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mx.microsoft.com 1; spf=pass smtp.mailfrom=citrix.com; dmarc=pass action=none header.from=citrix.com; dkim=pass header.d=citrix.com; arc=none DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=citrix.onmicrosoft.com; s=selector2-citrix-onmicrosoft-com; h=From:Date:Subject:Message-ID:Content-Type:MIME-Version:X-MS-Exchange-SenderADCheck; bh=dk0QKkx8KOD+aEZhcQ5FynwWqr2Jayi3cggBH2hmmCM=; b=glInX5RN0Nb1orRft62IEf8d7/b6KOXzV97WviHhtMG7dSspvr0WqsE8NUSqixH8Sapr1jvlmlPVFlsZ6gYENoyvqIjW+0ZZFW+968AlzFPeso15PW8APTyGa1qEBEQ3c95CzemvPrhL2qDlhGrlMpaw3zih9wAloTRyKCo5c9M= Date: Tue, 23 Feb 2021 16:37:15 +0100 From: Roger Pau =?utf-8?B?TW9ubsOp?= To: Jan Beulich CC: "xen-devel@lists.xenproject.org" , "Andrew Cooper" , Wei Liu , George Dunlap , Ian Jackson Subject: Re: [PATCH v2 8/8] x86/PV: use get_unsafe() instead of copy_from_unsafe() Message-ID: References: <0a59ae2f-448e-610d-e8a2-a7c3f9f3918f@suse.com> <76207250-1372-e7ab-2d03-b46020a7906b@suse.com> Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <76207250-1372-e7ab-2d03-b46020a7906b@suse.com> X-ClientProxiedBy: MR2P264CA0111.FRAP264.PROD.OUTLOOK.COM (2603:10a6:500:33::27) To DS7PR03MB5608.namprd03.prod.outlook.com (2603:10b6:5:2c9::18) MIME-Version: 1.0 X-MS-Exchange-MessageSentRepresentingType: 1 X-MS-PublicTrafficType: Email X-MS-Office365-Filtering-Correlation-Id: 7294105f-1963-4b62-578d-08d8d810e93a X-MS-TrafficTypeDiagnostic: DM6PR03MB4969: X-MS-Exchange-Transport-Forked: True X-Microsoft-Antispam-PRVS: X-MS-Oob-TLC-OOBClassifiers: OLM:2331; 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X-MS-Exchange-AntiSpam-MessageData: =?utf-8?B?bTVHTlBCbE5EUVphUm1wdzF1WmI1cFljRmhvdnNMVUFlQnI1d1ltRkZtQ0Fq?= =?utf-8?B?T1A2UG1ISXlQNGYrd3RSUDR1RTF2Z2tXbmdGQ2h3NDc2R1lYbGwzb2dDdnp4?= =?utf-8?B?cFIrZWZWdGhlOWF2RTVIZlJEM0ppR3hHcVBVUWUwZjI2cjYxcXlkSWtVOGkw?= =?utf-8?B?bHRmQzB5NkRJcVo2WCtXbnZHTmszQTYvL3RCSFRtVVVHY2NWbElyNVVtWXBi?= =?utf-8?B?dUNabzl1dzNFMi9iQkVwdmN3ZWtoWEVYaEFuaUtQL1BhMGFpR2hYRmlmQnhk?= =?utf-8?B?UDF2U1kzOElERXlvUjByS2xjY20yRnlYNTdWdWh5aDhrTm9pc0g1RmhuRTZv?= =?utf-8?B?Yys1TFFUOTdJc3dUeXMwb25IMU4vUkhVUmJYempwL1VGUDlQa280aHV5emwv?= =?utf-8?B?Y2tkOUdJN2E3S1M3UTBWaWNaYzBTNkV5Z09EMUs5TjVNKzlOb0pvNkdBMzM2?= =?utf-8?B?NGlyT25scGlKZmhRVG1Yck1la2s2Y1hzYm1KR2NmUERmS2xmODlVakJaWjlY?= =?utf-8?B?MXpUUkxhSnNNOWZpc3pRbWpSWXZtSnJHSVRCYnZkMDk1Y2p3b0Z4eXo3NExw?= =?utf-8?B?Zk1VVUk2d3hoaEJORkdJdkpMNU1LeDJKL2xoUXVWYlNjMFp2eEVPek5VTWQz?= =?utf-8?B?VmpLZjhJeGZMWTQ0SWlsa3I1YlBMc2hFVFdIQ0Z3dDM5VWFSY25IMUNBa3lT?= =?utf-8?B?ZVNPTll2a3g2UndaNEQ5KzNEUTNiRThCUVU2aC8wNnhxRUhtU2lhRkc0Sll3?= =?utf-8?B?SEFibnJ5dS8wbU9YWnNteGVnb2dTd2ptUXNvZ29zL1R1UVQrNTZuYWhodFRj?= =?utf-8?B?QlY5ZlNtL3BXalkxWWpzWmlxSVNsbjNiRjhBMmozSWpKMHgzY0R5dEtYVEhw?= =?utf-8?B?SUhlelZxaXNpZnVGOW9lcUxNUWNtNkxWSXZtVjN4Mm8xYkgzZHVQNEI5ZFJi?= =?utf-8?B?VTRJK2IzMDBKalgycTNSS3Z1cTdmdTl6MmxMSjM1aGR2MjZicnBYRGpKeTN5?= =?utf-8?B?WG0yNDJHUFc1YWY5RmtRT29IbmRJb0N4STZxY2FHQXJKQWJKeGlCYlR1VXc5?= =?utf-8?B?eWlHTFowRy94UWhjMGUvSyt2UHgxWEtJT1RaY08wbFB1VGluU3AzUGh2WS9T?= =?utf-8?B?d0JuZWlDVkZveGtQc1Fjc0FmMlpiNmRuVkFOY0sxYXJHTDBQODVIK1QwcWVV?= =?utf-8?B?YUlvV0JXNmlrelROL3hsVXhyWDU2YlducVQ2UlFwSk9aMEp6K0pXeXRGeDNW?