vhost-user: don't merge regions with different fds

vhost currently merges regions with contiguious virtual and physical
addresses.  This breaks for vhost-user since that also needs fds to
match.

Add a vhost_ops entry to compare the fds for vhost-user only.

Cc: qemu-stable@nongnu.org
Cc: Victor Kaplansky <victork@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
(cherry picked from commit ffe42cc14c)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>

hw/virtio/vhost-user.c

@@ -612,6 +612,25 @@ static int vhost_user_migration_done(struct vhost_dev *dev, char* mac_addr)
     return -1;
 }
 
+static bool vhost_user_can_merge(struct vhost_dev *dev,
+                                 uint64_t start1, uint64_t size1,
+                                 uint64_t start2, uint64_t size2)
+{
+    ram_addr_t ram_addr;
+    int mfd, rfd;
+    MemoryRegion *mr;
+
+    mr = qemu_ram_addr_from_host((void *)(uintptr_t)start1, &ram_addr);
+    assert(mr);
+    mfd = qemu_get_ram_fd(ram_addr);
+
+    mr = qemu_ram_addr_from_host((void *)(uintptr_t)start2, &ram_addr);
+    assert(mr);
+    rfd = qemu_get_ram_fd(ram_addr);
+
+    return mfd == rfd;
+}
+
 const VhostOps user_ops = {
         .backend_type = VHOST_BACKEND_TYPE_USER,
         .vhost_backend_init = vhost_user_init,
@@ -634,4 +653,5 @@ const VhostOps user_ops = {
         .vhost_set_vring_enable = vhost_user_set_vring_enable,
         .vhost_requires_shm_log = vhost_user_requires_shm_log,
         .vhost_migration_done = vhost_user_migration_done,
+        .vhost_backend_can_merge = vhost_user_can_merge,
 };

hw/virtio/vhost.c

@@ -259,6 +259,13 @@ static void vhost_dev_assign_memory(struct vhost_dev *dev,
             continue;
         }
 
+        if (dev->vhost_ops->vhost_backend_can_merge &&
+            !dev->vhost_ops->vhost_backend_can_merge(dev, uaddr, size,
+                                                     reg->userspace_addr,
+                                                     reg->memory_size)) {
+            continue;
+        }
+
         if (merged) {
             --to;
             assert(to >= 0);

include/hw/virtio/vhost-backend.h

@@ -70,6 +70,9 @@ typedef int (*vhost_set_vring_enable_op)(struct vhost_dev *dev,
 typedef bool (*vhost_requires_shm_log_op)(struct vhost_dev *dev);
 typedef int (*vhost_migration_done_op)(struct vhost_dev *dev,
                                        char *mac_addr);
+typedef bool (*vhost_backend_can_merge_op)(struct vhost_dev *dev,
+                                           uint64_t start1, uint64_t size1,
+                                           uint64_t start2, uint64_t size2);
 
 typedef struct VhostOps {
     VhostBackendType backend_type;
@@ -97,6 +100,7 @@ typedef struct VhostOps {
     vhost_set_vring_enable_op vhost_set_vring_enable;
     vhost_requires_shm_log_op vhost_requires_shm_log;
     vhost_migration_done_op vhost_migration_done;
+    vhost_backend_can_merge_op vhost_backend_can_merge;
 } VhostOps;
 
 extern const VhostOps user_ops;