3 files changed, 64 insertions, 2 deletions

diff --git a/docs/interop/vhost-user.rst b/docs/interop/vhost-user.rst
index 3b1b6602c7..b3cf5c3cb5 100644
--- a/docs/interop/vhost-user.rst
+++ b/docs/interop/vhost-user.rst
@@ -815,6 +815,7 @@ Protocol features
   #define VHOST_USER_PROTOCOL_F_INFLIGHT_SHMFD       12
   #define VHOST_USER_PROTOCOL_F_RESET_DEVICE         13
   #define VHOST_USER_PROTOCOL_F_INBAND_NOTIFICATIONS 14
+  #define VHOST_USER_PROTOCOL_F_CONFIGURE_MEM_SLOTS  15
 
 Master message types
 --------------------
@@ -1263,6 +1264,21 @@ Master message types
 
   The state.num field is currently reserved and must be set to 0.
 
+``VHOST_USER_GET_MAX_MEM_SLOTS``
+  :id: 36
+  :equivalent ioctl: N/A
+  :slave payload: u64
+
+  When the ``VHOST_USER_PROTOCOL_F_CONFIGURE_MEM_SLOTS`` protocol
+  feature has been successfully negotiated, this message is submitted
+  by master to the slave. The slave should return the message with a
+  u64 payload containing the maximum number of memory slots for
+  QEMU to expose to the guest. At this point, the value returned
+  by the backend will be capped at the maximum number of ram slots
+  which can be supported by vhost-user. Currently that limit is set
+  at VHOST_USER_MAX_RAM_SLOTS = 8 because of underlying protocol
+  limitations.
+
 Slave message types
 -------------------
 
diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c
index 442b0d650a..754ad885cf 100644
--- a/hw/virtio/vhost-user.c
+++ b/hw/virtio/vhost-user.c
@@ -59,6 +59,8 @@ enum VhostUserProtocolFeature {
     VHOST_USER_PROTOCOL_F_HOST_NOTIFIER = 11,
     VHOST_USER_PROTOCOL_F_INFLIGHT_SHMFD = 12,
     VHOST_USER_PROTOCOL_F_RESET_DEVICE = 13,
+    /* Feature 14 reserved for VHOST_USER_PROTOCOL_F_INBAND_NOTIFICATIONS. */
+    VHOST_USER_PROTOCOL_F_CONFIGURE_MEM_SLOTS = 15,
     VHOST_USER_PROTOCOL_F_MAX
 };
 
@@ -100,6 +102,8 @@ typedef enum VhostUserRequest {
     VHOST_USER_SET_INFLIGHT_FD = 32,
     VHOST_USER_GPU_SET_SOCKET = 33,
     VHOST_USER_RESET_DEVICE = 34,
+    /* Message number 35 reserved for VHOST_USER_VRING_KICK. */
+    VHOST_USER_GET_MAX_MEM_SLOTS = 36,
     VHOST_USER_MAX
 } VhostUserRequest;
 
@@ -895,6 +899,23 @@ static int vhost_user_set_owner(struct vhost_dev *dev)
     return 0;
 }
 
+static int vhost_user_get_max_memslots(struct vhost_dev *dev,
+                                       uint64_t *max_memslots)
+{
+    uint64_t backend_max_memslots;
+    int err;
+
+    err = vhost_user_get_u64(dev, VHOST_USER_GET_MAX_MEM_SLOTS,
+                             &backend_max_memslots);
+    if (err < 0) {
+        return err;
+    }
+
+    *max_memslots = backend_max_memslots;
+
+    return 0;
+}
+
 static int vhost_user_reset_device(struct vhost_dev *dev)
 {
     VhostUserMsg msg = {
@@ -1392,7 +1413,7 @@ static int vhost_user_postcopy_notifier(NotifierWithReturn *notifier,
 
 static int vhost_user_backend_init(struct vhost_dev *dev, void *opaque)
 {
-    uint64_t features, protocol_features;
+    uint64_t features, protocol_features, ram_slots;
     struct vhost_user *u;
     int err;
 
@@ -1454,6 +1475,27 @@ static int vhost_user_backend_init(struct vhost_dev *dev, void *opaque)
                          "slave-req protocol features.");
             return -1;
         }
+
+        /* get max memory regions if backend supports configurable RAM slots */
+        if (!virtio_has_feature(dev->protocol_features,
+                                VHOST_USER_PROTOCOL_F_CONFIGURE_MEM_SLOTS)) {
+            u->user->memory_slots = VHOST_MEMORY_MAX_NREGIONS;
+        } else {
+            err = vhost_user_get_max_memslots(dev, &ram_slots);
+            if (err < 0) {
+                return err;
+            }
+
+            if (ram_slots < u->user->memory_slots) {
+                error_report("The backend specified a max ram slots limit "
+                             "of %" PRIu64", when the prior validated limit was %d. "
+                             "This limit should never decrease.", ram_slots,
+                             u->user->memory_slots);
+                return -1;
+            }
+
+            u->user->memory_slots = MIN(ram_slots, VHOST_MEMORY_MAX_NREGIONS);
+        }
     }
 
     if (dev->migration_blocker == NULL &&
@@ -1519,7 +1561,9 @@ static int vhost_user_get_vq_index(struct vhost_dev *dev, int idx)
 
 static int vhost_user_memslots_limit(struct vhost_dev *dev)
 {
-    return VHOST_MEMORY_MAX_NREGIONS;
+    struct vhost_user *u = dev->opaque;
+
+    return u->user->memory_slots;
 }
 
 static bool vhost_user_requires_shm_log(struct vhost_dev *dev)
@@ -1904,6 +1948,7 @@ bool vhost_user_init(VhostUserState *user, CharBackend *chr, Error **errp)
         return false;
     }
     user->chr = chr;
+    user->memory_slots = 0;
     return true;
 }
 
diff --git a/include/hw/virtio/vhost-user.h b/include/hw/virtio/vhost-user.h
index 811e325f42..a9abca3288 100644
--- a/include/hw/virtio/vhost-user.h
+++ b/include/hw/virtio/vhost-user.h
@@ -20,6 +20,7 @@ typedef struct VhostUserHostNotifier {
 typedef struct VhostUserState {
     CharBackend *chr;
     VhostUserHostNotifier notifier[VIRTIO_QUEUE_MAX];
+    int memory_slots;
 } VhostUserState;
 
 bool vhost_user_init(VhostUserState *user, CharBackend *chr, Error **errp);