From: Vitaly Kuznetsov vkuznets@redhat.com
commit 250552b925ce400c17d166422fde9bb215958481 upstream.
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 --- arch/x86/kvm/vmx/vmx.c | 22 +++++++++++++--------- 1 file changed, 13 insertions(+), 9 deletions(-)
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c index c37cbd3fdd85..eefd6387a99d 100644 --- a/arch/x86/kvm/vmx/vmx.c +++ b/arch/x86/kvm/vmx/vmx.c @@ -2725,15 +2725,6 @@ 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)); @@ -7029,6 +7020,19 @@ 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);