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Wed, 13 May 2026 01:31:53 +0000 Date: Wed, 13 May 2026 09:31:43 +0800 From: Chao Gao To: Sean Christopherson CC: Reinette Chatre , , , , , , , , , , , , Subject: Re: VMX Preemption Timer appears to be buggy on SKX, CLX, and ICX Message-ID: References: <2fccf35715b5ba8aec5e5708d86ad7015b8d74e6.1718214999.git.reinette.chatre@intel.com> Content-Type: text/plain; charset="iso-8859-1" Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-ClientProxiedBy: TPYP295CA0006.TWNP295.PROD.OUTLOOK.COM (2603:1096:7d0:9::16) To CH3PR11MB8660.namprd11.prod.outlook.com (2603:10b6:610:1ce::13) Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: CH3PR11MB8660:EE_|SJ1PR11MB6154:EE_ X-MS-Office365-Filtering-Correlation-Id: 698e9f47-ddc6-4417-d057-08deb08f69d5 X-LD-Processed: 46c98d88-e344-4ed4-8496-4ed7712e255d,ExtAddr X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|1800799024|376014|366016|22082099003|18002099003|56012099003|11063799003; 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It looks like something APICv related is clobbering internal VMX timer state? >Or maybe there's a tearing or truncation issue? > >As mentioned ad nauseum at this point, I'm offline all of next week, so hopefully >there's enough info here to get a root cause... > > >A spurious VM-Exit will occur after programming a vmcs.PREEMPTION_TIMER_VALUE that >shouldn't exit. Every observed failure occurs when bits 27:16 are zero, with a >small value in bits 15:0, e.g. VM-Enter with a timer value of 0xe0003bf7 or >0xa0006db6 will cause a near-immediate VM-Exit. This behavior is documented as a CPU erratum. See https://cdrdv2.intel.com/v1/dl/getContent/793902 EMR158. VMX-Preemption Timer May Expire Earlier With Certain Large Timer Values Problem: When the VMX-preemption timer is programmed with certain large values, the timer may expire earlier than expected. Actual values vary by platform and Time Stamp Counter (TSC) frequency. Implication: Due to this erratum, software that relies on long duration VMXpreemption timers may observe VM exits significantly earlier than the programmed interval. Intel has not observed this erratum with any commercially available software. Workaround: A mitigation for this erratum is for software to program the VMXpreemption timer for values below 2^25 × CPUID.15H:EBX[31:0] / CPUID.15H:EAX[31:0].