[PATCH v6 18/19] i386: Hyper-V SynIC requires POST_MESSAGES/SIGNAL_EVENTS priviliges

[Vitaly Kuznetsov]

When Hyper-V SynIC has its QEMU part enabled (no 'x-hv-synic-kvm-only'),
we may need to allow Windows guest 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.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
---
 target/i386/kvm/hyperv-proto.h | 6 ++++++
 target/i386/kvm/kvm.c          | 5 +++++
 2 files changed, 11 insertions(+)

diff --git a/target/i386/kvm/hyperv-proto.h b/target/i386/kvm/hyperv-proto.h
index e30d64b4ade4..5fbb385cc136 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 d81451276cd8..88c64909a2ff 100644
--- a/target/i386/kvm/kvm.c
+++ b/target/i386/kvm/kvm.c
@@ -1341,6 +1341,11 @@ static int hyperv_fill_cpuids(CPUState *cs,
     c->ebx = hv_build_cpuid_leaf(cs, HV_CPUID_FEATURES, R_EBX);
     c->edx = hv_build_cpuid_leaf(cs, HV_CPUID_FEATURES, R_EDX);
 
+    /* In-QEMU SynIC and Vmbus devices require messages/signals hypercalls */
+    if (!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;
 
-- 
2.30.2