[PATCH 2/2] i386: Hyper-V Enlightened MSR bitmap feature

[Vitaly Kuznetsov <vkuznets@redhat.com>]

The newly introduced enlightenment allow L0 (KVM) and L1 (Hyper-V)
hypervisors to collaborate to avoid unnecessary updates to L2
MSR-Bitmap upon vmexits.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
---
 docs/hyperv.txt                | 10 ++++++++++
 target/i386/cpu.c              |  2 ++
 target/i386/cpu.h              |  1 +
 target/i386/kvm/hyperv-proto.h |  5 +++++
 target/i386/kvm/kvm.c          |  7 +++++++
 5 files changed, 25 insertions(+)

diff --git a/docs/hyperv.txt b/docs/hyperv.txt
index 0417c183a3b0..08429124a634 100644
--- a/docs/hyperv.txt
+++ b/docs/hyperv.txt
@@ -225,6 +225,16 @@ default (WS2016).
 Note: hv-version-id-* are not enlightenments and thus don't enable Hyper-V
 identification when specified without any other enlightenments.
 
+3.21. hv-emsr-bitmap
+=====================
+The enlightenment is nested specific, it targets Hyper-V on KVM guests. When
+enabled, it allows L0 (KVM) and L1 (Hyper-V) hypervisors to collaborate to
+avoid unnecessary updates to L2 MSR-Bitmap upon vmexits. While the protocol is
+supported for both VMX (Intel) and SVM (AMD), the VMX implementation requires
+Enlightened VMCS ('hv-evmcs') feature to also be enabled.
+
+Recommended: hv-evmcs (Intel)
+
 4. Supplementary features
 =========================
 
diff --git a/target/i386/cpu.c b/target/i386/cpu.c
index aa9e6368004c..f7405fdf4fa5 100644
--- a/target/i386/cpu.c
+++ b/target/i386/cpu.c
@@ -6839,6 +6839,8 @@ static Property x86_cpu_properties[] = {
                       HYPERV_FEAT_STIMER_DIRECT, 0),
     DEFINE_PROP_BIT64("hv-avic", X86CPU, hyperv_features,
                       HYPERV_FEAT_AVIC, 0),
+    DEFINE_PROP_BIT64("hv-emsr-bitmap", X86CPU, hyperv_features,
+                      HYPERV_FEAT_MSR_BITMAP, 0),
     DEFINE_PROP_ON_OFF_AUTO("hv-no-nonarch-coresharing", X86CPU,
                             hyperv_no_nonarch_cs, ON_OFF_AUTO_OFF),
     DEFINE_PROP_BOOL("hv-passthrough", X86CPU, hyperv_passthrough, false),
diff --git a/target/i386/cpu.h b/target/i386/cpu.h
index a1165215d972..04e3b38abf25 100644
--- a/target/i386/cpu.h
+++ b/target/i386/cpu.h
@@ -1060,6 +1060,7 @@ typedef uint64_t FeatureWordArray[FEATURE_WORDS];
 #define HYPERV_FEAT_IPI                 13
 #define HYPERV_FEAT_STIMER_DIRECT       14
 #define HYPERV_FEAT_AVIC                15
+#define HYPERV_FEAT_MSR_BITMAP          16
 
 #ifndef HYPERV_SPINLOCK_NEVER_NOTIFY
 #define HYPERV_SPINLOCK_NEVER_NOTIFY             0xFFFFFFFF
diff --git a/target/i386/kvm/hyperv-proto.h b/target/i386/kvm/hyperv-proto.h
index 89f81afda7c6..38e25468122d 100644
--- a/target/i386/kvm/hyperv-proto.h
+++ b/target/i386/kvm/hyperv-proto.h
@@ -72,6 +72,11 @@
 #define HV_ENLIGHTENED_VMCS_RECOMMENDED     (1u << 14)
 #define HV_NO_NONARCH_CORESHARING           (1u << 18)
 
+/*
+ * HV_CPUID_NESTED_FEATURES.EAX bits
+ */
+#define HV_NESTED_MSR_BITMAP                (1u << 19)
+
 /*
  * Basic virtualized MSRs
  */
diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c
index c8f4956a4e0e..2baa9609e181 100644
--- a/target/i386/kvm/kvm.c
+++ b/target/i386/kvm/kvm.c
@@ -933,6 +933,13 @@ static struct {
              .bits = HV_DEPRECATING_AEOI_RECOMMENDED}
         }
     },
+    [HYPERV_FEAT_MSR_BITMAP] = {
+        .desc = "enlightened MSR-Bitmap (hv-emsr-bitmap)",
+        .flags = {
+            {.func = HV_CPUID_NESTED_FEATURES, .reg = R_EAX,
+             .bits = HV_NESTED_MSR_BITMAP}
+        }
+    },
 };
 
 static struct kvm_cpuid2 *try_get_hv_cpuid(CPUState *cs, int max,
-- 
2.33.1