[PATCH] vmbus: Print a warning when enabled without the recommended set of features

["Maciej S. Szmigiero" <mail@xxxxxxxxxxxxxxxxxxxxx>]

From: "Maciej S. Szmigiero" <maciej.szmigiero@xxxxxxxxxx>

Some Windows versions crash at boot or fail to enable the VMBus device if
they don't see the expected set of Hyper-V features (enlightenments).

Since this provides poor user experience let's warn user if the VMBus
device is enabled without the recommended set of Hyper-V features.

The recommended set is the minimum set of Hyper-V features required to make
the VMBus device work properly in Windows Server versions 2016, 2019 and
2022.

Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@xxxxxxxxxx>
---
 hw/hyperv/hyperv.c            | 12 ++++++++++++
 hw/hyperv/vmbus.c             |  6 ++++++
 include/hw/hyperv/hyperv.h    |  4 ++++
 target/i386/kvm/hyperv-stub.c |  4 ++++
 target/i386/kvm/hyperv.c      |  5 +++++
 target/i386/kvm/hyperv.h      |  2 ++
 target/i386/kvm/kvm.c         |  7 +++++++
 7 files changed, 40 insertions(+)

diff --git a/hw/hyperv/hyperv.c b/hw/hyperv/hyperv.c
index 57b402b95610..2c91de7ff4a8 100644
--- a/hw/hyperv/hyperv.c
+++ b/hw/hyperv/hyperv.c
@@ -947,3 +947,15 @@ uint64_t hyperv_syndbg_query_options(void)
 
     return msg.u.query_options.options;
 }
+
+static bool vmbus_recommended_features_enabled;
+
+bool hyperv_are_vmbus_recommended_features_enabled(void)
+{
+    return vmbus_recommended_features_enabled;
+}
+
+void hyperv_set_vmbus_recommended_features_enabled(void)
+{
+    vmbus_recommended_features_enabled = true;
+}
diff --git a/hw/hyperv/vmbus.c b/hw/hyperv/vmbus.c
index 380239af2c7b..f33afeeea27d 100644
--- a/hw/hyperv/vmbus.c
+++ b/hw/hyperv/vmbus.c
@@ -2631,6 +2631,12 @@ static void vmbus_bridge_realize(DeviceState *dev, Error **errp)
         return;
     }
 
+    if (!hyperv_are_vmbus_recommended_features_enabled()) {
+        warn_report("VMBus enabled without the recommended set of Hyper-V features: "
+                    "hv-stimer, hv-vapic and hv-runtime. "
+                    "Some Windows versions might not boot or enable the VMBus device");
+    }
+
     bridge->bus = VMBUS(qbus_new(TYPE_VMBUS, dev, "vmbus"));
 }
 
diff --git a/include/hw/hyperv/hyperv.h b/include/hw/hyperv/hyperv.h
index 015c3524b1c2..d717b4e13d40 100644
--- a/include/hw/hyperv/hyperv.h
+++ b/include/hw/hyperv/hyperv.h
@@ -139,4 +139,8 @@ typedef struct HvSynDbgMsg {
 } HvSynDbgMsg;
 typedef uint16_t (*HvSynDbgHandler)(void *context, HvSynDbgMsg *msg);
 void hyperv_set_syndbg_handler(HvSynDbgHandler handler, void *context);
+
+bool hyperv_are_vmbus_recommended_features_enabled(void);
+void hyperv_set_vmbus_recommended_features_enabled(void);
+
 #endif
diff --git a/target/i386/kvm/hyperv-stub.c b/target/i386/kvm/hyperv-stub.c
index 778ed782e6fc..3263dcf05d31 100644
--- a/target/i386/kvm/hyperv-stub.c
+++ b/target/i386/kvm/hyperv-stub.c
@@ -52,3 +52,7 @@ void hyperv_x86_synic_reset(X86CPU *cpu)
 void hyperv_x86_synic_update(X86CPU *cpu)
 {
 }
+
+void hyperv_x86_set_vmbus_recommended_features_enabled(void)
+{
+}
diff --git a/target/i386/kvm/hyperv.c b/target/i386/kvm/hyperv.c
index 6825c89af374..f2a3fe650a18 100644
--- a/target/i386/kvm/hyperv.c
+++ b/target/i386/kvm/hyperv.c
@@ -149,3 +149,8 @@ int kvm_hv_handle_exit(X86CPU *cpu, struct kvm_hyperv_exit *exit)
         return -1;
     }
 }
+
+void hyperv_x86_set_vmbus_recommended_features_enabled(void)
+{
+    hyperv_set_vmbus_recommended_features_enabled();
+}
diff --git a/target/i386/kvm/hyperv.h b/target/i386/kvm/hyperv.h
index 67543296c3a4..e3982c8f4dd1 100644
--- a/target/i386/kvm/hyperv.h
+++ b/target/i386/kvm/hyperv.h
@@ -26,4 +26,6 @@ int hyperv_x86_synic_add(X86CPU *cpu);
 void hyperv_x86_synic_reset(X86CPU *cpu);
 void hyperv_x86_synic_update(X86CPU *cpu);
 
+void hyperv_x86_set_vmbus_recommended_features_enabled(void);
+
 #endif
diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c
index e88e65fe014c..d3d01b3cf82d 100644
--- a/target/i386/kvm/kvm.c
+++ b/target/i386/kvm/kvm.c
@@ -1650,6 +1650,13 @@ static int hyperv_init_vcpu(X86CPU *cpu)
         }
     }
 
+    /* Skip SynIC and VP_INDEX since they are hard deps already */
+    if (hyperv_feat_enabled(cpu, HYPERV_FEAT_STIMER) &&
+        hyperv_feat_enabled(cpu, HYPERV_FEAT_VAPIC) &&
+        hyperv_feat_enabled(cpu, HYPERV_FEAT_RUNTIME)) {
+        hyperv_x86_set_vmbus_recommended_features_enabled();
+    }
+
     return 0;
 }