From: Vitaly Kuznetsov <vkuznets@redhat.com>

When Hyper-V SynIC is enabled, we may need to allow Windows guests to make
hypercalls (POST_MESSAGES/SIGNAL_EVENTS). No issue is currently observed
because KVM is very permissive, allowing these hypercalls regarding of
guest visible CPUid bits.

Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Message-Id: <20210608120817.1325125-9-vkuznets@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
---
 target/i386/kvm/hyperv-proto.h | 6 ++++++
 target/i386/kvm/kvm.c          | 6 ++++++
 2 files changed, 12 insertions(+)

diff --git a/target/i386/kvm/hyperv-proto.h b/target/i386/kvm/hyperv-proto.h
index e30d64b4ade..5fbb385cc13 100644
--- a/target/i386/kvm/hyperv-proto.h
+++ b/target/i386/kvm/hyperv-proto.h
@@ -38,6 +38,12 @@
 #define HV_ACCESS_FREQUENCY_MSRS     (1u << 11)
 #define HV_ACCESS_REENLIGHTENMENTS_CONTROL  (1u << 13)
 
+/*
+ * HV_CPUID_FEATURES.EBX bits
+ */
+#define HV_POST_MESSAGES             (1u << 4)
+#define HV_SIGNAL_EVENTS             (1u << 5)
+
 /*
  * HV_CPUID_FEATURES.EDX bits
  */
diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c
index eee1a6b46ea..59ed8327ac1 100644
--- a/target/i386/kvm/kvm.c
+++ b/target/i386/kvm/kvm.c
@@ -1346,6 +1346,12 @@ static int hyperv_fill_cpuids(CPUState *cs,
     /* Unconditionally required with any Hyper-V enlightenment */
     c->eax |= HV_HYPERCALL_AVAILABLE;
 
+    /* SynIC and Vmbus devices require messages/signals hypercalls */
+    if (hyperv_feat_enabled(cpu, HYPERV_FEAT_SYNIC) &&
+        !cpu->hyperv_synic_kvm_only) {
+        c->ebx |= HV_POST_MESSAGES | HV_SIGNAL_EVENTS;
+    }
+
     /* Not exposed by KVM but needed to make CPU hotplug in Windows work */
     c->edx |= HV_CPU_DYNAMIC_PARTITIONING_AVAILABLE;
 
-- 
2.31.1