= =?utf-8?B?dEl1U01hMDJCcHlldVc0cWJQU1FERktRQUFDRXZPZ0RobGl0QXoxUDNGakp0?= =?utf-8?B?bkw0VEV6OXJ6TUF0d0ZNMHRrYkFJRndhdGhvMGhzMTNuU0hjbTBSNGc4K2RK?= =?utf-8?B?MHpVNkU1ZzZyU1ZldVphV3M4T2NXaitHdTdrc2tZKzFnYTF1aFJPazBFaHlB?= =?utf-8?B?UHcvU0M5MU9iLy9DUDdhUmlBWW9BWEVXMzVaYnRRRlBIWnBvM0tzaitMb0w0?= =?utf-8?B?dXJLUEt0RkJ1T3BiY1NIbW9RUk1BRENrZFdBRVhVajNQZFZ3ckNOdlFKZ2w0?= =?utf-8?B?L0N2dlhUZmd4akVZR2ZkMURiVFhNUUxkWXNTTXdyb2FyWG5zRmxiVzJYNVFZ?= =?utf-8?B?aUZRa1JLKzgrUE81aFVYVnlzSXRoY3VjWTUySjN4eWV4VWVOVVd1a0o3eHd5?= =?utf-8?B?NnpzWlowdURqOGd4eGgrcUh2QUpCRzVucmZRWXRlV0s1NzhLaXIzOUxXeStL?= =?utf-8?B?MkppY0JLb0IrcjdoMnNuaTNOUUZ3VDNCMEpOZWhneGQ5NEVFa3JOenhBM1M5?= =?utf-8?B?a3BpaXZmNGpJN3Qzd0Q4Zz09?= X-MS-Exchange-CrossTenant-Network-Message-Id: 7294105f-1963-4b62-578d-08d8d810e93a X-MS-Exchange-CrossTenant-AuthSource: DS7PR03MB5608.namprd03.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-OriginalArrivalTime: 23 Feb 2021 15:37:21.9495 (UTC) X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted X-MS-Exchange-CrossTenant-Id: 335836de-42ef-43a2-b145-348c2ee9ca5b X-MS-Exchange-CrossTenant-MailboxType: HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: uuh8zY6wl47UQ48aQVUXtJtbe6f0s3FFHj/3UR1PPagr3ZEUvrPCHTQnY+5ooF0W2aeNgLy5f/ebM67o4d/H4Q== X-MS-Exchange-Transport-CrossTenantHeadersStamped: DM6PR03MB4969 X-OriginatorOrg: citrix.com On Tue, Feb 23, 2021 at 04:25:00PM +0100, Jan Beulich wrote: > On 23.02.2021 12:59, Roger Pau Monné wrote: > > On Wed, Feb 17, 2021 at 09:23:33AM +0100, Jan Beulich wrote: > >> The former expands to a single (memory accessing) insn, which the latter > >> does not guarantee. Yet we'd prefer to read consistent PTEs rather than > >> risking a split read racing with an update done elsewhere. > >> > >> Signed-off-by: Jan Beulich > > > > Reviewed-by: Roger Pau Monné > > > > Albeit I wonder why the __builtin_constant_p check done in > > copy_from_unsafe is not enough to take the get_unsafe_size branch in > > there. Doesn't sizeof(l{1,2}_pgentry_t) qualify as a built time > > constant? > > > > Or the fact that n it's a parameter to an inline function hides this, > > in which case the __builtin_constant_p is pointless? > > Without (enough) optimization, __builtin_constant_p() may indeed > yield false in such cases. But that wasn't actually what I had > in mind when making this change (and the original similar on in > shadow code). Instead, at the time I made the shadow side change, > I had removed this optimization from the new function flavors. > With that removal, things are supposed to still be correct - it's > an optimization only, after all. Meanwhile the optimizations are > back, so there's no immediate problem as long as the optimizer > doesn't decide to out-of-line the function invocations (we > shouldn't forget that even always_inline is not a guarantee for > inlining to actually occur). I'm fine with you switching those use cases to get_unsafe, but I think the commit message should be slightly adjusted to notice that copy_from_unsafe will likely do the right thing, but that it's simply clearer to call get_unsafe directly, also in case copy_from_unsafe gets changed in the future to drop the get_unsafe paths. Thanks, Roger.