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Fri, 5 Jun 2026 02:56:18 +0000 Date: Fri, 5 Jun 2026 10:56:06 +0800 From: Chao Gao To: Jim Mattson CC: Sean Christopherson , Reinette Chatre , , , , , , , , , , , , Yosry Ahmed 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="utf-8" Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-ClientProxiedBy: SI2PR01CA0036.apcprd01.prod.exchangelabs.com (2603:1096:4:192::22) To BN7PR11MB2836.namprd11.prod.outlook.com (2603:10b6:406:ad::26) 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: BN7PR11MB2836:EE_|PH0PR11MB7544:EE_ X-MS-Office365-Filtering-Correlation-Id: e27df6d1-c29e-4009-b2fc-08dec2ae039d 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|7416014|376014|366016|1800799024|6133799003|22082099003|18002099003|56012099006|4143699003|5023799004|11063799006; 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VMX-Preemption Timer May Expire Earlier With Certain Large Timer Values >> > > > >> > > >I assume the same erratum applies to previous generations as well? >> > > >> > > Yes. >> > >> > This test still fails on our SKX, CLX, and ICX systems. >> > >> > Sean, >> > >> > Were you thinking of enforcing a cap on delta_tsc in vmx_set_hv_timer()? >> >> Heh, to be honest, I wasn't thinking of a whole lot of nothing. Falling back to >> hrtimers does seem like the easiest solution. > >I think vmx_set_hv_timer() should return -EINVAL for values impacted >by this erratum. However, the only documented issue is for EMR, and we >have not observed the problem on EMR. That's unsettling. Could you clarify what tests you ran? I am using the reproducer from Yuan: https://lore.kernel.org/kvm/20240708055559.rl4w5xfhj3uru6j2@yy-desk-7060/ I write -1 to the VMX preemption timer, do VM-Enter, and have the guest execute VMCALL to force a VM-Exit. On VM-Exit, we read back the preemption timer. The delta should be very small; otherwise, the platform likely has the same issue. I tested several platforms, including EMR. The results are consistent with the erratum, i.e., I observed premature VMX preemption-timer VM-Exits, and the documented limit did not trigger premature VMX preemption-timer VM-Exits in my testing. > >Chao: > >1) Should we just assume that all Intel CPUs are affected? I think that is reasonable unless we have explicit evidence to exclude specific parts. > >2) Is there any compelling reason not to simplify the limit to 2^25? We can use 2^25 as a conservative bound, but it is much lower than necessary. The current bound comes from theoretical analysis and was validated on multiple platforms. > >3) Is it just coincidence that 25 + IA32_VMX_MISC[4:0] (on EMR) == 32, >or should the limit be calculated as 32 - IA32_VMX_MISC[4:0]? My understanding is that hardware scales the preemption-timer value and converts it to a 32-bit core crystal clock counter, rather than directly using a 32-bit TSC delta. IA32_VMX_MISC[4:0] likely participates in that calculation.