Before, the virtio-mem device would unplug all the memory with any reset
of the device, including during the wake-up of the guest from a
suspended state. Due to this, the virtio-mem driver in the Linux kernel
disallowed suspend-to-ram requests in the guest when the
VIRTIO_MEM_F_PERSISTENT_SUSPEND feature is not exposed by QEMU.

This patch adds the code to skip the reset on wake-up and exposes
theVIRTIO_MEM_F_PERSISTENT_SUSPEND feature to the guest kernel driver
when suspending is possible in QEMU (currently only x86).

Signed-off-by: Juraj Marcin <jmarcin@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
---
 hw/virtio/virtio-mem.c | 10 ++++++++++
 hw/virtio/virtio-qmp.c |  3 +++
 2 files changed, 13 insertions(+)

diff --git a/hw/virtio/virtio-mem.c b/hw/virtio/virtio-mem.c
index aeee381eb2..ce228da369 100644
--- a/hw/virtio/virtio-mem.c
+++ b/hw/virtio/virtio-mem.c
@@ -883,6 +883,9 @@ static uint64_t virtio_mem_get_features(VirtIODevice *vdev, uint64_t features,
     if (vmem->unplugged_inaccessible == ON_OFF_AUTO_ON) {
         virtio_add_feature(&features, VIRTIO_MEM_F_UNPLUGGED_INACCESSIBLE);
     }
+    if (qemu_wakeup_suspend_enabled()) {
+        virtio_add_feature(&features, VIRTIO_MEM_F_PERSISTENT_SUSPEND);
+    }
     return features;
 }
 
@@ -1841,6 +1844,13 @@ static void virtio_mem_system_reset_hold(Object *obj, ResetType type)
 {
     VirtIOMEM *vmem = VIRTIO_MEM(obj);
 
+    /*
+     * When waking up from standby/suspend-to-ram, do not unplug any memory.
+     */
+    if (type == RESET_TYPE_WAKEUP) {
+        return;
+    }
+
     /*
      * During usual resets, we will unplug all memory and shrink the usable
      * region size. This is, however, not possible in all scenarios. Then,
diff --git a/hw/virtio/virtio-qmp.c b/hw/virtio/virtio-qmp.c
index 1dd96ed20f..cccc6fe761 100644
--- a/hw/virtio/virtio-qmp.c
+++ b/hw/virtio/virtio-qmp.c
@@ -450,6 +450,9 @@ static const qmp_virtio_feature_map_t virtio_mem_feature_map[] = {
     FEATURE_ENTRY(VIRTIO_MEM_F_UNPLUGGED_INACCESSIBLE, \
             "VIRTIO_MEM_F_UNPLUGGED_INACCESSIBLE: Unplugged memory cannot be "
             "accessed"),
+    FEATURE_ENTRY(VIRTIO_MEM_F_PERSISTENT_SUSPEND, \
+            "VIRTIO_MEM_F_PERSISTENT_SUSPND: Plugged memory will remain "
+            "plugged when suspending+resuming"),
     { -1, "" }
 };
 #endif
-- 
2.46.0