Add Hyper-V Dynamic Memory Protocol driver (hv-balloon) base

This driver is like virtio-balloon on steroids: it allows both changing the
guest memory allocation via ballooning and (in the next patch) inserting
pieces of extra RAM into it on demand from a provided memory backend.

The actual resizing is done via ballooning interface (for example, via
the "balloon" HMP command).
This includes resizing the guest past its boot size - that is, hot-adding
additional memory in granularity limited only by the guest alignment
requirements, as provided by the next patch.

In contrast with ACPI DIMM hotplug where one can only request to unplug a
whole DIMM stick this driver allows removing memory from guest in single
page (4k) units via ballooning.

After a VM reboot the guest is back to its original (boot) size.

In the future, the guest boot memory size might be changed on reboot
instead, taking into account the effective size that VM had before that
reboot (much like Hyper-V does).

For performance reasons, the guest-released memory is tracked in a few
range trees, as a series of (start, count) ranges.
Each time a new page range is inserted into such tree its neighbors are
checked as candidates for possible merging with it.

Besides performance reasons, the Dynamic Memory protocol itself uses page
ranges as the data structure in its messages, so relevant pages need to be
merged into such ranges anyway.

One has to be careful when tracking the guest-released pages, since the
guest can maliciously report returning pages outside its current address
space, which later clash with the address range of newly added memory.
Similarly, the guest can report freeing the same page twice.

The above design results in much better ballooning performance than when
using virtio-balloon with the same guest: 230 GB / minute with this driver
versus 70 GB / minute with virtio-balloon.

During a ballooning operation most of time is spent waiting for the guest
to come up with newly freed page ranges, processing the received ranges on
the host side (in QEMU and KVM) is nearly instantaneous.

The unballoon operation is also pretty much instantaneous:
thanks to the merging of the ballooned out page ranges 200 GB of memory can
be returned to the guest in about 1 second.
With virtio-balloon this operation takes about 2.5 minutes.

These tests were done against a Windows Server 2019 guest running on a
Xeon E5-2699, after dirtying the whole memory inside guest before each
balloon operation.

Using a range tree instead of a bitmap to track the removed memory also
means that the solution scales well with the guest size: even a 1 TB range
takes just a few bytes of such metadata.

Since the required GTree operations aren't present in every Glib version
a check for them was added to the meson build script, together with new
"--enable-hv-balloon" and "--disable-hv-balloon" configure arguments.
If these GTree operations are missing in the system's Glib version this
driver will be skipped during QEMU build.

An optional "status-report=on" device parameter requests memory status
events from the guest (typically sent every second), which allow the host
to learn both the guest memory available and the guest memory in use
counts.

Following commits will add support for their external emission as
"HV_BALLOON_STATUS_REPORT" QMP events.

The driver is named hv-balloon since the Linux kernel client driver for
the Dynamic Memory Protocol is named as such and to follow the naming
pattern established by the virtio-balloon driver.
The whole protocol runs over Hyper-V VMBus.

The driver was tested against Windows Server 2012 R2, Windows Server 2016
and Windows Server 2019 guests and obeys the guest alignment requirements
reported to the host via DM_CAPABILITIES_REPORT message.

