vhost: switch to use new message format

We use to have message like:

struct vhost_msg {
int type;
union {
struct vhost_iotlb_msg iotlb;
__u8 padding[64];
};
};

Unfortunately, there will be a hole of 32bit in 64bit machine because
of the alignment. This leads a different formats between 32bit API and
64bit API. What's more it will break 32bit program running on 64bit
machine.

So fixing this by introducing a new message type with an explicit
32bit reserved field after type like:

struct vhost_msg_v2 {
__u32 type;
__u32 reserved;
union {
struct vhost_iotlb_msg iotlb;
__u8 padding[64];
};
};

We will have a consistent ABI after switching to use this. To enable
this capability, introduce a new ioctl (VHOST_SET_BAKCEND_FEATURE) for
userspace to enable this feature (VHOST_BACKEND_F_IOTLB_V2).

Fixes: 6b1e6cc7855b ("vhost: new device IOTLB API")
Signed-off-by: Jason Wang <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/vhost/net.c

···
 };
 
 enum {
+	VHOST_NET_BACKEND_FEATURES = (1ULL << VHOST_BACKEND_F_IOTLB_MSG_V2)
+};
+
+enum {
 	VHOST_NET_VQ_RX = 0,
 	VHOST_NET_VQ_TX = 1,
 	VHOST_NET_VQ_MAX = 2,
···
 	return err;
 }
 
+static int vhost_net_set_backend_features(struct vhost_net *n, u64 features)
+{
+	int i;
+
+	mutex_lock(&n->dev.mutex);
+	for (i = 0; i < VHOST_NET_VQ_MAX; ++i) {
+		mutex_lock(&n->vqs[i].vq.mutex);
+		n->vqs[i].vq.acked_backend_features = features;
+		mutex_unlock(&n->vqs[i].vq.mutex);
+	}
+	mutex_unlock(&n->dev.mutex);
+
+	return 0;
+}
+
 static int vhost_net_set_features(struct vhost_net *n, u64 features)
 {
 	size_t vhost_hlen, sock_hlen, hdr_len;
···
 		if (features & ~VHOST_NET_FEATURES)
 			return -EOPNOTSUPP;
 		return vhost_net_set_features(n, features);
+	case VHOST_GET_BACKEND_FEATURES:
+		features = VHOST_NET_BACKEND_FEATURES;
+		if (copy_to_user(featurep, &features, sizeof(features)))
+			return -EFAULT;
+		return 0;
+	case VHOST_SET_BACKEND_FEATURES:
+		if (copy_from_user(&features, featurep, sizeof(features)))
+			return -EFAULT;
+		if (features & ~VHOST_NET_BACKEND_FEATURES)
+			return -EOPNOTSUPP;
+		return vhost_net_set_backend_features(n, features);
 	case VHOST_RESET_OWNER:
 		return vhost_net_reset_owner(n);
 	case VHOST_SET_OWNER:

drivers/vhost/vhost.c

···
 	vq->log_addr = -1ull;
 	vq->private_data = NULL;
 	vq->acked_features = 0;
+	vq->acked_backend_features = 0;
 	vq->log_base = NULL;
 	vq->error_ctx = NULL;
 	vq->kick = NULL;
···
 ssize_t vhost_chr_write_iter(struct vhost_dev *dev,
 			     struct iov_iter *from)
 {
-	struct vhost_msg_node node;
-	unsigned size = sizeof(struct vhost_msg);
-	size_t ret;
-	int err;
+	struct vhost_iotlb_msg msg;
+	size_t offset;
+	int type, ret;
 
-	if (iov_iter_count(from) < size)
-		return 0;
-	ret = copy_from_iter(&node.msg, size, from);
-	if (ret != size)
+	ret = copy_from_iter(&type, sizeof(type), from);
+	if (ret != sizeof(type))
 		goto done;
 
-	switch (node.msg.type) {
+	switch (type) {
 	case VHOST_IOTLB_MSG:
-		err = vhost_process_iotlb_msg(dev, &node.msg.iotlb);
-		if (err)
-			ret = err;
+		/* There maybe a hole after type for V1 message type,
+		 * so skip it here.
+		 */
+		offset = offsetof(struct vhost_msg, iotlb) - sizeof(int);
+		break;
+	case VHOST_IOTLB_MSG_V2:
+		offset = sizeof(__u32);
 		break;
 	default:
 		ret = -EINVAL;
-		break;
+		goto done;
 	}
 
