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Fri, 10 Jul 2026 05:56:08 -0700 (PDT) Received: from fedora ([91.219.240.20]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-47aa0f213e8sm58123582f8f.34.2026.07.10.05.56.07 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 10 Jul 2026 05:56:07 -0700 (PDT) From: Vitaly Kuznetsov To: Sean Christopherson , Paolo Bonzini Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2 1/3] KVM: x86: Reject nested CAP enablement if nested virtualization is disabled In-Reply-To: <20260630202828.440724-2-seanjc@google.com> References: <20260630202828.440724-1-seanjc@google.com> <20260630202828.440724-2-seanjc@google.com> Date: Fri, 10 Jul 2026 14:56:07 +0200 Message-ID: <87qzlbf1g8.fsf@redhat.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain Sean Christopherson writes: > Add a flag to explicitly track if nested virtualization is enabled, and use > it enumerate that various nested CAPs are unsupported, and to reject > enablement of said CAPs. When the nested ops hooks were moved to their > own structure, KVM's NULL-by-default behavior was deliberately dropped, > with the changelog asserting that all was well. That wasn't quite true; > there is no danger to KVM, but now KVM is over-reporting support for > KVM_CAP_NESTED_STATE and KVM_CAP_HYPERV_ENLIGHTENED_VMCS. > > Fixes: 33b22172452f ("KVM: x86: move nested-related kvm_x86_ops to a separate struct") > Signed-off-by: Sean Christopherson > --- > arch/x86/include/asm/kvm_host.h | 2 ++ > arch/x86/kvm/hyperv.c | 3 ++- > arch/x86/kvm/svm/svm.c | 1 + > arch/x86/kvm/vmx/vmx.c | 1 + > arch/x86/kvm/x86.c | 12 +++++++----- > 5 files changed, 13 insertions(+), 6 deletions(-) > > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h > index b517257a6315..36de689a579c 100644 > --- a/arch/x86/include/asm/kvm_host.h > +++ b/arch/x86/include/asm/kvm_host.h > @@ -1906,6 +1906,8 @@ struct kvm_x86_ops { > }; > > struct kvm_x86_nested_ops { > + bool enabled; > + > void (*leave_nested)(struct kvm_vcpu *vcpu); > bool (*is_exception_vmexit)(struct kvm_vcpu *vcpu, u8 vector, > u32 error_code); > diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c > index 1ee0d23f8949..7184622992d0 100644 > --- a/arch/x86/kvm/hyperv.c > +++ b/arch/x86/kvm/hyperv.c > @@ -2787,7 +2787,8 @@ int kvm_get_hv_cpuid(struct kvm_vcpu *vcpu, struct kvm_cpuid2 *cpuid, > }; > int i, nent = ARRAY_SIZE(cpuid_entries); > > - if (kvm_x86_ops.nested_ops->get_evmcs_version) > + if (kvm_x86_ops.nested_ops->enabled && > + kvm_x86_ops.nested_ops->get_evmcs_version) > evmcs_ver = kvm_x86_ops.nested_ops->get_evmcs_version(vcpu); > > if (cpuid->nent < nent) > diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c > index ef69a51ab27f..163440fca59f 100644 > --- a/arch/x86/kvm/svm/svm.c > +++ b/arch/x86/kvm/svm/svm.c > @@ -5646,6 +5646,7 @@ static __init int svm_hardware_setup(void) > if (r) > return r; > } > + svm_nested_ops.enabled = nested; > > /* > * KVM's MMU doesn't support using 2-level paging for itself, and thus > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c > index 3681d565f177..ef9e1bc39edb 100644 > --- a/arch/x86/kvm/vmx/vmx.c > +++ b/arch/x86/kvm/vmx/vmx.c > @@ -8786,6 +8786,7 @@ __init int vmx_hardware_setup(void) > if (r) > return r; > } > + vmx_nested_ops.enabled = nested; > > kvm_set_posted_intr_wakeup_handler(pi_wakeup_handler); > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c > index 0626e835e9eb..cd9a5ff47f6f 100644 > --- a/arch/x86/kvm/x86.c > +++ b/arch/x86/kvm/x86.c > @@ -2346,7 +2346,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext) > r &= ~KVM_X2APIC_ENABLE_SUPPRESS_EOI_BROADCAST; > break; > case KVM_CAP_NESTED_STATE: > - r = kvm_x86_ops.nested_ops->get_state ? > + r = kvm_x86_ops.nested_ops->enabled ? > kvm_x86_ops.nested_ops->get_state(NULL, NULL, 0) : 0; > break; > #ifdef CONFIG_KVM_HYPERV > @@ -2354,7 +2354,8 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext) > r = kvm_x86_ops.enable_l2_tlb_flush != NULL; > break; > case KVM_CAP_HYPERV_ENLIGHTENED_VMCS: > - r = kvm_x86_ops.nested_ops->enable_evmcs != NULL; > + r = kvm_x86_ops.nested_ops->enabled && > + kvm_x86_ops.nested_ops->enable_evmcs != NULL; > break; > #endif > case KVM_CAP_SMALLER_MAXPHYADDR: > @@ -3366,7 +3367,8 @@ static int kvm_vcpu_ioctl_enable_cap(struct kvm_vcpu *vcpu, > uint16_t vmcs_version; > void __user *user_ptr; > > - if (!kvm_x86_ops.nested_ops->enable_evmcs) > + if (!kvm_x86_ops.nested_ops->enabled || > + !kvm_x86_ops.nested_ops->enable_evmcs) > return -ENOTTY; > r = kvm_x86_ops.nested_ops->enable_evmcs(vcpu, &vmcs_version); > if (!r) { > @@ -3732,7 +3734,7 @@ long kvm_arch_vcpu_ioctl(struct file *filp, > u32 user_data_size; > > r = -EINVAL; > - if (!kvm_x86_ops.nested_ops->get_state) > + if (!kvm_x86_ops.nested_ops->enabled) > break; > > BUILD_BUG_ON(sizeof(user_data_size) != sizeof(user_kvm_nested_state->size)); > @@ -3762,7 +3764,7 @@ long kvm_arch_vcpu_ioctl(struct file *filp, > int idx; > > r = -EINVAL; > - if (!kvm_x86_ops.nested_ops->set_state) > + if (!kvm_x86_ops.nested_ops->enabled) > break; > > r = -EFAULT; Reviewed-by: Vitaly Kuznetsov -- Vitaly