Acked-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>

7 files changed, 1563 insertions, 0 deletions

diff --git a/hw/hyperv/Kconfig b/hw/hyperv/Kconfig
index fcf6590..41dd827 100644
--- a/hw/hyperv/Kconfig
+++ b/hw/hyperv/Kconfig
@@ -16,3 +16,13 @@ config SYNDBG
     bool
     default y
     depends on VMBUS
+
+config HV_BALLOON_SUPPORTED
+    bool
+
+config HV_BALLOON
+    bool
+    default y
+    depends on VMBUS
+    depends on HV_BALLOON_POSSIBLE
+    depends on HV_BALLOON_SUPPORTED
diff --git a/hw/hyperv/hv-balloon-internal.h b/hw/hyperv/hv-balloon-internal.h
new file mode 100644
index 0000000..164c2e5
--- /dev/null
+++ b/hw/hyperv/hv-balloon-internal.h
@@ -0,0 +1,33 @@
+/*
+ * QEMU Hyper-V Dynamic Memory Protocol driver
+ *
+ * Copyright (C) 2020-2023 Oracle and/or its affiliates.
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2 or later.
+ * See the COPYING file in the top-level directory.
+ */
+
+#ifndef HW_HYPERV_HV_BALLOON_INTERNAL_H
+#define HW_HYPERV_HV_BALLOON_INTERNAL_H
+
+#include "qemu/osdep.h"
+
+#define HV_BALLOON_PFN_SHIFT 12
+#define HV_BALLOON_PAGE_SIZE (1 << HV_BALLOON_PFN_SHIFT)
+
+#define SUM_OVERFLOW_U64(in1, in2) ((in1) > UINT64_MAX - (in2))
+#define SUM_SATURATE_U64(in1, in2)              \
+    ({                                          \
+        uint64_t _in1 = (in1), _in2 = (in2);    \
+        uint64_t _result;                       \
+                                                \
+        if (!SUM_OVERFLOW_U64(_in1, _in2)) {    \
+            _result = _in1 + _in2;              \
+        } else {                                \
+            _result = UINT64_MAX;               \
+        }                                       \
+                                                \
+        _result;                                \
+    })
+
+#endif
diff --git a/hw/hyperv/hv-balloon-page_range_tree.c b/hw/hyperv/hv-balloon-page_range_tree.c
new file mode 100644
index 0000000..e178d8b
--- /dev/null
+++ b/hw/hyperv/hv-balloon-page_range_tree.c
@@ -0,0 +1,228 @@
+/*
+ * QEMU Hyper-V Dynamic Memory Protocol driver
+ *
+ * Copyright (C) 2020-2023 Oracle and/or its affiliates.
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2 or later.
+ * See the COPYING file in the top-level directory.
+ */
+
+#include "hv-balloon-internal.h"
+#include "hv-balloon-page_range_tree.h"
+
+/*
+ * temporarily avoid warnings about enhanced GTree API usage requiring a
+ * too recent Glib version until GLIB_VERSION_MAX_ALLOWED finally reaches
+ * the Glib version with this API
+ */
+#pragma GCC diagnostic ignored "-Wdeprecated-declarations"
+
+/* PageRangeTree */
+static gint page_range_tree_key_compare(gconstpointer leftp,
+                                        gconstpointer rightp,
+                                        gpointer user_data)
+{
+    const uint64_t *left = leftp, *right = rightp;
+
+    if (*left < *right) {
+        return -1;
+    } else if (*left > *right) {
+        return 1;
+    } else { /* *left == *right */
+        return 0;
+    }
+}
+
+static GTreeNode *page_range_tree_insert_new(PageRangeTree tree,
+                                             uint64_t start, uint64_t count)
+{
+    uint64_t *key = g_malloc(sizeof(*key));
+    PageRange *range = g_malloc(sizeof(*range));
+
+    assert(count > 0);
+
+    *key = range->start = start;
+    range->count = count;
+
+    return g_tree_insert_node(tree.t, key, range);
+}
+
+void hvb_page_range_tree_insert(PageRangeTree tree,
+                                uint64_t start, uint64_t count,
+                                uint64_t *dupcount)
+{
+    GTreeNode *node;
+    bool joinable;
+    uint64_t intersection;
+    PageRange *range;
+
+    assert(!SUM_OVERFLOW_U64(start, count));
+    if (count == 0) {
+        return;
+    }
+
+    node = g_tree_upper_bound(tree.t, &start);
+    if (node) {
+        node = g_tree_node_previous(node);
+    } else {
+        node = g_tree_node_last(tree.t);
+    }
+
+    if (node) {
+        range = g_tree_node_value(node);
+        assert(range);
+        intersection = page_range_intersection_size(range, start, count);
+        joinable = page_range_joinable_right(range, start, count);
+    }
+
+    if (!node ||
+        (!intersection && !joinable)) {
+        /*
+         * !node case: the tree is empty or the very first node in the tree
+         * already has a higher key (the start of its range).
+         * the other case: there is a gap in the tree between the new range
+         * and the previous one.
+         * anyway, let's just insert the new range into the tree.
+         */
+        node = page_range_tree_insert_new(tree, start, count);
+        assert(node);
+        range = g_tree_node_value(node);
+        assert(range);
+    } else {
+        /*
+         * the previous range in the tree either partially covers the new
+         * range or ends just at its beginning - extend it
+         */
+        if (dupcount) {
+            *dupcount += intersection;
+        }
+
+        count += start - range->start;
+        range->count = MAX(range->count, count);
+    }
+
+    /* check next nodes for possible merging */
+    for (node = g_tree_node_next(node); node; ) {
+        PageRange *rangecur;
+
+        rangecur = g_tree_node_value(node);
+        assert(rangecur);
+
+        intersection = page_range_intersection_size(rangecur,
+                                                    range->start, range->count);
+        joinable = page_range_joinable_left(rangecur,
+                                            range->start, range->count);
+        if (!intersection && !joinable) {
+            /* the current node is disjoint */
+            break;
+        }
+
+        if (dupcount) {
+            *dupcount += intersection;
+        }
+
+        count = rangecur->count + (rangecur->start - range->start);
+        range->count = MAX(range->count, count);
+
+        /* the current node was merged in, remove it */
+        start = rangecur->start;
+        node = g_tree_node_next(node);
+        /* no hinted removal in GTree... */
+        g_tree_remove(tree.t, &start);
+    }
+}
+
+bool hvb_page_range_tree_pop(PageRangeTree tree, PageRange *out,
+                             uint64_t maxcount)
+{
+    GTreeNode *node;
+    PageRange *range;
+
+    node = g_tree_node_last(tree.t);
+    if (!node) {
+        return false;
+    }
+
+    range = g_tree_node_value(node);
+    assert(range);
+
+    out->start = range->start;
+
+    /* can't modify range->start as it is the node key */
+    if (range->count > maxcount) {
+        out->start += range->count - maxcount;
+        out->count = maxcount;
+        range->count -= maxcount;
+    } else {
+        out->count = range->count;
+        /* no hinted removal in GTree... */
+        g_tree_remove(tree.t, &out->start);
+    }
+
+    return true;
+}
+
+bool hvb_page_range_tree_intree_any(PageRangeTree tree,
+                                    uint64_t start, uint64_t count)
+{
+    GTreeNode *node;
+
+    if (count == 0) {
+        return false;
+    }
+
+    /* find the first node that can possibly intersect our range */
+    node = g_tree_upper_bound(tree.t, &start);
+    if (node) {
+        /*
+         * a NULL node below means that the very first node in the tree
+         * already has a higher key (the start of its range).
+         */
+        node = g_tree_node_previous(node);
