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h=From:Date:Subject:Message-ID:Content-Type:MIME-Version:X-MS-Exchange-SenderADCheck; bh=Nnkr2XUcJy0cU69vf0NDpq+bxw60h+0QWEzvKe0tSlQ=; b=SVvS4BSwDKmrBZYsbwjLtB8Byg9G369ToKaO9t7Dl8RFMr5OTM9BBwsaQ+n4PMJmVpVGdUV6yXYp923HT6WOzbDagWqa8eI+Sr2YdTrHgrSrGmPXjJecTX+84qFO87ByCXQn8EGIXGDQfkL48QVLUCeTFGvzUkFeTol+dRnKRvc= 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: Tue, 8 Jul 2025 11:54:32 +0200 Message-ID: To: Stefano Stabellini , CC: , , , , Subject: Re: [PATCH 1/2] xen/x86: don't send IPI to sync TSC when it is reliable From: Alejandro Vallejo X-Mailer: aerc 0.20.1 References: <20250708000712.2731666-1-stefano.stabellini@amd.com> In-Reply-To: <20250708000712.2731666-1-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: BN2PEPF00004FBD:EE_|LV8PR12MB9109:EE_ X-MS-Office365-Filtering-Correlation-Id: 9de288bf-5f4c-4e1f-198d-08ddbe0571f8 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|376014|36860700013|82310400026|1800799024; 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X-OriginatorOrg: amd.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 08 Jul 2025 09:54:35.2244 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 9de288bf-5f4c-4e1f-198d-08ddbe0571f8 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: BN2PEPF00004FBD.namprd04.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: LV8PR12MB9109 On Tue Jul 8, 2025 at 2:07 AM 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. Remove the unnecessary > IPI on all physical CPUs to sync the TSC when the TSC is known to be > reliable. > > Signed-off-by: Stefano Stabellini > --- > xen/arch/x86/time.c | 4 ++++ > 1 file changed, 4 insertions(+) > > diff --git a/xen/arch/x86/time.c b/xen/arch/x86/time.c > index 59129f419d..bfd022174a 100644 > --- a/xen/arch/x86/time.c > +++ b/xen/arch/x86/time.c > @@ -2303,6 +2303,10 @@ static void cf_check time_calibration(void *unused= ) > local_irq_enable(); > } > =20 > + if ( boot_cpu_has(X86_FEATURE_CONSTANT_TSC) && > + boot_cpu_has(X86_FEATURE_TSC_RELIABLE) ) > + return; > + This should check "(tsc_flags & TSC_RELIABLE_SOCKET)" as well. The TSCs mig= ht still be unsynchronized across sockets. I'm still quite confused as to how Xen (mis)handles time, but wouldn't this= need to go inside the branch above? If the clocksource is not the TSC as well th= e TSC can still drift with respect to the actual clocksource (PIT, HPET or ACPI t= imer). If so, we could probably do an early return in the branch above ignoring th= e conditions (they are required for picking the TSC clocksource already, incl= uding synchronization across sockets). Another matter is whether we could drop the "master_stime" write. Would we care about it at all? > cpumask_copy(&r.cpu_calibration_map, &cpu_online_map); > =20 > /* @wait=3D1 because we must wait for all cpus before freeing @r. */ Cheers, Aljeandro