KVM: nVMX: Don't use Enlightened MSR Bitmap for L3

When KVM runs as a nested hypervisor on top of Hyper-V it uses Enlightened
VMCS and enables Enlightened MSR Bitmap feature for its L1s and L2s (which
are actually L2s and L3s from Hyper-V's perspective). When MSR bitmap is
updated, KVM has to reset HV_VMX_ENLIGHTENED_CLEAN_FIELD_MSR_BITMAP from
clean fields to make Hyper-V aware of the change. For KVM's L1s, this is
done in vmx_disable_intercept_for_msr()/vmx_enable_intercept_for_msr().
MSR bitmap for L2 is build in nested_vmx_prepare_msr_bitmap() by blending
MSR bitmap for L1 and L1's idea of MSR bitmap for L2. KVM, however, doesn't
check if the resulting bitmap is different and never cleans
HV_VMX_ENLIGHTENED_CLEAN_FIELD_MSR_BITMAP in eVMCS02. This is incorrect and
may result in Hyper-V missing the update.

The issue could've been solved by calling evmcs_touch_msr_bitmap() for
eVMCS02 from nested_vmx_prepare_msr_bitmap() unconditionally but doing so
would not give any performance benefits (compared to not using Enlightened
MSR Bitmap at all). 3-level nesting is also not a very common setup
nowadays.

Don't enable 'Enlightened MSR Bitmap' feature for KVM's L2s (real L3s) for
now.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Message-Id: <20211129094704.326635-2-vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 9453743ce0c410e439ab6074b15c140cc993f03a..5aadad3e736752f0930e9ec60d95038ae8850f18 100644 (file)
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -2646,15 +2646,6 @@ int alloc_loaded_vmcs(struct loaded_vmcs *loaded_vmcs)
                if (!loaded_vmcs->msr_bitmap)
                        goto out_vmcs;
                memset(loaded_vmcs->msr_bitmap, 0xff, PAGE_SIZE);
-
-               if (IS_ENABLED(CONFIG_HYPERV) &&
-                   static_branch_unlikely(&enable_evmcs) &&
-                   (ms_hyperv.nested_features & HV_X64_NESTED_MSR_BITMAP)) {
-                       struct hv_enlightened_vmcs *evmcs =
-                               (struct hv_enlightened_vmcs *)loaded_vmcs->vmcs;
-
-                       evmcs->hv_enlightenments_control.msr_bitmap = 1;
-               }
        }
 
        memset(&loaded_vmcs->host_state, 0, sizeof(struct vmcs_host_state));
@@ -6842,6 +6833,19 @@ static int vmx_create_vcpu(struct kvm_vcpu *vcpu)
        if (err < 0)
                goto free_pml;
 
+       /*
+        * Use Hyper-V 'Enlightened MSR Bitmap' feature when KVM runs as a
+        * nested (L1) hypervisor and Hyper-V in L0 supports it. Enable the
+        * feature only for vmcs01, KVM currently isn't equipped to realize any
+        * performance benefits from enabling it for vmcs02.
+        */
+       if (IS_ENABLED(CONFIG_HYPERV) && static_branch_unlikely(&enable_evmcs) &&
+           (ms_hyperv.nested_features & HV_X64_NESTED_MSR_BITMAP)) {
+               struct hv_enlightened_vmcs *evmcs = (void *)vmx->vmcs01.vmcs;
+
+               evmcs->hv_enlightenments_control.msr_bitmap = 1;
+       }
+
        /* The MSR bitmap starts with all ones */
        bitmap_fill(vmx->shadow_msr_intercept.read, MAX_POSSIBLE_PASSTHROUGH_MSRS);
        bitmap_fill(vmx->shadow_msr_intercept.write, MAX_POSSIBLE_PASSTHROUGH_MSRS);