+    } else {
+        /* a NULL node below means that the tree is empty */
+        node = g_tree_node_last(tree.t);
+    }
+    /* node range start <= range start */
+
+    if (!node) {
+        /* node range start > range start */
+        node = g_tree_node_first(tree.t);
+    }
+
+    for ( ; node; node = g_tree_node_next(node)) {
+        PageRange *range = g_tree_node_value(node);
+
+        assert(range);
+        /*
+         * if this node starts beyond or at the end of our range so does
+         * every next one
+         */
+        if (range->start >= start + count) {
+            break;
+        }
+
+        if (page_range_intersection_size(range, start, count) > 0) {
+            return true;
+        }
+    }
+
+    return false;
+}
+
+void hvb_page_range_tree_init(PageRangeTree *tree)
+{
+    tree->t = g_tree_new_full(page_range_tree_key_compare, NULL,
+                              g_free, g_free);
+}
+
+void hvb_page_range_tree_destroy(PageRangeTree *tree)
+{
+    /* g_tree_destroy() is not NULL-safe */
+    if (!tree->t) {
+        return;
+    }
+
+    g_tree_destroy(tree->t);
+    tree->t = NULL;
+}
diff --git a/hw/hyperv/hv-balloon-page_range_tree.h b/hw/hyperv/hv-balloon-page_range_tree.h
new file mode 100644
index 0000000..07a9ae0
--- /dev/null
+++ b/hw/hyperv/hv-balloon-page_range_tree.h
@@ -0,0 +1,118 @@
+/*
+ * QEMU Hyper-V Dynamic Memory Protocol driver
+ *
+ * Copyright (C) 2020-2023 Oracle and/or its affiliates.
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2 or later.
+ * See the COPYING file in the top-level directory.
+ */
+
+#ifndef HW_HYPERV_HV_BALLOON_PAGE_RANGE_TREE_H
+#define HW_HYPERV_HV_BALLOON_PAGE_RANGE_TREE_H
+
+#include "qemu/osdep.h"
+
+/* PageRange */
+typedef struct PageRange {
+    uint64_t start;
+    uint64_t count;
+} PageRange;
+
+/* return just the part of range before (start) */
+static inline void page_range_part_before(const PageRange *range,
+                                          uint64_t start, PageRange *out)
+{
+    uint64_t endr = range->start + range->count;
+    uint64_t end = MIN(endr, start);
+
+    out->start = range->start;
+    if (end > out->start) {
+        out->count = end - out->start;
+    } else {
+        out->count = 0;
+    }
+}
+
+/* return just the part of range after (start, count) */
+static inline void page_range_part_after(const PageRange *range,
+                                         uint64_t start, uint64_t count,
+                                         PageRange *out)
+{
+    uint64_t end = range->start + range->count;
+    uint64_t ends = start + count;
+
+    out->start = MAX(range->start, ends);
+    if (end > out->start) {
+        out->count = end - out->start;
+    } else {
+        out->count = 0;
+    }
+}
+
+static inline void page_range_intersect(const PageRange *range,
+                                        uint64_t start, uint64_t count,
+                                        PageRange *out)
+{
+    uint64_t end1 = range->start + range->count;
+    uint64_t end2 = start + count;
+    uint64_t end = MIN(end1, end2);
+
+    out->start = MAX(range->start, start);
+    out->count = out->start < end ? end - out->start : 0;
+}
+
+static inline uint64_t page_range_intersection_size(const PageRange *range,
+                                                    uint64_t start, uint64_t count)
+{
+    PageRange trange;
+
+    page_range_intersect(range, start, count, &trange);
+    return trange.count;
+}
+
+static inline bool page_range_joinable_left(const PageRange *range,
+                                            uint64_t start, uint64_t count)
+{
+    return start + count == range->start;
+}
+
+static inline bool page_range_joinable_right(const PageRange *range,
+                                             uint64_t start, uint64_t count)
+{
+    return range->start + range->count == start;
+}
+
+static inline bool page_range_joinable(const PageRange *range,
+                                       uint64_t start, uint64_t count)
+{
+    return page_range_joinable_left(range, start, count) ||
+        page_range_joinable_right(range, start, count);
+}
+
+/* PageRangeTree */
+/* type safety */
+typedef struct PageRangeTree {
+    GTree *t;
+} PageRangeTree;
+
+static inline bool page_range_tree_is_empty(PageRangeTree tree)
+{
+    guint nnodes = g_tree_nnodes(tree.t);
+
+    return nnodes == 0;
+}
+
+void hvb_page_range_tree_init(PageRangeTree *tree);
+void hvb_page_range_tree_destroy(PageRangeTree *tree);
+
+bool hvb_page_range_tree_intree_any(PageRangeTree tree,
+                                    uint64_t start, uint64_t count);
+
+bool hvb_page_range_tree_pop(PageRangeTree tree, PageRange *out,
+                             uint64_t maxcount);
+
+void hvb_page_range_tree_insert(PageRangeTree tree,
+                                uint64_t start, uint64_t count,
+                                uint64_t *dupcount);
+
+#endif
diff --git a/hw/hyperv/hv-balloon.c b/hw/hyperv/hv-balloon.c
new file mode 100644
index 0000000..5f3674b
--- /dev/null
+++ b/hw/hyperv/hv-balloon.c
@@ -0,0 +1,1160 @@
+/*
+ * QEMU Hyper-V Dynamic Memory Protocol driver
+ *
+ * Copyright (C) 2020-2023 Oracle and/or its affiliates.
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2 or later.
+ * See the COPYING file in the top-level directory.
+ */
+
+#include "hv-balloon-internal.h"
+
+#include "exec/address-spaces.h"
+#include "exec/cpu-common.h"
+#include "exec/ramblock.h"
+#include "hw/boards.h"
+#include "hw/hyperv/dynmem-proto.h"
+#include "hw/hyperv/hv-balloon.h"
+#include "hw/hyperv/vmbus.h"
+#include "hw/mem/memory-device.h"
+#include "hw/mem/pc-dimm.h"
+#include "hw/qdev-core.h"
+#include "hw/qdev-properties.h"
+#include "monitor/qdev.h"
+#include "qapi/error.h"
+#include "qapi/qapi-commands-machine.h"
+#include "qapi/qapi-events-machine.h"
+#include "qapi/qapi-types-machine.h"
+#include "qapi/qmp/qdict.h"
+#include "qapi/visitor.h"
+#include "qemu/error-report.h"
+#include "qemu/module.h"
+#include "qemu/units.h"
+#include "qemu/timer.h"
+#include "sysemu/balloon.h"
+#include "sysemu/hostmem.h"
+#include "sysemu/reset.h"
+#include "hv-balloon-page_range_tree.h"
+#include "trace.h"
+
+#define HV_BALLOON_GUID "525074DC-8985-46e2-8057-A307DC18A502"
+
+/*
+ * Some Windows versions (at least Server 2019) will crash with various
+ * error codes when receiving DM protocol requests (at least
+ * DM_MEM_HOT_ADD_REQUEST) immediately after boot.
+ *
+ * It looks like Hyper-V from Server 2016 uses a 50-second after-boot
+ * delay, probably to workaround this issue, so we'll use this value, too.
+ */
+#define HV_BALLOON_POST_INIT_WAIT (50 * 1000)
+
+#define HV_BALLOON_HA_CHUNK_SIZE (2 * GiB)
+#define HV_BALLOON_HA_CHUNK_PAGES (HV_BALLOON_HA_CHUNK_SIZE / HV_BALLOON_PAGE_SIZE)
+
+#define HV_BALLOON_HR_CHUNK_PAGES 585728
+/*
+ *                                ^ that's the maximum number of pages
+ * that Windows returns in one hot remove response
+ *
+ * If the number requested is too high Windows will no longer honor
+ * these requests
+ */
+
+struct HvBalloonClass {
+    VMBusDeviceClass parent_class;
+} HvBalloonClass;
+
+typedef enum State {
+    /* not a real state */
+    S_NO_CHANGE = 0,
+
+    S_WAIT_RESET,
+    S_POST_RESET_CLOSED,
+
+    /* init flow */
+    S_VERSION,
+    S_CAPS,
+    S_POST_INIT_WAIT,
+
+    S_IDLE,
+
+    /* balloon op flow */
+    S_BALLOON_POSTING,
+    S_BALLOON_RB_WAIT,
