vdpa_sim_blk: support shared backend

The vdpa_sim_blk simulator uses a ramdisk as the backend. To test live
migration, we need two devices that share the backend to have the data
synchronized with each other.

Add a new module parameter to make the buffer shared between all devices.

The shared_buffer_mutex is used just to ensure that each operation is
atomic, but it is up to the user to use the devices knowing that the
underlying ramdisk is shared.

For example, when we do a migration, the VMM (e.g., QEMU) will guarantee
to write to the destination device, only after completing operations with
the source device.

Signed-off-by: Stefano Garzarella <[e-mail]>
Message-Id: <[e-mail]>
Signed-off-by: Michael S. Tsirkin <[e-mail]>

drivers/vdpa/vdpa_sim/vdpa_sim_blk.c

···
 struct vdpasim_blk {
 	struct vdpasim vdpasim;
 	void *buffer;
+	bool shared_backend;
 };
 
 static struct vdpasim_blk *sim_to_blk(struct vdpasim *vdpasim)
···
 }
 
 static char vdpasim_blk_id[VIRTIO_BLK_ID_BYTES] = "vdpa_blk_sim";
+
+static bool shared_backend;
+module_param(shared_backend, bool, 0444);
+MODULE_PARM_DESC(shared_backend, "Enable the shared backend between virtio-blk devices");
+
+static void *shared_buffer;
+/* mutex to synchronize shared_buffer access */
+static DEFINE_MUTEX(shared_buffer_mutex);
+
+static void vdpasim_blk_buffer_lock(struct vdpasim_blk *blk)
+{
+	if (blk->shared_backend)
+		mutex_lock(&shared_buffer_mutex);
+}
+
+static void vdpasim_blk_buffer_unlock(struct vdpasim_blk *blk)
+{
+	if (blk->shared_backend)
+		mutex_unlock(&shared_buffer_mutex);
+}
 
 static bool vdpasim_blk_check_range(struct vdpasim *vdpasim, u64 start_sector,
 				    u64 num_sectors, u64 max_sectors)
···
 			break;
 		}
 
+		vdpasim_blk_buffer_lock(blk);
 		bytes = vringh_iov_push_iotlb(&vq->vring, &vq->in_iov,
 					      blk->buffer + offset, to_push);
+		vdpasim_blk_buffer_unlock(blk);
 		if (bytes < 0) {
 			dev_dbg(&vdpasim->vdpa.dev,
 				"vringh_iov_push_iotlb() error: %zd offset: 0x%llx len: 0x%zx\n",
···
 			break;
 		}
 
+		vdpasim_blk_buffer_lock(blk);
 		bytes = vringh_iov_pull_iotlb(&vq->vring, &vq->out_iov,
 					      blk->buffer + offset, to_pull);
+		vdpasim_blk_buffer_unlock(blk);
 		if (bytes < 0) {
 			dev_dbg(&vdpasim->vdpa.dev,
 				"vringh_iov_pull_iotlb() error: %zd offset: 0x%llx len: 0x%zx\n",
···
 		}
 
 		if (type == VIRTIO_BLK_T_WRITE_ZEROES) {
+			vdpasim_blk_buffer_lock(blk);
 			memset(blk->buffer + offset, 0,
 			       num_sectors << SECTOR_SHIFT);
+			vdpasim_blk_buffer_unlock(blk);
 		}
 
 		break;
···
 {
 	struct vdpasim_blk *blk = sim_to_blk(vdpasim);
 
-	kvfree(blk->buffer);
+	if (!blk->shared_backend)
+		kvfree(blk->buffer);
 }
 
 static void vdpasim_blk_mgmtdev_release(struct device *dev)
···
 		return PTR_ERR(simdev);
 
 	blk = sim_to_blk(simdev);
+	blk->shared_backend = shared_backend;
 
-	blk->buffer = kvmalloc(VDPASIM_BLK_CAPACITY << SECTOR_SHIFT,
-			       GFP_KERNEL);
-	if (!blk->buffer) {
-		ret = -ENOMEM;
-		goto put_dev;
+	if (blk->shared_backend) {
+		blk->buffer = shared_buffer;
+	} else {
+		blk->buffer = kvmalloc(VDPASIM_BLK_CAPACITY << SECTOR_SHIFT,
+				       GFP_KERNEL);
+		if (!blk->buffer) {
+			ret = -ENOMEM;
+			goto put_dev;
+		}
 	}
 
 	ret = _vdpa_register_device(&simdev->vdpa, VDPASIM_BLK_VQ_NUM);
···
 	if (ret)
 		goto parent_err;
 
+	if (shared_backend) {
+		shared_buffer = kvmalloc(VDPASIM_BLK_CAPACITY << SECTOR_SHIFT,
+					 GFP_KERNEL);
+		if (!shared_buffer) {
+			ret = -ENOMEM;
+			goto parent_err;
+		}
+	}
+
 	return 0;
 
 parent_err:
···
 
 static void __exit vdpasim_blk_exit(void)
 {
+	kvfree(shared_buffer);
 	vdpa_mgmtdev_unregister(&mgmt_dev);
 	device_unregister(&vdpasim_blk_mgmtdev);
 }