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Message-ID: Date: Thu, 22 Dec 2022 11:03:06 +0100 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:102.0) Gecko/20100101 Thunderbird/102.6.0 Subject: Re: [PATCH v5 08/10] x86/mm: make code robust to future PAT changes Content-Language: en-US To: Demi Marie Obenour Cc: =?UTF-8?Q?Marek_Marczykowski-G=c3=b3recki?= , Andrew Cooper , =?UTF-8?Q?Roger_Pau_Monn=c3=a9?= , Wei Liu , Jun Nakajima , Kevin Tian , George Dunlap , Tim Deegan , xen-devel@lists.xenproject.org References: <06dff83a-b120-a2b4-c61f-7864935d4c3e@suse.com> <700e86ac-7efc-9d6d-43dd-a06bb541a1fb@suse.com> From: Jan Beulich In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-ClientProxiedBy: FR3P281CA0185.DEUP281.PROD.OUTLOOK.COM (2603:10a6:d10:a4::18) To AM6PR04MB6551.eurprd04.prod.outlook.com (2603:10a6:20b:fa::20) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: AM6PR04MB6551:EE_|AS8PR04MB8820:EE_ X-MS-Office365-Filtering-Correlation-Id: f1951c8a-19f1-48b4-851a-08dae403ba56 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0; 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>>>>> + >>>>> + /* >>>>> + * _PAGE_WB must be zero for several reasons, not least because Linux >>>>> + * assumes it. >>>>> + */ >>>>> + BUILD_BUG_ON(_PAGE_WB); >>>> >>>> Strictly speaking this is a requirement only for PV guests. We may not >>>> want to go as far as putting "#ifdef CONFIG_PV" around it, but at least >>>> the code comment (and then also the part of the description referring >>>> to this) will imo want to say so. >>> >>> Does Xen itself depend on this? >> >> With the wording in the description I was going from the assumption that >> you have audited code and found that we properly use _PAGE_* constants >> everywhere. > > There could be other hard-coded uses of magic numbers I haven’t found, > and _PAGE_WB is currently zero so I would be quite surpised if no code > in Xen omits it. Linux also has a BUILD_BUG_ON() to check the same > thing. Fair enough - please adjust description and comment text accordingly then. Jan