+    S_BALLOON_REPLY_WAIT,
+
+    /* unballoon + hot add ops flow */
+    S_UNBALLOON_POSTING,
+    S_UNBALLOON_RB_WAIT,
+    S_UNBALLOON_REPLY_WAIT,
+} State;
+
+typedef struct StateDesc {
+    State state;
+    const char *desc;
+} StateDesc;
+
+typedef struct HvBalloon {
+    VMBusDevice parent;
+    State state;
+
+    union dm_version version;
+    union dm_caps caps;
+
+    QEMUTimer post_init_timer;
+
+    unsigned int trans_id;
+
+    struct {
+        bool enabled;
+        bool received;
+        uint64_t committed;
+        uint64_t available;
+    } status_report;
+
+    /* Guest target size */
+    uint64_t target;
+    bool target_changed;
+
+    /* Current (un)balloon */
+    union {
+        uint64_t balloon_diff;
+
+        struct {
+            uint64_t unballoon_diff;
+        };
+    };
+
+    /* Nominal size of each memslot (the last one might be smaller) */
+    uint64_t memslot_size;
+
+    PageRangeTree removed_guest, removed_both;
+
+    uint64_t removed_guest_ctr, removed_both_ctr;
+} HvBalloon;
+
+OBJECT_DEFINE_TYPE_WITH_INTERFACES(HvBalloon, hv_balloon, HV_BALLOON, VMBUS_DEVICE, \
+                                   { })
+
+#define HV_BALLOON_SET_STATE(hvb, news)             \
+    do {                                            \
+        assert(news != S_NO_CHANGE);                \
+        hv_balloon_state_set(hvb, news, # news);    \
+    } while (0)
+
+#define HV_BALLOON_STATE_DESC_SET(stdesc, news)         \
+    _hv_balloon_state_desc_set(stdesc, news, # news)
+
+#define HV_BALLOON_STATE_DESC_INIT \
+    {                              \
+        .state = S_NO_CHANGE,      \
+    }
+
+typedef struct HvBalloonReq {
+    VMBusChanReq vmreq;
+} HvBalloonReq;
+
+/* TODO: unify the code below with virtio-balloon and cache the value */
+static int build_dimm_list(Object *obj, void *opaque)
+{
+    GSList **list = opaque;
+
+    if (object_dynamic_cast(obj, TYPE_PC_DIMM)) {
+        DeviceState *dev = DEVICE(obj);
+        if (dev->realized) { /* only realized DIMMs matter */
+            *list = g_slist_prepend(*list, dev);
+        }
+    }
+
+    object_child_foreach(obj, build_dimm_list, opaque);
+    return 0;
+}
+
+static ram_addr_t get_current_ram_size(void)
+{
+    GSList *list = NULL, *item;
+    ram_addr_t size = current_machine->ram_size;
+
+    build_dimm_list(qdev_get_machine(), &list);
+    for (item = list; item; item = g_slist_next(item)) {
+        Object *obj = OBJECT(item->data);
+        if (!strcmp(object_get_typename(obj), TYPE_PC_DIMM))
+            size += object_property_get_int(obj, PC_DIMM_SIZE_PROP,
+                                            &error_abort);
+    }
+    g_slist_free(list);
+
+    return size;
+}
+
+/* total RAM includes memory currently removed from the guest */
+static uint64_t hv_balloon_total_ram(HvBalloon *balloon)
+{
+    ram_addr_t ram_size = get_current_ram_size();
+    uint64_t ram_size_pages = ram_size >> HV_BALLOON_PFN_SHIFT;
+
+    assert(ram_size_pages > 0);
+
+    return ram_size_pages;
+}
+
+/*
+ * calculating the total RAM size is a slow operation,
+ * avoid it as much as possible
+ */
+static uint64_t hv_balloon_total_removed_rs(HvBalloon *balloon,
+                                            uint64_t ram_size_pages)
+{
+    uint64_t total_removed;
+
+    total_removed = SUM_SATURATE_U64(balloon->removed_guest_ctr,
+                                     balloon->removed_both_ctr);
+
+    /* possible if guest returns pages outside actual RAM */
+    if (total_removed > ram_size_pages) {
+        total_removed = ram_size_pages;
+    }
+
+    return total_removed;
+}
+
+/* Returns whether the state has actually changed */
+static bool hv_balloon_state_set(HvBalloon *balloon,
+                                 State newst, const char *newststr)
+{
+    if (newst == S_NO_CHANGE || balloon->state == newst) {
+        return false;
+    }
+
+    balloon->state = newst;
+    trace_hv_balloon_state_change(newststr);
+    return true;
+}
+
+static void _hv_balloon_state_desc_set(StateDesc *stdesc,
+                                       State newst, const char *newststr)
+{
+    /* state setting is only permitted on a freshly init desc */
+    assert(stdesc->state == S_NO_CHANGE);
+
+    assert(newst != S_NO_CHANGE);
+
+    stdesc->state = newst;
+    stdesc->desc = newststr;
+}
+
+static VMBusChannel *hv_balloon_get_channel_maybe(HvBalloon *balloon)
+{
+    return vmbus_device_channel(&balloon->parent, 0);
+}
+
+static VMBusChannel *hv_balloon_get_channel(HvBalloon *balloon)
+{
+    VMBusChannel *chan;
+
+    chan = hv_balloon_get_channel_maybe(balloon);
+    assert(chan != NULL);
+    return chan;
+}
+
+static ssize_t hv_balloon_send_packet(VMBusChannel *chan,
+                                      struct dm_message *msg)
+{
+    int ret;
+
+    ret = vmbus_channel_reserve(chan, 0, msg->hdr.size);
+    if (ret < 0) {
+        return ret;
+    }
+
+    return vmbus_channel_send(chan, VMBUS_PACKET_DATA_INBAND,
+                              NULL, 0, msg, msg->hdr.size, false,
+                              msg->hdr.trans_id);
+}
+
+static bool hv_balloon_unballoon_get_source(HvBalloon *balloon,
+                                            PageRangeTree *dtree,
+                                            uint64_t **dctr)
+{
+    if (g_tree_nnodes(balloon->removed_guest.t) > 0) {
+        *dtree = balloon->removed_guest;
+        *dctr = &balloon->removed_guest_ctr;
+    } else if (g_tree_nnodes(balloon->removed_both.t) > 0) {
+        *dtree = balloon->removed_both;
+        *dctr = &balloon->removed_both_ctr;
+    } else {
+        return false;
+    }
+
+    return true;
+}
+
+static void hv_balloon_unballoon_rb_wait(HvBalloon *balloon, StateDesc *stdesc)
+{
+    VMBusChannel *chan = hv_balloon_get_channel(balloon);
+    struct dm_unballoon_request *ur;
+    size_t ur_size = sizeof(*ur) + sizeof(ur->range_array[0]);
+
+    assert(balloon->state == S_UNBALLOON_RB_WAIT);
+
+    if (vmbus_channel_reserve(chan, 0, ur_size) < 0) {
+        return;
+    }
+
+    HV_BALLOON_STATE_DESC_SET(stdesc, S_UNBALLOON_POSTING);
+}
+
+static void hv_balloon_unballoon_posting(HvBalloon *balloon, StateDesc *stdesc)
+{
+    VMBusChannel *chan = hv_balloon_get_channel(balloon);
+    PageRangeTree dtree;
+    uint64_t *dctr;
+    struct dm_unballoon_request *ur;
+    size_t ur_size = sizeof(*ur) + sizeof(ur->range_array[0]);
+    PageRange range;
+    bool bret;
+    ssize_t ret;
+
+    assert(balloon->state == S_UNBALLOON_POSTING);
+    assert(balloon->unballoon_diff > 0);
+
+    if (!hv_balloon_unballoon_get_source(balloon, &dtree, &dctr)) {
+        error_report("trying to unballoon but nothing seems to be ballooned");
+        /*
+         * there is little we can do as we might have already
+         * sent the guest a partial request we can't cancel
+         */
+        return;
+    }
+
+    assert(dtree.t);
+    assert(dctr);
+
+    ur = alloca(ur_size);
+    memset(ur, 0, ur_size);
+    ur->hdr.type = DM_UNBALLOON_REQUEST;
+    ur->hdr.size = ur_size;
+    ur->hdr.trans_id = balloon->trans_id;
+
+    bret = hvb_page_range_tree_pop(dtree, &range, MIN(balloon->unballoon_diff,
+                                                      HV_BALLOON_HA_CHUNK_PAGES));
+    assert(bret);
+    /* TODO: madvise? */
+
+    *dctr -= range.count;
+    balloon->unballoon_diff -= range.count;
+
+    ur->range_count = 1;
