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Wed, 31 Dec 2025 03:31:47 +0000 Date: Wed, 31 Dec 2025 11:31:37 +0800 From: Chao Gao To: Sean Christopherson CC: Paolo Bonzini , , , Xin Li , Yosry Ahmed Subject: Re: [PATCH v2 2/2] KVM: nVMX: Remove explicit filtering of GUEST_INTR_STATUS from shadow VMCS fields Message-ID: References: <20251230220220.4122282-1-seanjc@google.com> <20251230220220.4122282-3-seanjc@google.com> Content-Type: text/plain; charset="us-ascii" Content-Disposition: inline In-Reply-To: <20251230220220.4122282-3-seanjc@google.com> X-ClientProxiedBy: KL1PR0401CA0006.apcprd04.prod.outlook.com (2603:1096:820:f::11) 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_|LV2PR11MB5976:EE_ X-MS-Office365-Filtering-Correlation-Id: da67c2fe-6876-423d-1bca-08de481d20be X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|1800799024|376014|366016|7053199007; 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IIUC, the construction of the shadow VMCS bitmap and fields doesn't reference "the set of supported vmcs12 fields". So, with the filtering dropped, copy_shadow_to_vmcs12() and copy_vmcs12_to_shadow() may access GUEST_INTR_STATUS on unsupported hardware. Do we need something like this (i.e., don't shadow unsupported vmcs12 fields) diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c index f50d21a6a2d7..08433b3713d2 100644 --- a/arch/x86/kvm/vmx/nested.c +++ b/arch/x86/kvm/vmx/nested.c @@ -127,6 +127,8 @@ static void init_vmcs_shadow_fields(void) continue; break; default: + if (!cpu_has_vmcs12_field(field)) + continue; break; } > >Note, there is technically a small functional change here, as the vmcs12 >filtering only requires support for Virtual Interrupt Delivery, whereas >the shadow VMCS code being removed required "full" APICv support, i.e. >required Virtual Interrupt Delivery *and* APIC Register Virtualizaton *and* >Posted Interrupt support. > >Opportunistically tweak the comment to more precisely explain why the >PML and VMX preemption timer fields need to be explicitly checked. > >Signed-off-by: Sean Christopherson >--- > arch/x86/kvm/vmx/nested.c | 11 ++++------- > 1 file changed, 4 insertions(+), 7 deletions(-) > >diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c >index 9d8f84e3f2da..f50d21a6a2d7 100644 >--- a/arch/x86/kvm/vmx/nested.c >+++ b/arch/x86/kvm/vmx/nested.c >@@ -112,9 +112,10 @@ static void init_vmcs_shadow_fields(void) > "Update vmcs12_write_any() to drop reserved bits from AR_BYTES"); > > /* >- * PML and the preemption timer can be emulated, but the >- * processor cannot vmwrite to fields that don't exist >- * on bare metal. >+ * KVM emulates PML and the VMX preemption timer irrespective >+ * of hardware support, but shadowing their related VMCS fields >+ * requires hardware support as the CPU will reject VMWRITEs to >+ * fields that don't exist. > */ > switch (field) { > case GUEST_PML_INDEX: >@@ -125,10 +126,6 @@ static void init_vmcs_shadow_fields(void) > if (!cpu_has_vmx_preemption_timer()) > continue; > break; >- case GUEST_INTR_STATUS: >- if (!cpu_has_vmx_apicv()) >- continue; >- break; > default: > break; > } >-- >2.52.0.351.gbe84eed79e-goog >