vhost: Return number of free memslots

Let's return the number of free slots instead of only checking if there
is a free slot. Required to support memory devices that consume multiple
memslots.

This is a preparation for memory devices that consume multiple memslots.

Message-ID: <20230926185738.277351-6-david@redhat.com>
Reviewed-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>

hw/mem/memory-device.c

@@ -63,7 +63,7 @@ static void memory_device_check_addable(MachineState *ms, MemoryRegion *mr,
         error_setg(errp, "hypervisor has no free memory slots left");
         return;
     }
-    if (!vhost_has_free_slot()) {
+    if (!vhost_get_free_memslots()) {
         error_setg(errp, "a used vhost backend has no free memory slots left");
         return;
     }

hw/virtio/vhost-stub.c

@@ -2,9 +2,9 @@
 #include "hw/virtio/vhost.h"
 #include "hw/virtio/vhost-user.h"
 
-bool vhost_has_free_slot(void)
+unsigned int vhost_get_free_memslots(void)
 {
-    return true;
+    return UINT_MAX;
 }
 
 bool vhost_user_init(VhostUserState *user, CharBackend *chr, Error **errp)

hw/virtio/vhost.c

@@ -54,7 +54,7 @@ static unsigned int used_shared_memslots;
 static QLIST_HEAD(, vhost_dev) vhost_devices =
     QLIST_HEAD_INITIALIZER(vhost_devices);
 
-bool vhost_has_free_slot(void)
+unsigned int vhost_get_free_memslots(void)
 {
     unsigned int free = UINT_MAX;
     struct vhost_dev *hdev;
@@ -71,7 +71,7 @@ bool vhost_has_free_slot(void)
         }
         free = MIN(free, cur_free);
     }
-    return free > 0;
+    return free;
 }
 
 static void vhost_dev_sync_region(struct vhost_dev *dev,

include/hw/virtio/vhost.h

@@ -315,7 +315,7 @@ uint64_t vhost_get_features(struct vhost_dev *hdev, const int *feature_bits,
  */
 void vhost_ack_features(struct vhost_dev *hdev, const int *feature_bits,
                         uint64_t features);
-bool vhost_has_free_slot(void);
+unsigned int vhost_get_free_memslots(void);
 
 int vhost_net_set_backend(struct vhost_dev *hdev,
                           struct vhost_vring_file *file);