+    ur->range_array[0].finfo.start_page = range.start;
+    ur->range_array[0].finfo.page_cnt = range.count;
+    ur->more_pages = balloon->unballoon_diff > 0;
+
+    trace_hv_balloon_outgoing_unballoon(ur->hdr.trans_id,
+                                        range.count, range.start,
+                                        balloon->unballoon_diff);
+
+    if (ur->more_pages) {
+        HV_BALLOON_STATE_DESC_SET(stdesc, S_UNBALLOON_RB_WAIT);
+    } else {
+        HV_BALLOON_STATE_DESC_SET(stdesc, S_UNBALLOON_REPLY_WAIT);
+    }
+
+    ret = vmbus_channel_send(chan, VMBUS_PACKET_DATA_INBAND,
+                             NULL, 0, ur, ur_size, false,
+                             ur->hdr.trans_id);
+    if (ret <= 0) {
+        error_report("error %zd when posting unballoon msg, expect problems",
+                     ret);
+    }
+}
+
+static void hv_balloon_balloon_rb_wait(HvBalloon *balloon, StateDesc *stdesc)
+{
+    VMBusChannel *chan = hv_balloon_get_channel(balloon);
+    size_t bl_size = sizeof(struct dm_balloon);
+
+    assert(balloon->state == S_BALLOON_RB_WAIT);
+
+    if (vmbus_channel_reserve(chan, 0, bl_size) < 0) {
+        return;
+    }
+
+    HV_BALLOON_STATE_DESC_SET(stdesc, S_BALLOON_POSTING);
+}
+
+static void hv_balloon_balloon_posting(HvBalloon *balloon, StateDesc *stdesc)
+{
+    VMBusChannel *chan = hv_balloon_get_channel(balloon);
+    struct dm_balloon bl;
+    size_t bl_size = sizeof(bl);
+    ssize_t ret;
+
+    assert(balloon->state == S_BALLOON_POSTING);
+    assert(balloon->balloon_diff > 0);
+
+    memset(&bl, 0, sizeof(bl));
+    bl.hdr.type = DM_BALLOON_REQUEST;
+    bl.hdr.size = bl_size;
+    bl.hdr.trans_id = balloon->trans_id;
+    bl.num_pages = MIN(balloon->balloon_diff, HV_BALLOON_HR_CHUNK_PAGES);
+
+    trace_hv_balloon_outgoing_balloon(bl.hdr.trans_id, bl.num_pages,
+                                      balloon->balloon_diff);
+
+    ret = vmbus_channel_send(chan, VMBUS_PACKET_DATA_INBAND,
+                             NULL, 0, &bl, bl_size, false,
+                             bl.hdr.trans_id);
+    if (ret <= 0) {
+        error_report("error %zd when posting balloon msg, expect problems",
+                     ret);
+    }
+
+    HV_BALLOON_STATE_DESC_SET(stdesc, S_BALLOON_REPLY_WAIT);
+}
+
+static void hv_balloon_idle_state_process_target(HvBalloon *balloon,
+                                                 StateDesc *stdesc)
+{
+    bool can_balloon = balloon->caps.cap_bits.balloon;
+    uint64_t ram_size_pages, total_removed;
+
+    ram_size_pages = hv_balloon_total_ram(balloon);
+    total_removed = hv_balloon_total_removed_rs(balloon, ram_size_pages);
+
+    /*
+     * we need to cache the values computed from the balloon target value when
+     * starting the adjustment procedure in case someone changes the target when
+     * the procedure is in progress
+     */
+    if (balloon->target > ram_size_pages - total_removed) {
+        uint64_t target_diff = balloon->target -
+            (ram_size_pages - total_removed);
+
+        balloon->unballoon_diff = MIN(target_diff, total_removed);
+
+        if (balloon->unballoon_diff > 0) {
+            assert(can_balloon);
+            HV_BALLOON_STATE_DESC_SET(stdesc, S_UNBALLOON_RB_WAIT);
+        }
+    } else if (can_balloon &&
+               balloon->target < ram_size_pages - total_removed) {
+        balloon->balloon_diff = ram_size_pages - total_removed -
+            balloon->target;
+        HV_BALLOON_STATE_DESC_SET(stdesc, S_BALLOON_RB_WAIT);
+    }
+}
+
+static void hv_balloon_idle_state(HvBalloon *balloon,
+                                  StateDesc *stdesc)
+{
+    assert(balloon->state == S_IDLE);
+
+    if (balloon->target_changed) {
+        balloon->target_changed = false;
+        hv_balloon_idle_state_process_target(balloon, stdesc);
+        return;
+    }
+}
+
+static const struct {
+    void (*handler)(HvBalloon *balloon, StateDesc *stdesc);
+} state_handlers[] = {
+    [S_IDLE].handler = hv_balloon_idle_state,
+    [S_BALLOON_POSTING].handler = hv_balloon_balloon_posting,
+    [S_BALLOON_RB_WAIT].handler = hv_balloon_balloon_rb_wait,
+    [S_UNBALLOON_POSTING].handler = hv_balloon_unballoon_posting,
+    [S_UNBALLOON_RB_WAIT].handler = hv_balloon_unballoon_rb_wait,
+};
+
+static void hv_balloon_handle_state(HvBalloon *balloon, StateDesc *stdesc)
+{
+    if (balloon->state >= ARRAY_SIZE(state_handlers) ||
+        !state_handlers[balloon->state].handler) {
+        return;
+    }
+
+    state_handlers[balloon->state].handler(balloon, stdesc);
+}
+
+static void hv_balloon_remove_response_insert_range(PageRangeTree tree,
+                                                    const PageRange *range,
+                                                    uint64_t *ctr1,
+                                                    uint64_t *ctr2,
+                                                    uint64_t *ctr3)
+{
+    uint64_t dupcount, effcount;
+
+    if (range->count == 0) {
+        return;
+    }
+
+    dupcount = 0;
+    hvb_page_range_tree_insert(tree, range->start, range->count, &dupcount);
+
+    assert(dupcount <= range->count);
+    effcount = range->count - dupcount;
+
+    *ctr1 += effcount;
+    *ctr2 += effcount;
+    if (ctr3) {
+        *ctr3 += effcount;
+    }
+}
+
+static void hv_balloon_remove_response_handle_range(HvBalloon *balloon,
+                                                    PageRange *range,
+                                                    bool both,
+                                                    uint64_t *removedctr)
+{
+    PageRangeTree globaltree =
+        both ? balloon->removed_both : balloon->removed_guest;
+    uint64_t *globalctr =
+        both ? &balloon->removed_both_ctr : &balloon->removed_guest_ctr;
+
+    trace_hv_balloon_remove_response(range->count, range->start, both);
+
+    if (range->count > 0) {
+        hv_balloon_remove_response_insert_range(globaltree, range,
+                                                globalctr, removedctr, NULL);
+        trace_hv_balloon_remove_response_remainder(range->count, range->start,
+                                                   both);
+        range->count = 0;
+    }
+}
+
+static void hv_balloon_remove_response_handle_pages(HvBalloon *balloon,
+                                                    PageRange *range,
+                                                    uint64_t start,
+                                                    uint64_t count,
+                                                    bool both,
+                                                    uint64_t *removedctr)
+{
+    assert(count > 0);
+
+    /*
+     * if there is an existing range that the new range can't be joined to
+     * dump it into tree(s)
+     */
+    if (range->count > 0 && !page_range_joinable(range, start, count)) {
+        hv_balloon_remove_response_handle_range(balloon, range, both,
+                                                removedctr);
+    }
+
+    if (range->count == 0) {
+        range->start = start;
+        range->count = count;
+    } else if (page_range_joinable_left(range, start, count)) {
+        range->start = start;
+        range->count += count;
+    } else { /* page_range_joinable_right() */
+        range->count += count;
+    }
+}
+
+static gboolean hv_balloon_handle_remove_host_addr_node(gpointer key,
+                                                        gpointer value,
