[PATCH v3 1/4] KVM: nVMX: Don't use Enlightened MSR Bitmap for L3

Vitaly Kuznetsov
2021-10-13T14:23:23Z

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>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
---
arch/x86/kvm/vmx/vmx.c | 21 ++++++++++++---------
1 file changed, 12 insertions(+), 9 deletions(-)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 1c8b2b6e7ed9..e82cdde58119 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -2655,15 +2655,6 @@ 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));
@@ -6903,6 +6894,18 @@ static int vmx_create_vcpu(struct kvm_vcpu *vcpu)

vmx->loaded_vmcs = &vmx->vmcs01;

+ /*
+ * Use Hyper-V 'Enlightened MSR Bitmap' feature when KVM runs as a
+ * nested (L1) hypervisor and Hyper-V in L0 supports it.
+ */
+ 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 *)vmx->loaded_vmcs->vmcs;
+
+ evmcs->hv_enlightenments_control.msr_bitmap = 1;
+ }
+
if (cpu_need_virtualize_apic_accesses(vcpu)) {
err = alloc_apic_access_page(vcpu->kvm);
if (err)
--
2.31.1