_running_time, >>>>> + &v->runstate.time[RUNSTATE_running], >>>>> + sizeof(v->runstate.time[RUNSTATE_running])); >>>>> + smp_wmb(); >>>>> + vcpustats_va->vcpu_info[v->vcpu_id].version = >>>>> + version_update_end(vcpustats_va->vcpu_info[v->vcpu_id].version); >>>>> + } >>>> >>>> A further aspect to consider here is cache line ping-pong. I think the >>>> per-vCPU elements of the array want to be big enough to not share a >>>> cache line. The interface being generic this presents some challenge >>>> in determining what the supposed size is to be. However, taking into >>>> account the extensibility question, maybe the route to take is to >>>> simply settle on a power-of-2 value somewhere between x86'es and Arm's >>>> cache line sizes and the pretty common page size of 4k, e.g. 512 bytes >>>> or 1k? >>>> >>> >>> I do not now how to address this. I was thinking to align each vcpu_stats >>> instance to a multiple of the cache-line. I would pick up the first multiple >>> that is bigger to the size of the vcpu_stats structure. For example, currently >>> the structure is 16 bytes so I would align each instance in a frame to 64 >>> bytes. Would it make sense? >> >> Well, 64 may be an option, but I gave higher numbers for a reason. One thing >> I don't know is what common cache line sizes are on Arm or e.g. RISC-V. > > Thanks. I found that structures that require cache-aligment are defined with > "__cacheline_aligned" that uses L1_CACHE_BYTES. For example, in x86, this > aligns to 128 bytes. What is the reason to use a higher value like 512 bytes or > 1k?. You cannot bake an x86 property (which may even change: at some point we may choose to drop the 128-byte special for the very few CPUs actually using such, when the majority uses 64-byte cache lines) into the public interface. You also don't want to make an aspect of the public interface arch-dependent when not really needed. My suggestion for a higher value was in the hope that we may never see a port to an architecture with cache lines wider than, say, 512 bytes. What exactly the value should be is of course up for discussion, but I think it wants to include some slack on top of what we currently support (arch-wise). Jan