+                                                        gpointer data)
+{
+    PageRange *range = value;
+    uint64_t pageoff;
+
+    for (pageoff = 0; pageoff < range->count; ) {
+        uint64_t addr_64 = (range->start + pageoff) * HV_BALLOON_PAGE_SIZE;
+        void *addr;
+        RAMBlock *rb;
+        ram_addr_t rb_offset;
+        size_t rb_page_size;
+        size_t discard_size;
+
+        assert(addr_64 <= UINTPTR_MAX);
+        addr = (void *)((uintptr_t)addr_64);
+        rb = qemu_ram_block_from_host(addr, false, &rb_offset);
+        rb_page_size = qemu_ram_pagesize(rb);
+
+        if (rb_page_size != HV_BALLOON_PAGE_SIZE) {
+            /* TODO: these should end in "removed_guest" */
+            warn_report("guest reported removed page backed by unsupported page size %zu",
+                        rb_page_size);
+            pageoff++;
+            continue;
+        }
+
+        discard_size = MIN(range->count - pageoff,
+                           (rb->max_length - rb_offset) /
+                           HV_BALLOON_PAGE_SIZE);
+        discard_size = MAX(discard_size, 1);
+
+        if (ram_block_discard_range(rb, rb_offset, discard_size *
+                                    HV_BALLOON_PAGE_SIZE) != 0) {
+            warn_report("guest reported removed page failed discard");
+        }
+
+        pageoff += discard_size;
+    }
+
+    return false;
+}
+
+static void hv_balloon_handle_remove_host_addr_tree(PageRangeTree tree)
+{
+    g_tree_foreach(tree.t, hv_balloon_handle_remove_host_addr_node, NULL);
+}
+
+static int hv_balloon_handle_remove_section(PageRangeTree tree,
+                                            const MemoryRegionSection *section,
+                                            uint64_t count)
+{
+    void *addr = memory_region_get_ram_ptr(section->mr) +
+        section->offset_within_region;
+    uint64_t addr_page;
+
+    assert(count > 0);
+
+    if ((uintptr_t)addr % HV_BALLOON_PAGE_SIZE) {
+        warn_report("guest reported removed pages at an unaligned host addr %p",
+                    addr);
+        return -EINVAL;
+    }
+
+    addr_page = (uintptr_t)addr / HV_BALLOON_PAGE_SIZE;
+    hvb_page_range_tree_insert(tree, addr_page, count, NULL);
+
+    return 0;
+}
+
+static void hv_balloon_handle_remove_ranges(HvBalloon *balloon,
+                                            union dm_mem_page_range ranges[],
+                                            uint32_t count)
+{
+    uint64_t removedcnt;
+    PageRangeTree removed_host_addr;
+    PageRange range_guest, range_both;
+
+    hvb_page_range_tree_init(&removed_host_addr);
+    range_guest.count = range_both.count = removedcnt = 0;
+    for (unsigned int ctr = 0; ctr < count; ctr++) {
+        union dm_mem_page_range *mr = &ranges[ctr];
+        hwaddr pa;
+        MemoryRegionSection section;
+
+        for (unsigned int offset = 0; offset < mr->finfo.page_cnt; ) {
+            int ret;
+            uint64_t pageno = mr->finfo.start_page + offset;
+            uint64_t pagecnt = 1;
+
+            pa = (hwaddr)pageno << HV_BALLOON_PFN_SHIFT;
+            section = memory_region_find(get_system_memory(), pa,
+                                         (mr->finfo.page_cnt - offset) *
+                                         HV_BALLOON_PAGE_SIZE);
+            if (!section.mr) {
+                warn_report("guest reported removed page %"PRIu64" not found in RAM",
+                            pageno);
+                ret = -EINVAL;
+                goto finish_page;
+            }
+
+            pagecnt = int128_get64(section.size) / HV_BALLOON_PAGE_SIZE;
+            if (pagecnt <= 0) {
+                warn_report("guest reported removed page %"PRIu64" in a section smaller than page size",
+                            pageno);
+                pagecnt = 1; /* skip the whole page */
+                ret = -EINVAL;
+                goto finish_page;
+            }
+
+            if (!memory_region_is_ram(section.mr) ||
+                memory_region_is_rom(section.mr) ||
+                memory_region_is_romd(section.mr)) {
+                warn_report("guest reported removed page %"PRIu64" in a section that is not an ordinary RAM",
+                            pageno);
+                ret = -EINVAL;
+                goto finish_page;
+            }
+
+            ret = hv_balloon_handle_remove_section(removed_host_addr, &section,
+                                                   pagecnt);
+
+        finish_page:
+            if (ret == 0) {
+                hv_balloon_remove_response_handle_pages(balloon,
+                                                        &range_both,
+                                                        pageno, pagecnt,
+                                                        true, &removedcnt);
+            } else {
+                hv_balloon_remove_response_handle_pages(balloon,
+                                                        &range_guest,
+                                                        pageno, pagecnt,
+                                                        false, &removedcnt);
+            }
+
+            if (section.mr) {
+                memory_region_unref(section.mr);
+            }
+
+            offset += pagecnt;
+        }
+    }
+
+    hv_balloon_remove_response_handle_range(balloon, &range_both, true,
+                                            &removedcnt);
+    hv_balloon_remove_response_handle_range(balloon, &range_guest, false,
+                                            &removedcnt);
+
+    hv_balloon_handle_remove_host_addr_tree(removed_host_addr);
+    hvb_page_range_tree_destroy(&removed_host_addr);
+
+    if (removedcnt > balloon->balloon_diff) {
+        warn_report("guest reported more pages removed than currently pending (%"PRIu64" vs %"PRIu64")",
+                    removedcnt, balloon->balloon_diff);
+        balloon->balloon_diff = 0;
+    } else {
+        balloon->balloon_diff -= removedcnt;
+    }
+}
+
+static bool hv_balloon_handle_msg_size(HvBalloonReq *req, size_t minsize,
+                                       const char *msgname)
+{
+    VMBusChanReq *vmreq = &req->vmreq;
+    uint32_t msglen = vmreq->msglen;
+
+    if (msglen >= minsize) {
+        return true;
+    }
+
+    warn_report("%s message too short (%u vs %zu), ignoring", msgname,
+                (unsigned int)msglen, minsize);
+    return false;
+}
+
+static void hv_balloon_handle_version_request(HvBalloon *balloon,
+                                              HvBalloonReq *req,
+                                              StateDesc *stdesc)
+{
+    VMBusChanReq *vmreq = &req->vmreq;
+    struct dm_version_request *msgVr = vmreq->msg;
+    struct dm_version_response respVr;
+
+    if (balloon->state != S_VERSION) {
+        warn_report("unexpected DM_VERSION_REQUEST in %d state",
+                    balloon->state);
+        return;
+    }
+
+    if (!hv_balloon_handle_msg_size(req, sizeof(*msgVr),
+                                    "DM_VERSION_REQUEST")) {
+        return;
+    }
+
+    trace_hv_balloon_incoming_version(msgVr->version.major_version,
+                                      msgVr->version.minor_version);
+
+    memset(&respVr, 0, sizeof(respVr));
+    respVr.hdr.type = DM_VERSION_RESPONSE;
+    respVr.hdr.size = sizeof(respVr);
+    respVr.hdr.trans_id = msgVr->hdr.trans_id;
