CPU hot-unplug with SMM requires firmware participation to prevent
guest crash (i.e. CPU can be removed only after OS _and_ firmware
were prepared for the action).
Previous patches introduced ICH9_LPC_SMI_F_CPU_HOT_UNPLUG_BIT
feature bit, which is advertised by firmware when it has support
for CPU hot-unplug. Use it to check if guest is able to handle
unplug and make device_del fail gracefully if hot-unplug feature
hasn't been negotiated.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Tested-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
---
v2:
 - fix typo in commit message
 - drop 5.1 version from hint message (Laszlo)
---
 hw/acpi/ich9.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/hw/acpi/ich9.c b/hw/acpi/ich9.c
index 0acc9a3107..95cb0f935b 100644
--- a/hw/acpi/ich9.c
+++ b/hw/acpi/ich9.c
@@ -460,6 +460,18 @@ void ich9_pm_device_unplug_request_cb(HotplugHandler *hotplug_dev,
                                       errp);
     } else if (object_dynamic_cast(OBJECT(dev), TYPE_CPU) &&
                !lpc->pm.cpu_hotplug_legacy) {
+        uint64_t negotiated = lpc->smi_negotiated_features;
+
+        if (negotiated & BIT_ULL(ICH9_LPC_SMI_F_BROADCAST_BIT) &&
+            !(negotiated & BIT_ULL(ICH9_LPC_SMI_F_CPU_HOT_UNPLUG_BIT))) {
+            error_setg(errp, "cpu hot-unplug with SMI wasn't enabled "
+                             "by firmware");
+            error_append_hint(errp, "update machine type to a version having "
+                                    "x-smi-cpu-hotunplug=on and firmware that "
+                                    "supports CPU hot-unplug with SMM");
+            return;
+        }
+
         acpi_cpu_unplug_request_cb(hotplug_dev, &lpc->pm.cpuhp_state,
                                    dev, errp);
     } else {
-- 
2.27.0