vhost: logs sharing

Currently we allocate one vhost log per vhost device. This is sub
optimal when:

- Guest has several device with vhost as backend
- Guest has multiqueue devices

In the above cases, we can avoid the memory allocation by sharing a
single vhost log among all the vhost devices. This is done through:

- Introducing a new vhost_log structure with refcnt inside.
- Using a global pointer to vhost_log structure that will be used. And
  introduce helper to get the log with expected log size and helper to
- drop the refcnt to the old log.
- Each vhost device still keep track of a pointer to the log that was
  used.

With above, if no resize happens, all vhost device will share a single
vhost log. During resize, a new vhost_log structure will be allocated
and made for the global pointer. And each vhost devices will drop the
refcnt to the old log.

Tested by doing scp during migration for a 2 queues virtio-net-pci.

Cc: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

include/hw/virtio/vhost.h

@@ -28,6 +28,12 @@ typedef unsigned long vhost_log_chunk_t;
 #define VHOST_LOG_CHUNK (VHOST_LOG_PAGE * VHOST_LOG_BITS)
 #define VHOST_INVALID_FEATURE_BIT   (0xff)
 
+struct vhost_log {
+    unsigned long long size;
+    int refcnt;
+    vhost_log_chunk_t log[0];
+};
+
 struct vhost_memory;
 struct vhost_dev {
     MemoryListener memory_listener;
@@ -43,7 +49,6 @@ struct vhost_dev {
     unsigned long long backend_features;
     bool started;
     bool log_enabled;
-    vhost_log_chunk_t *log;
     unsigned long long log_size;
     Error *migration_blocker;
     bool force;
@@ -52,6 +57,7 @@ struct vhost_dev {
     hwaddr mem_changed_end_addr;
     const VhostOps *vhost_ops;
     void *opaque;
+    struct vhost_log *log;
 };
 
 int vhost_dev_init(struct vhost_dev *hdev, void *opaque,