+    respVr.is_accepted = msgVr->version.version >= DYNMEM_PROTOCOL_VERSION_1 &&
+        msgVr->version.version <= DYNMEM_PROTOCOL_VERSION_3;
+
+    hv_balloon_send_packet(vmreq->chan, (struct dm_message *)&respVr);
+
+    if (respVr.is_accepted) {
+        HV_BALLOON_STATE_DESC_SET(stdesc, S_CAPS);
+    }
+}
+
+static void hv_balloon_handle_caps_report(HvBalloon *balloon,
+                                          HvBalloonReq *req,
+                                          StateDesc *stdesc)
+{
+    VMBusChanReq *vmreq = &req->vmreq;
+    struct dm_capabilities *msgCap = vmreq->msg;
+    struct dm_capabilities_resp_msg respCap;
+
+    if (balloon->state != S_CAPS) {
+        warn_report("unexpected DM_CAPABILITIES_REPORT in %d state",
+                    balloon->state);
+        return;
+    }
+
+    if (!hv_balloon_handle_msg_size(req, sizeof(*msgCap),
+                                    "DM_CAPABILITIES_REPORT")) {
+        return;
+    }
+
+    trace_hv_balloon_incoming_caps(msgCap->caps.caps);
+    balloon->caps = msgCap->caps;
+
+    memset(&respCap, 0, sizeof(respCap));
+    respCap.hdr.type = DM_CAPABILITIES_RESPONSE;
+    respCap.hdr.size = sizeof(respCap);
+    respCap.hdr.trans_id = msgCap->hdr.trans_id;
+    respCap.is_accepted = 1;
+    respCap.hot_remove = 1;
+    respCap.suppress_pressure_reports = !balloon->status_report.enabled;
+    hv_balloon_send_packet(vmreq->chan, (struct dm_message *)&respCap);
+
+    timer_mod(&balloon->post_init_timer,
+              qemu_clock_get_ms(QEMU_CLOCK_VIRTUAL) +
+              HV_BALLOON_POST_INIT_WAIT);
+
+    HV_BALLOON_STATE_DESC_SET(stdesc, S_POST_INIT_WAIT);
+}
+
+static void hv_balloon_handle_status_report(HvBalloon *balloon,
+                                            HvBalloonReq *req)
+{
+    VMBusChanReq *vmreq = &req->vmreq;
+    struct dm_status *msgStatus = vmreq->msg;
+
+    if (!hv_balloon_handle_msg_size(req, sizeof(*msgStatus),
+                                    "DM_STATUS_REPORT")) {
+        return;
+    }
+
+    if (!balloon->status_report.enabled) {
+        return;
+    }
+
+    balloon->status_report.committed = msgStatus->num_committed;
+    balloon->status_report.committed *= HV_BALLOON_PAGE_SIZE;
+    balloon->status_report.available = msgStatus->num_avail;
+    balloon->status_report.available *= HV_BALLOON_PAGE_SIZE;
+    balloon->status_report.received = true;
+
+    /* report event */
+}
+
+static void hv_balloon_handle_unballoon_response(HvBalloon *balloon,
+                                                 HvBalloonReq *req,
+                                                 StateDesc *stdesc)
+{
+    VMBusChanReq *vmreq = &req->vmreq;
+    struct dm_unballoon_response *msgUrR = vmreq->msg;
+
+    if (balloon->state != S_UNBALLOON_REPLY_WAIT) {
+        warn_report("unexpected DM_UNBALLOON_RESPONSE in %d state",
+                    balloon->state);
+        return;
+    }
+
+    if (!hv_balloon_handle_msg_size(req, sizeof(*msgUrR),
+                                    "DM_UNBALLOON_RESPONSE"))
+        return;
+
+    trace_hv_balloon_incoming_unballoon(msgUrR->hdr.trans_id);
+
+    balloon->trans_id++;
+
+    HV_BALLOON_STATE_DESC_SET(stdesc, S_IDLE);
+}
+
+static void hv_balloon_handle_balloon_response(HvBalloon *balloon,
+                                               HvBalloonReq *req,
+                                               StateDesc *stdesc)
+{
+    VMBusChanReq *vmreq = &req->vmreq;
+    struct dm_balloon_response *msgBR = vmreq->msg;
+
+    if (balloon->state != S_BALLOON_REPLY_WAIT) {
+        warn_report("unexpected DM_BALLOON_RESPONSE in %d state",
+                    balloon->state);
+        return;
+    }
+
+    if (!hv_balloon_handle_msg_size(req, sizeof(*msgBR),
+                                    "DM_BALLOON_RESPONSE"))
+        return;
+
+    trace_hv_balloon_incoming_balloon(msgBR->hdr.trans_id, msgBR->range_count,
+                                      msgBR->more_pages);
+
+    if (vmreq->msglen < sizeof(*msgBR) +
+        (uint64_t)sizeof(msgBR->range_array[0]) * msgBR->range_count) {
+        warn_report("DM_BALLOON_RESPONSE too short for the range count");
+        return;
+    }
+
+    if (msgBR->range_count == 0) {
+        /* The guest is already at its minimum size */
+        balloon->balloon_diff = 0;
+        goto ret_end_trans;
+    } else {
+        hv_balloon_handle_remove_ranges(balloon,
+                                        msgBR->range_array,
+                                        msgBR->range_count);
+    }
+
+    /* More responses expected? */
+    if (msgBR->more_pages) {
+        return;
+    }
+
+ret_end_trans:
+    balloon->trans_id++;
+
+    if (balloon->balloon_diff > 0) {
+        HV_BALLOON_STATE_DESC_SET(stdesc, S_BALLOON_RB_WAIT);
+    } else {
+        HV_BALLOON_STATE_DESC_SET(stdesc, S_IDLE);
+    }
+}
+
+static void hv_balloon_handle_packet(HvBalloon *balloon, HvBalloonReq *req,
+                                     StateDesc *stdesc)
+{
+    VMBusChanReq *vmreq = &req->vmreq;
+    struct dm_message *msg = vmreq->msg;
+
+    if (vmreq->msglen < sizeof(msg->hdr)) {
+        return;
+    }
+
+    switch (msg->hdr.type) {
+    case DM_VERSION_REQUEST:
+        hv_balloon_handle_version_request(balloon, req, stdesc);
+        break;
+
+    case DM_CAPABILITIES_REPORT:
+        hv_balloon_handle_caps_report(balloon, req, stdesc);
+        break;
+
+    case DM_STATUS_REPORT:
+        hv_balloon_handle_status_report(balloon, req);
+        break;
+
+    case DM_UNBALLOON_RESPONSE:
+        hv_balloon_handle_unballoon_response(balloon, req, stdesc);
+        break;
+
+    case DM_BALLOON_RESPONSE:
+        hv_balloon_handle_balloon_response(balloon, req, stdesc);
+        break;
+
+    default:
+        warn_report("unknown DM message %u", msg->hdr.type);
+        break;
+    }
+}
+
+static bool hv_balloon_recv_channel(HvBalloon *balloon, StateDesc *stdesc)
+{
+    VMBusChannel *chan;
+    HvBalloonReq *req;
+
+    if (balloon->state == S_WAIT_RESET ||
+        balloon->state == S_POST_RESET_CLOSED) {
+        return false;
+    }
+
+    chan = hv_balloon_get_channel(balloon);
+    if (vmbus_channel_recv_start(chan)) {
+        return false;
+    }
+
+    while ((req = vmbus_channel_recv_peek(chan, sizeof(*req)))) {
+        hv_balloon_handle_packet(balloon, req, stdesc);
+        vmbus_free_req(req);
+        vmbus_channel_recv_pop(chan);
+
+        if (stdesc->state != S_NO_CHANGE) {
+            break;
+        }
+    }
+
+    return vmbus_channel_recv_done(chan) > 0;
+}
+
+/* old state handler -> new state transition (potential) */
+static bool hv_balloon_event_loop_state(HvBalloon *balloon)
+{
+    StateDesc state_new = HV_BALLOON_STATE_DESC_INIT;
+
+    hv_balloon_handle_state(balloon, &state_new);
+    return hv_balloon_state_set(balloon, state_new.state, state_new.desc);
+}
+
+/* VMBus message -> new state transition (potential) */
+static bool hv_balloon_event_loop_recv(HvBalloon *balloon)
+{
+    StateDesc state_new = HV_BALLOON_STATE_DESC_INIT;
+    bool any_recv, state_changed;
+
+    any_recv = hv_balloon_recv_channel(balloon, &state_new);
+    state_changed = hv_balloon_state_set(balloon,
+                                         state_new.state, state_new.desc);
+
+    return state_changed || any_recv;
+}
+
+static void hv_balloon_event_loop(HvBalloon *balloon)
+{
+    bool state_repeat, recv_repeat;
+
+    do {
+        state_repeat = hv_balloon_event_loop_state(balloon);
