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Message-ID: Date: Thu, 27 Oct 2022 08:56:31 +0200 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:102.0) Gecko/20100101 Thunderbird/102.4.0 Subject: Re: [PATCH 1/4] xen: Introduce non-broken hypercalls for the p2m pool size Content-Language: en-US To: Julien Grall Cc: Roger Pau Monne , Wei Liu , Stefano Stabellini , Volodymyr Babchuk , Bertrand Marquis , Henry Wang , Anthony Perard , Xen-devel , Andrew Cooper References: <20221026102018.4144-1-andrew.cooper3@citrix.com> <20221026102018.4144-2-andrew.cooper3@citrix.com> <0f048bd2-d08c-8bd5-2a20-7e49e794c679@citrix.com> <4c713750-670b-eac1-5f5b-376de79192eb@xen.org> From: Jan Beulich In-Reply-To: <4c713750-670b-eac1-5f5b-376de79192eb@xen.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-ClientProxiedBy: FR3P281CA0043.DEUP281.PROD.OUTLOOK.COM (2603:10a6:d10:4a::15) To VE1PR04MB6560.eurprd04.prod.outlook.com (2603:10a6:803:122::25) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: VE1PR04MB6560:EE_|AM7PR04MB6902:EE_ X-MS-Office365-Filtering-Correlation-Id: 77e1e7e9-c2a1-4dff-60f1-08dab7e8620d X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0; 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>>>> } >>>> >>>> +int hap_get_allocation_bytes(struct domain *d, uint64_t *size) >>>> +{ >>>> + unsigned long pages = (d->arch.paging.hap.total_pages + >>>> + d->arch.paging.hap.p2m_pages); >>> Unlike for Arm no ACCESS_ONCE() here? Also the addition can in >>> principle overflow, because being done only in 32 bits. >> >> I'm not actually convinced ARM needs ACCESS_ONCE() to begin with.  I >> can't see any legal transformation of that logic which could result in a >> torn load. > > AFAIU, ACCESS_ONCE() is not only about torn load but also making sure > that the compiler will only read the value once. > > When LTO is enabled (not yet supported) in Xen, can we guarantee the > compiler will not try to access total_pages twice (obviously it would be > caller dependent)? Aren't all accesses (supposed to be) under paging lock? At which point there's no issue with multiple (or torn) accesses? Jan