+	iov_iter_advance(from, offset);
+	ret = copy_from_iter(&msg, sizeof(msg), from);
+	if (ret != sizeof(msg))
+		goto done;
+	if (vhost_process_iotlb_msg(dev, &msg)) {
+		ret = -EFAULT;
+		goto done;
+	}
+
+	ret = (type == VHOST_IOTLB_MSG) ? sizeof(struct vhost_msg) :
+	      sizeof(struct vhost_msg_v2);
 done:
 	return ret;
 }
···
 		finish_wait(&dev->wait, &wait);
 
 	if (node) {
-		ret = copy_to_iter(&node->msg, size, to);
+		struct vhost_iotlb_msg *msg;
+		void *start = &node->msg;
 
-		if (ret != size || node->msg.type != VHOST_IOTLB_MISS) {
+		switch (node->msg.type) {
+		case VHOST_IOTLB_MSG:
+			size = sizeof(node->msg);
+			msg = &node->msg.iotlb;
+			break;
+		case VHOST_IOTLB_MSG_V2:
+			size = sizeof(node->msg_v2);
+			msg = &node->msg_v2.iotlb;
+			break;
+		default:
+			BUG();
+			break;
+		}
+
+		ret = copy_to_iter(start, size, to);
+		if (ret != size || msg->type != VHOST_IOTLB_MISS) {
 			kfree(node);
 			return ret;
 		}
-
 		vhost_enqueue_msg(dev, &dev->pending_list, node);
 	}
 
···
 	struct vhost_dev *dev = vq->dev;
 	struct vhost_msg_node *node;
 	struct vhost_iotlb_msg *msg;
+	bool v2 = vhost_backend_has_feature(vq, VHOST_BACKEND_F_IOTLB_MSG_V2);
 
-	node = vhost_new_msg(vq, VHOST_IOTLB_MISS);
+	node = vhost_new_msg(vq, v2 ? VHOST_IOTLB_MSG_V2 : VHOST_IOTLB_MSG);
 	if (!node)
 		return -ENOMEM;
 
-	msg = &node->msg.iotlb;
+	if (v2) {
+		node->msg_v2.type = VHOST_IOTLB_MSG_V2;
+		msg = &node->msg_v2.iotlb;
+	} else {
+		msg = &node->msg.iotlb;
+	}
+
 	msg->type = VHOST_IOTLB_MISS;
 	msg->iova = iova;
 	msg->perm = access;

drivers/vhost/vhost.h

···
 	struct vhost_umem *iotlb;
 	void *private_data;
 	u64 acked_features;
+	u64 acked_backend_features;
 	/* Log write descriptors */
 	void __user *log_base;
 	struct vhost_log *log;
···
 };
 
 struct vhost_msg_node {
-  struct vhost_msg msg;
+  union {
+	  struct vhost_msg msg;
+	  struct vhost_msg_v2 msg_v2;
+  };
   struct vhost_virtqueue *vq;
   struct list_head node;
 };
···
 static inline bool vhost_has_feature(struct vhost_virtqueue *vq, int bit)
 {
 	return vq->acked_features & (1ULL << bit);
+}
+
+static inline bool vhost_backend_has_feature(struct vhost_virtqueue *vq, int bit)
+{
+	return vq->acked_backend_features & (1ULL << bit);
 }
 
 #ifdef CONFIG_VHOST_CROSS_ENDIAN_LEGACY

include/uapi/linux/vhost.h

···
 };
 
 #define VHOST_IOTLB_MSG 0x1
+#define VHOST_IOTLB_MSG_V2 0x2
 
 struct vhost_msg {
 	int type;
+	union {
+		struct vhost_iotlb_msg iotlb;
+		__u8 padding[64];
+	};
+};
+
+struct vhost_msg_v2 {
+	__u32 type;
+	__u32 reserved;
 	union {
 		struct vhost_iotlb_msg iotlb;
 		__u8 padding[64];
···
 /* Get busy loop timeout (in us) */
 #define VHOST_GET_VRING_BUSYLOOP_TIMEOUT _IOW(VHOST_VIRTIO, 0x24,	\
 					 struct vhost_vring_state)
+
+/* Set or get vhost backend capability */
+
+/* Use message type V2 */
+#define VHOST_BACKEND_F_IOTLB_MSG_V2 0x1
+
+#define VHOST_SET_BACKEND_FEATURES _IOW(VHOST_VIRTIO, 0x25, __u64)
+#define VHOST_GET_BACKEND_FEATURES _IOW(VHOST_VIRTIO, 0x26, __u64)
 
 /* VHOST_NET specific defines */