+        recv_repeat = hv_balloon_event_loop_recv(balloon);
+    } while (state_repeat || recv_repeat);
+}
+
+static void hv_balloon_vmdev_chan_notify(VMBusChannel *chan)
+{
+    HvBalloon *balloon = HV_BALLOON(vmbus_channel_device(chan));
+
+    hv_balloon_event_loop(balloon);
+}
+
+static void hv_balloon_stat(void *opaque, BalloonInfo *info)
+{
+    HvBalloon *balloon = opaque;
+    info->actual = (hv_balloon_total_ram(balloon) - balloon->removed_both_ctr)
+        << HV_BALLOON_PFN_SHIFT;
+}
+
+static void hv_balloon_to_target(void *opaque, ram_addr_t target)
+{
+    HvBalloon *balloon = opaque;
+    uint64_t target_pages = target >> HV_BALLOON_PFN_SHIFT;
+
+    if (!target_pages) {
+        return;
+    }
+
+    /*
+     * always set target_changed, even with unchanged target, as the user
+     * might be asking us to try again reaching it
+     */
+    balloon->target = target_pages;
+    balloon->target_changed = true;
+
+    hv_balloon_event_loop(balloon);
+}
+
+static int hv_balloon_vmdev_open_channel(VMBusChannel *chan)
+{
+    HvBalloon *balloon = HV_BALLOON(vmbus_channel_device(chan));
+
+    if (balloon->state != S_POST_RESET_CLOSED) {
+        warn_report("guest trying to open a DM channel in invalid %d state",
+                    balloon->state);
+        return -EINVAL;
+    }
+
+    HV_BALLOON_SET_STATE(balloon, S_VERSION);
+    hv_balloon_event_loop(balloon);
+
+    return 0;
+}
+
+static void hv_balloon_vmdev_close_channel(VMBusChannel *chan)
+{
+    HvBalloon *balloon = HV_BALLOON(vmbus_channel_device(chan));
+
+    timer_del(&balloon->post_init_timer);
+
+    /* Don't report stale data */
+    balloon->status_report.received = false;
+
+    HV_BALLOON_SET_STATE(balloon, S_WAIT_RESET);
+    hv_balloon_event_loop(balloon);
+}
+
+static void hv_balloon_post_init_timer(void *opaque)
+{
+    HvBalloon *balloon = opaque;
+
+    if (balloon->state != S_POST_INIT_WAIT) {
+        return;
+    }
+
+    HV_BALLOON_SET_STATE(balloon, S_IDLE);
+    hv_balloon_event_loop(balloon);
+}
+
+static void hv_balloon_vmdev_realize(VMBusDevice *vdev, Error **errp)
+{
+    ERRP_GUARD();
+    HvBalloon *balloon = HV_BALLOON(vdev);
+    int ret;
+
+    balloon->state = S_WAIT_RESET;
+
+    ret = qemu_add_balloon_handler(hv_balloon_to_target, hv_balloon_stat,
+                                   balloon);
+    if (ret < 0) {
+        /* This also protects against having multiple hv-balloon instances */
+        error_setg(errp, "Only one balloon device is supported");
+        return;
+    }
+
+    timer_init_ms(&balloon->post_init_timer, QEMU_CLOCK_VIRTUAL,
+                  hv_balloon_post_init_timer, balloon);
+}
+
+/*
+ * VMBus device reset has to be implemented in case the guest decides to
+ * disconnect and reconnect to the VMBus without rebooting the whole system.
+ */
+static void hv_balloon_vmdev_reset(VMBusDevice *vdev)
+{
+    HvBalloon *balloon = HV_BALLOON(vdev);
+
+    if (balloon->state == S_POST_RESET_CLOSED) {
+        return;
+    }
+
+    hvb_page_range_tree_destroy(&balloon->removed_guest);
+    hvb_page_range_tree_destroy(&balloon->removed_both);
+    hvb_page_range_tree_init(&balloon->removed_guest);
+    hvb_page_range_tree_init(&balloon->removed_both);
+
+    balloon->trans_id = 0;
+    balloon->removed_guest_ctr = 0;
+    balloon->removed_both_ctr = 0;
+
+    HV_BALLOON_SET_STATE(balloon, S_POST_RESET_CLOSED);
+    hv_balloon_event_loop(balloon);
+}
+
+static void hv_balloon_vmdev_unrealize(VMBusDevice *vdev)
+{
+    HvBalloon *balloon = HV_BALLOON(vdev);
+
+    qemu_remove_balloon_handler(balloon);
+
+    hvb_page_range_tree_destroy(&balloon->removed_guest);
+    hvb_page_range_tree_destroy(&balloon->removed_both);
+}
+
+static void hv_balloon_init(Object *obj)
+{
+}
+
+static void hv_balloon_finalize(Object *obj)
+{
+}
+
+static Property hv_balloon_properties[] = {
+    DEFINE_PROP_BOOL("status-report", HvBalloon,
+                     status_report.enabled, false),
+
+    DEFINE_PROP_END_OF_LIST(),
+};
+
+static void hv_balloon_class_init(ObjectClass *klass, void *data)
+{
+    DeviceClass *dc = DEVICE_CLASS(klass);
+    VMBusDeviceClass *vdc = VMBUS_DEVICE_CLASS(klass);
+
+    device_class_set_props(dc, hv_balloon_properties);
+    qemu_uuid_parse(HV_BALLOON_GUID, &vdc->classid);
+    set_bit(DEVICE_CATEGORY_MISC, dc->categories);
+
+    vdc->vmdev_realize = hv_balloon_vmdev_realize;
+    vdc->vmdev_unrealize = hv_balloon_vmdev_unrealize;
+    vdc->vmdev_reset = hv_balloon_vmdev_reset;
+    vdc->open_channel = hv_balloon_vmdev_open_channel;
+    vdc->close_channel = hv_balloon_vmdev_close_channel;
+    vdc->chan_notify_cb = hv_balloon_vmdev_chan_notify;
+}
diff --git a/hw/hyperv/meson.build b/hw/hyperv/meson.build
index b43f119..b5be1cb 100644
--- a/hw/hyperv/meson.build
+++ b/hw/hyperv/meson.build
@@ -2,3 +2,4 @@ specific_ss.add(when: 'CONFIG_HYPERV', if_true: files('hyperv.c'))
 specific_ss.add(when: 'CONFIG_HYPERV_TESTDEV', if_true: files('hyperv_testdev.c'))
 specific_ss.add(when: 'CONFIG_VMBUS', if_true: files('vmbus.c'))
 specific_ss.add(when: 'CONFIG_SYNDBG', if_true: files('syndbg.c'))
+specific_ss.add(when: 'CONFIG_HV_BALLOON', if_true: files('hv-balloon.c', 'hv-balloon-page_range_tree.c'))
diff --git a/hw/hyperv/trace-events b/hw/hyperv/trace-events
index b4c35ca..86c724b 100644
--- a/hw/hyperv/trace-events
+++ b/hw/hyperv/trace-events
@@ -16,3 +16,16 @@ vmbus_gpadl_torndown(uint32_t gpadl_id) "gpadl #%d"
 vmbus_open_channel(uint32_t chan_id, uint32_t gpadl_id, uint32_t target_vp) "channel #%d gpadl #%d target vp %d"
 vmbus_channel_open(uint32_t chan_id, uint32_t status) "channel #%d status %d"
 vmbus_close_channel(uint32_t chan_id) "channel #%d"
+
+# hv-balloon
+hv_balloon_state_change(const char *tostr) "-> %s"
+hv_balloon_incoming_version(uint16_t major, uint16_t minor) "incoming proto version %u.%u"
+hv_balloon_incoming_caps(uint32_t caps) "incoming caps 0x%x"
+hv_balloon_outgoing_unballoon(uint32_t trans_id, uint64_t count, uint64_t start, uint64_t rempages) "posting unballoon %"PRIu32" for %"PRIu64" @ 0x%"PRIx64", remaining %"PRIu64
+hv_balloon_incoming_unballoon(uint32_t trans_id) "incoming unballoon response %"PRIu32
+hv_balloon_outgoing_balloon(uint32_t trans_id, uint64_t count, uint64_t rempages) "posting balloon %"PRIu32" for %"PRIu64", remaining %"PRIu64
+hv_balloon_incoming_balloon(uint32_t trans_id, uint32_t range_count, uint32_t more_pages) "incoming balloon response %"PRIu32", ranges %"PRIu32", more %"PRIu32
+hv_balloon_remove_response(uint64_t count, uint64_t start, unsigned int both) "processing remove response range %"PRIu64" @ 0x%"PRIx64", both %u"
+hv_balloon_remove_response_hole(uint64_t counthole, uint64_t starthole, uint64_t countrange, uint64_t startrange, uint64_t starthpr, unsigned int both) "response range hole %"PRIu64" @ 0x%"PRIx64" from range %"PRIu64" @ 0x%"PRIx64", before our start 0x%"PRIx64", both %u"
+hv_balloon_remove_response_common(uint64_t countcommon, uint64_t startcommon, uint64_t countrange, uint64_t startrange, uint64_t counthpr, uint64_t starthpr, uint64_t removed, unsigned int both) "response common range %"PRIu64" @ 0x%"PRIx64" from range %"PRIu64" @ 0x%"PRIx64" with our %"PRIu64" @ 0x%"PRIx64", removed %"PRIu64", both %u"
+hv_balloon_remove_response_remainder(uint64_t count, uint64_t start, unsigned int both) "remove response remaining range %"PRIu64" @ 0x%"PRIx64", both %u"