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Message-ID: Date: Thu, 15 Dec 2022 09:46:41 +0100 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:102.0) Gecko/20100101 Thunderbird/102.5.1 Subject: Re: [PATCH v3 01/12] x86/mm: Avoid hard-coding PAT in get_page_from_l1e() 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: <6a0d969d0aade0258e108009ef33ad5d2663e969.1671057808.git.demi@invisiblethingslab.com> From: Jan Beulich In-Reply-To: <6a0d969d0aade0258e108009ef33ad5d2663e969.1671057808.git.demi@invisiblethingslab.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-ClientProxiedBy: FR3P281CA0093.DEUP281.PROD.OUTLOOK.COM (2603:10a6:d10:a1::13) To VE1PR04MB6560.eurprd04.prod.outlook.com (2603:10a6:803:122::25) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: VE1PR04MB6560:EE_|PA4PR04MB7760:EE_ X-MS-Office365-Filtering-Correlation-Id: 93ea7075-1c7b-4970-6474-08dade78e453 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0; 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Use the proper _PAGE_* constants > instead. Also, treat the two unused cases as if they are cacheable, as > future changes may make them cacheable. This still does not cover ... > --- a/xen/arch/x86/mm.c > +++ b/xen/arch/x86/mm.c > @@ -959,14 +959,19 @@ get_page_from_l1e( > flip = _PAGE_RW; > } > > + /* Force cacheable memtypes to UC */ > switch ( l1f & PAGE_CACHE_ATTRS ) > { > - case 0: /* WB */ > - flip |= _PAGE_PWT | _PAGE_PCD; > + case _PAGE_UC: > + case _PAGE_UCM: > + case _PAGE_WC: > + /* not cached */ > break; > - case _PAGE_PWT: /* WT */ > - case _PAGE_PWT | _PAGE_PAT: /* WP */ > - flip |= _PAGE_PCD | (l1f & _PAGE_PAT); > + case _PAGE_WB: > + case _PAGE_WT: > + case _PAGE_WP: > + default: > + flip |= (l1f & PAGE_CACHE_ATTRS) ^ _PAGE_UC; > break; ... the three cases here assuming certain properties wrt the flipping of _PAGE_UC. As said before - going from one kind of assumption to another _may_ be a good thing to do, but needs justifying as actually being an improvement. Alternatively such assumptions could be checked by suitable BUILD_BUG_ON(), which then at the same serve as documentation thereof. Jan