Page reporting is a feature which enables the virtual machine to report
chunk of free pages to the hypervisor.
This patch enables QEMU to process these reports from the VM and discard the
unused memory range.

Signed-off-by: Nitesh Narayan Lal <nit...@redhat.com>
---
 hw/virtio/virtio-balloon.c         | 41 ++++++++++++++++++++++++++++++
 include/hw/virtio/virtio-balloon.h |  2 +-
 2 files changed, 42 insertions(+), 1 deletion(-)

diff --git a/hw/virtio/virtio-balloon.c b/hw/virtio/virtio-balloon.c
index 25de154307..1132e47ee0 100644
--- a/hw/virtio/virtio-balloon.c
+++ b/hw/virtio/virtio-balloon.c
@@ -320,6 +320,39 @@ static void balloon_stats_set_poll_interval(Object *obj, 
Visitor *v,
     balloon_stats_change_timer(s, 0);
 }
 
+static void virtio_balloon_handle_reporting(VirtIODevice *vdev, VirtQueue *vq)
+{
+    VirtQueueElement *elem;
+
+    while ((elem = virtqueue_pop(vq, sizeof(VirtQueueElement)))) {
+        unsigned int i;
+
+        for (i = 0; i < elem->in_num; i++) {
+            void *gaddr = elem->in_sg[i].iov_base;
+            size_t size = elem->in_sg[i].iov_len;
+            ram_addr_t ram_offset;
+            size_t rb_page_size;
+           RAMBlock *rb;
+
+            if (qemu_balloon_is_inhibited())
+                continue;
+
+            rb = qemu_ram_block_from_host(gaddr, false, &ram_offset);
+            rb_page_size = qemu_ram_pagesize(rb);
+
+            /* For now we will simply ignore unaligned memory regions */
+            if ((ram_offset | size) & (rb_page_size - 1))
+                continue;
+
+            ram_block_discard_range(rb, ram_offset, size);
+        }
+
+        virtqueue_push(vq, elem, 0);
+        virtio_notify(vdev, vq);
+        g_free(elem);
+    }
+}
+
 static void virtio_balloon_handle_output(VirtIODevice *vdev, VirtQueue *vq)
 {
     VirtIOBalloon *s = VIRTIO_BALLOON(vdev);
@@ -792,6 +825,12 @@ static void virtio_balloon_device_realize(DeviceState 
*dev, Error **errp)
     s->dvq = virtio_add_queue(vdev, 128, virtio_balloon_handle_output);
     s->svq = virtio_add_queue(vdev, 128, virtio_balloon_receive_stats);
 
+    if (virtio_has_feature(s->host_features,
+                           VIRTIO_BALLOON_F_REPORTING)) {
+        s->reporting_vq = virtio_add_queue(vdev, 16,
+                                          virtio_balloon_handle_reporting);
+    }
+
     if (virtio_has_feature(s->host_features,
                            VIRTIO_BALLOON_F_FREE_PAGE_HINT)) {
         s->free_page_vq = virtio_add_queue(vdev, VIRTQUEUE_MAX_SIZE,
@@ -912,6 +951,8 @@ static Property virtio_balloon_properties[] = {
      * is disabled, resulting in QEMU 3.1 migration incompatibility.  This
      * property retains this quirk for QEMU 4.1 machine types.
      */
+    DEFINE_PROP_BIT("free-page-reporting", VirtIOBalloon, host_features,
+                    VIRTIO_BALLOON_F_REPORTING, true),
     DEFINE_PROP_BOOL("qemu-4-0-config-size", VirtIOBalloon,
                      qemu_4_0_config_size, false),
     DEFINE_PROP_LINK("iothread", VirtIOBalloon, iothread, TYPE_IOTHREAD,
diff --git a/include/hw/virtio/virtio-balloon.h 
b/include/hw/virtio/virtio-balloon.h
index d1c968d237..15a05e6435 100644
--- a/include/hw/virtio/virtio-balloon.h
+++ b/include/hw/virtio/virtio-balloon.h
@@ -42,7 +42,7 @@ enum virtio_balloon_free_page_report_status {
 
 typedef struct VirtIOBalloon {
     VirtIODevice parent_obj;
-    VirtQueue *ivq, *dvq, *svq, *free_page_vq;
+    VirtQueue *ivq, *dvq, *svq, *free_page_vq, *reporting_vq;
     uint32_t free_page_report_status;
     uint32_t num_pages;
     uint32_t actual;
-- 
2.21.0


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