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h=From:Date:Subject:Message-ID:Content-Type:MIME-Version:X-MS-Exchange-SenderADCheck; bh=eVfrjj51kzoFCKFr0dUpXaVy/wgbKEn9Bgu7PzAwDOQ=; b=NHDV0ons+hJZMpDVwWsQ9s5hrdorh4gcPZ+EVYe7isRZEc1kBiYER3z6eE+kzJSEyaskYUOXk5dF+bm9Qc5H3DlD1frGFlW0C0tsBNBrV+8eswkB7d2L09A43OSnzSs8q3zg/BuV6/tjZYHMElc9IIX8vJVOF6y6azdgWLc5yKg= 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 19:53:02 +0200 Message-ID: From: Alejandro Vallejo To: Stefano Stabellini CC: , , , , , Subject: Re: [PATCH 1/2] xen/x86: don't send IPI to sync TSC when it is reliable X-Mailer: aerc 0.20.1 References: <20250708000712.2731666-1-stefano.stabellini@amd.com> In-Reply-To: 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: BN3PEPF0000B06C:EE_|LV8PR12MB9084:EE_ X-MS-Office365-Filtering-Correlation-Id: ff243a00-355c-4aa9-3c83-08ddbe484b90 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|82310400026|376014|36860700013|1800799024; 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X-OriginatorOrg: amd.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 08 Jul 2025 17:53:07.0716 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: ff243a00-355c-4aa9-3c83-08ddbe484b90 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: BN3PEPF0000B06C.namprd21.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: LV8PR12MB9084 On Tue Jul 8, 2025 at 7:40 PM CEST, Stefano Stabellini wrote: > On Tue, 8 Jul 2025, Alejandro Vallejo wrote: >> 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 cou= ld >> > 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 *unu= sed) >> > local_irq_enable(); >> > } >> > =20 >> > + if ( boot_cpu_has(X86_FEATURE_CONSTANT_TSC) && >> > + boot_cpu_has(X86_FEATURE_TSC_RELIABLE) ) >> > + return; >> > + >>=20 >> This should check "(tsc_flags & TSC_RELIABLE_SOCKET)" as well. The TSCs = might >> still be unsynchronized across sockets. >> >> I'm still quite confused as to how Xen (mis)handles time, but wouldn't t= his need >> to go inside the branch above? If the clocksource is not the TSC as well= the TSC >> can still drift with respect to the actual clocksource (PIT, HPET or ACP= I timer). > > I can move it inside the previous if > > >> If so, we could probably do an early return in the branch above ignoring= the >> conditions (they are required for picking the TSC clocksource already, i= ncluding >> synchronization across sockets). >>=20 >> Another matter is whether we could drop the "master_stime" write. Would = we >> care about it at all? > > I'll drop it. > > Is this what you had in mind? > > > 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 Yes, I think that would do. Cheers, Alejandro