On Fri, May 10, 2019 at 02:58:36PM +0200, Stefano Garzarella wrote:
> +struct virtio_vsock_buf {

Please add a comment describing the purpose of this struct and to
differentiate its use from struct virtio_vsock_pkt.

> +static struct virtio_vsock_buf *
> +virtio_transport_alloc_buf(struct virtio_vsock_pkt *pkt, bool zero_copy)
> +{
> +	struct virtio_vsock_buf *buf;
> +
> +	if (pkt->len == 0)
> +		return NULL;
> +
> +	buf = kzalloc(sizeof(*buf), GFP_KERNEL);
> +	if (!buf)
> +		return NULL;
> +
> +	/* If the buffer in the virtio_vsock_pkt is full, we can move it to
> +	 * the new virtio_vsock_buf avoiding the copy, because we are sure that
> +	 * we are not use more memory than that counted by the credit mechanism.
> +	 */
> +	if (zero_copy && pkt->len == pkt->buf_len) {
> +		buf->addr = pkt->buf;
> +		pkt->buf = NULL;
> +	} else {
> +		buf->addr = kmalloc(pkt->len, GFP_KERNEL);

buf and buf->addr could be allocated in a single call, though I'm not
sure how big an optimization this is.

> @@ -841,20 +882,24 @@ virtio_transport_recv_connected(struct sock *sk,
>  {
>  	struct vsock_sock *vsk = vsock_sk(sk);
>  	struct virtio_vsock_sock *vvs = vsk->trans;
> +	struct virtio_vsock_buf *buf;
>  	int err = 0;
>  
>  	switch (le16_to_cpu(pkt->hdr.op)) {
>  	case VIRTIO_VSOCK_OP_RW:
>  		pkt->len = le32_to_cpu(pkt->hdr.len);
> -		pkt->off = 0;
> +		buf = virtio_transport_alloc_buf(pkt, true);
>  
> -		spin_lock_bh(&vvs->rx_lock);
> -		virtio_transport_inc_rx_pkt(vvs, pkt);
> -		list_add_tail(&pkt->list, &vvs->rx_queue);
> -		spin_unlock_bh(&vvs->rx_lock);
> +		if (buf) {
> +			spin_lock_bh(&vvs->rx_lock);
> +			virtio_transport_inc_rx_pkt(vvs, pkt->len);
> +			list_add_tail(&buf->list, &vvs->rx_queue);
> +			spin_unlock_bh(&vvs->rx_lock);
>  
> -		sk->sk_data_ready(sk);
> -		return err;
> +			sk->sk_data_ready(sk);
> +		}

The return value of this function isn't used but the code still makes an
effort to return errors.  Please return -ENOMEM when buf == NULL.

If you'd like to remove the return value that's fine too, but please do
it for the whole function to be consistent.