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h=From:Date:Subject:Message-ID:Content-Type:MIME-Version:X-MS-Exchange-SenderADCheck; bh=BLdhdSRqr4ds7QWEI04Rz6nogbT2BOCixOAhIDszOg8=; b=5n0jZjcvcAx8OHzgIId9CVLx9YueAM180xQQ4/F7XjvOSTNRQwXRnBtCSJrZASUmIPJDuspe/n8honrAjOG3JzXG+tYx6dXYI7w0QeRZ1DYZGTdkhcAFvCEKWubrmNCVXpUiA9cyY0x55/oNiWsCKoTK+5YufqL/6smpODAxAFM= X-MS-Exchange-Authentication-Results: spf=pass (sender IP is 165.204.84.17) smtp.mailfrom=amd.com; dkim=none (message not signed) header.d=none;dmarc=pass action=none header.from=amd.com; Received-SPF: Pass (protection.outlook.com: domain of amd.com designates 165.204.84.17 as permitted sender) receiver=protection.outlook.com; client-ip=165.204.84.17; helo=SATLEXMB04.amd.com; pr=C MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="UTF-8" Date: Wed, 9 Jul 2025 11:44:38 +0200 Message-ID: Subject: Re: [PATCH v2 1/2] xen/x86: don't send IPI to sync TSC when it is reliable From: Alejandro Vallejo To: Stefano Stabellini , CC: , , , , X-Mailer: aerc 0.20.1 References: <20250708183238.3104638-1-stefano.stabellini@amd.com> In-Reply-To: <20250708183238.3104638-1-stefano.stabellini@amd.com> X-Originating-IP: [10.180.168.240] X-ClientProxiedBy: SATLEXMB03.amd.com (10.181.40.144) To SATLEXMB04.amd.com (10.181.40.145) X-EOPAttributedMessage: 0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: BL6PEPF00020E65:EE_|MN0PR12MB5739:EE_ X-MS-Office365-Filtering-Correlation-Id: 22343639-1bb0-4853-9f45-08ddbecd3a81 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|36860700013|376014|82310400026|1800799024; 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X-OriginatorOrg: amd.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 09 Jul 2025 09:44:41.5187 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 22343639-1bb0-4853-9f45-08ddbecd3a81 X-MS-Exchange-CrossTenant-Id: 3dd8961f-e488-4e60-8e11-a82d994e183d X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=3dd8961f-e488-4e60-8e11-a82d994e183d;Ip=[165.204.84.17];Helo=[SATLEXMB04.amd.com] X-MS-Exchange-CrossTenant-AuthSource: BL6PEPF00020E65.namprd04.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: MN0PR12MB5739 On Tue Jul 8, 2025 at 8:32 PM CEST, Stefano Stabellini wrote: > On real time configuration with the null scheduler, we shouldn't > interrupt the guest execution unless strictly necessary: the guest could > be a real time guest (e.g. FreeRTOS) and interrupting its execution > could lead to a missed deadline. The principal source of interruptions > is IPIs. > > When TSC is the chosen clocksource, we know it is reliable and > synchronized across cpus and clusters. Thus, we can return early > time_calibration because the calibration is not needed, removing the > related Xen timer and IPIs. > > Also remove the master_stime write as it is unnecessary. > > Signed-off-by: Stefano Stabellini > --- > Changes in v2: > - simplify the patch simply by returning early if clocksource_is_tsc() > - also remove setting r.master_stime as it is not needed > --- > xen/arch/x86/time.c | 6 +----- > 1 file changed, 1 insertion(+), 5 deletions(-) > > diff --git a/xen/arch/x86/time.c b/xen/arch/x86/time.c > index 59129f419d..d72e640f72 100644 > --- a/xen/arch/x86/time.c > +++ b/xen/arch/x86/time.c > @@ -2297,11 +2297,7 @@ static void cf_check time_calibration(void *unused= ) > }; > =20 > if ( clocksource_is_tsc() ) > - { > - local_irq_disable(); > - r.master_stime =3D read_platform_stime(&r.master_tsc_stamp); > - local_irq_enable(); > - } > + return; > =20 > cpumask_copy(&r.cpu_calibration_map, &cpu_online_map); > =20 As far as I can tell, this shouldn't cause problems. But I'd prefer if some= one knowledgeable in the calibration code (Jan?) pitches in as to the effects o= f master_stime in the absence of calibration. Otherwise: Reviewed-by: Alejandro Vallejo Cheers, Alejandro