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Mon, 18 Aug 2025 15:07:11 +0000 Message-ID: <4bd07d7d-f4f8-4175-9384-d90b0e3c6e57@intel.com> Date: Mon, 18 Aug 2025 18:07:06 +0300 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH RFC 1/2] KVM: TDX: Disable general support for MWAIT in guest To: "Brown, Len" CC: , , , , , , , , , , , , , , "Sean Christopherson" References: <20250816144436.83718-1-adrian.hunter@intel.com> <20250816144436.83718-2-adrian.hunter@intel.com> Content-Language: en-US From: Adrian Hunter Organization: Intel Finland Oy, Registered Address: c/o Alberga Business Park, 6 krs, Bertel Jungin Aukio 5, 02600 Espoo, Business Identity Code: 0357606 - 4, Domiciled in Helsinki In-Reply-To: Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit X-ClientProxiedBy: DB9PR02CA0015.eurprd02.prod.outlook.com (2603:10a6:10:1d9::20) To IA1PR11MB7198.namprd11.prod.outlook.com (2603:10b6:208:419::15) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: IA1PR11MB7198:EE_|IA0PR11MB7815:EE_ X-MS-Office365-Filtering-Correlation-Id: e114cc34-3415-4e63-b0da-08ddde68e837 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|1800799024|366016|376014; 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To >> virtualize those, KVM would need the guest not to enable #VE reduction, >> which is not something that KVM can control or even be aware of. Note, >> however, that the consequent unchecked MSR access errors might be harmless. >> >> Without #VE reduction enabled, the TDX Module will inject #VE for MSRs that >> it does not virtualize itself. The guest can then hypercall the host VMM >> for a resolution. >> >> With #VE reduction enabled, accessing MSRs such as the 2 above, results in >> the TDX Module injecting #GP. >> >> Currently, Linux guest opts for #VE reduction unconditionally if it is >> available, refer reduce_unnecessary_ve(). However, the #VE reduction >> feature was not added to the TDX Module until versions 1.5.09 and 2.0.04. >> Refer https://github.com/intel/tdx-module/releases >> >> There is also a further issue experienced by a Linux guest. Prior to >> TDX Module versions 1.5.09 and 2.0.04, the Always-Running-APIC-Timer (ARAT) >> feature (CPUID leaf 6: EAX bit 2) is not exposed. That results in cpuidle >> disabling the timer interrupt and invoking the Tick Broadcast framework >> to provide a wake-up. Currently, that falls back to the PIT timer which >> does not work for TDX, resulting in the guest becoming stuck in the idle >> loop. >> >> Conclusion >> >> User's may expect TDX support of MWAIT in a guest to be similar to VMX >> support, but KVM cannot ensure that. Consequently KVM should not expose >> the capability. >> >> Fixes: 0186dd29a2518 ("KVM: TDX: add ioctl to initialize VM with TDX specific parameters") >> Signed-off-by: Adrian Hunter >> --- > > NAK. > > Fix the guest, or wherever else in the pile there are issues. KVM is NOT carrying > hack-a-fixes to workaround buggy software/firmware. Been there, done that. Thanks for the quick reply. Adding Len Brown for intel_idle. Len, you may recall that MSR_PKG_CST_CONFIG_CONTROL came up in the following context: https://bugzilla.kernel.org/show_bug.cgi?id=218792 https://lore.kernel.org/kvm/bug-218792-28872-5sylPIVpHD@https.bugzilla.kernel.org%2F/ For TDX platforms we would need _safe() MSR access for MSR_POWER_CTL and MSR_PKG_CST_CONFIG_CONTROL. Would that be OK?