This can be used to force-synchronize the time in guest after a long
stop-cont pause, which can be useful for serverless-type workload.

Also add a comment to highlight the fact that this (and one other QMP
command) only works for the MC146818 RTC controller.

Signed-off-by: Daniil Tatianin <d-tatianin@yandex-team.ru>
---

Changes since v0:
- Rename to rtc-inject-irq to match other similar API
- Add a comment to highlight that this only works for the I386 RTC

Changes since v1:
- Added a description below the QMP command to explain how it can be
  used and what it does.

Changes since v2:
- Add a 'broadcast' suffix.
- Change the comments to explain the flags we're setting.
- Change the command description to fix styling & explain that it's a broadcast command.

---
 hw/rtc/mc146818rtc.c         | 20 ++++++++++++++++++++
 include/hw/rtc/mc146818rtc.h |  1 +
 qapi/misc-target.json        | 19 +++++++++++++++++++
 3 files changed, 40 insertions(+)

diff --git a/hw/rtc/mc146818rtc.c b/hw/rtc/mc146818rtc.c
index 3379f92748..2b3754f5c6 100644
--- a/hw/rtc/mc146818rtc.c
+++ b/hw/rtc/mc146818rtc.c
@@ -107,6 +107,11 @@ static void rtc_coalesced_timer_update(MC146818RtcState *s)
 static QLIST_HEAD(, MC146818RtcState) rtc_devices =
     QLIST_HEAD_INITIALIZER(rtc_devices);
 
+/*
+ * NOTE:
+ * The two QMP functions below are _only_ implemented for the MC146818.
+ * All other RTC devices ignore this.
+ */
 void qmp_rtc_reset_reinjection(Error **errp)
 {
     MC146818RtcState *s;
@@ -116,6 +121,21 @@ void qmp_rtc_reset_reinjection(Error **errp)
     }
 }
 
+void qmp_rtc_inject_irq_broadcast(Error **errp)
+{
+    MC146818RtcState *s;
+
+    QLIST_FOREACH(s, &rtc_devices, link) {
+        // Update-ended interrupt enable
+        s->cmos_data[RTC_REG_B] |= REG_B_UIE;
+
+        // Interrupt request flag | update interrupt flag
+        s->cmos_data[RTC_REG_C] |= REG_C_IRQF | REG_C_UF;
+
+        qemu_irq_raise(s->irq);
+    }
+}
+
 static bool rtc_policy_slew_deliver_irq(MC146818RtcState *s)
 {
     kvm_reset_irq_delivered();
diff --git a/include/hw/rtc/mc146818rtc.h b/include/hw/rtc/mc146818rtc.h
index 97cec0b3e8..e9dd0f9c72 100644
--- a/include/hw/rtc/mc146818rtc.h
+++ b/include/hw/rtc/mc146818rtc.h
@@ -56,5 +56,6 @@ MC146818RtcState *mc146818_rtc_init(ISABus *bus, int base_year,
 void mc146818rtc_set_cmos_data(MC146818RtcState *s, int addr, int val);
 int mc146818rtc_get_cmos_data(MC146818RtcState *s, int addr);
 void qmp_rtc_reset_reinjection(Error **errp);
+void qmp_rtc_inject_irq_broadcast(Error **errp);
 
 #endif /* HW_RTC_MC146818RTC_H */
diff --git a/qapi/misc-target.json b/qapi/misc-target.json
index 4e0a6492a9..7d388a3753 100644
--- a/qapi/misc-target.json
+++ b/qapi/misc-target.json
@@ -19,6 +19,25 @@
 { 'command': 'rtc-reset-reinjection',
   'if': 'TARGET_I386' }
 
+##
+# @rtc-inject-irq-broadcast:
+#
+# Inject an RTC interrupt for all existing RTCs on the system.
+# The interrupt forces the guest to synchronize the time with RTC.
+# This is useful after a long stop-cont pause, which is common for
+# serverless-type workload.
+#
+# Since: 9.1
+#
+# Example:
+#
+#     -> { "execute": "rtc-inject-irq-broadcast" }
+#     <- { "return": {} }
+#
+##
+{ 'command': 'rtc-inject-irq-broadcast',
+  'if': 'TARGET_I386' }
+
 ##
 # @SevState:
 #
-- 
2.34.1