Re: [RFC PATCH v7 12/22] virtio/vsock: fetch length for SEQPACKET record

[Stefano Garzarella]

On Tue, Mar 23, 2021 at 04:12:55PM +0300, Arseny Krasnov wrote:
> This adds transport callback which tries to fetch record begin marker
> from socket's rx queue. It is called from af_vsock.c before reading data
> packets of record.
>
> Signed-off-by: Arseny Krasnov <arseny.krasnov@xxxxxxxxxxxxx>
> ---
> v6 -> v7:
> 1) Now 'virtio_transport_seqpacket_seq_get_len()' returns 0, if rx
>    queue of socket is empty. Else it returns length of current message
>    to handle.
> 2) If dequeue callback is called, but there is no detected length of
>    message to dequeue, EAGAIN is returned, and outer loop restarts
>    receiving.
>
> net/vmw_vsock/virtio_transport_common.c | 61 +++++++++++++++++++++++++
> 1 file changed, 61 insertions(+)
>
> diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
> index a8f4326e45e8..41f05034593e 100644
> --- a/net/vmw_vsock/virtio_transport_common.c
> +++ b/net/vmw_vsock/virtio_transport_common.c
> @@ -399,6 +399,62 @@ static inline void virtio_transport_remove_pkt(struct virtio_vsock_pkt *pkt)
> 	virtio_transport_free_pkt(pkt);
> }
>
> +static size_t virtio_transport_drop_until_seq_begin(struct 
> virtio_vsock_sock *vvs)
> +{
> +	struct virtio_vsock_pkt *pkt, *n;
> +	size_t bytes_dropped = 0;
> +
> +	list_for_each_entry_safe(pkt, n, &vvs->rx_queue, list) {
> +		if (le16_to_cpu(pkt->hdr.op) == VIRTIO_VSOCK_OP_SEQ_BEGIN)
> +			break;
> +
> +		bytes_dropped += le32_to_cpu(pkt->hdr.len);
> +		virtio_transport_dec_rx_pkt(vvs, pkt);
> +		virtio_transport_remove_pkt(pkt);
> +	}
> +
> +	return bytes_dropped;
> +}
> +
> +static size_t virtio_transport_seqpacket_seq_get_len(struct vsock_sock *vsk)
> +{
> +	struct virtio_vsock_seq_hdr *seq_hdr;
> +	struct virtio_vsock_sock *vvs;
> +	struct virtio_vsock_pkt *pkt;
> +	size_t bytes_dropped = 0;
> +
> +	vvs = vsk->trans;
> +
> +	spin_lock_bh(&vvs->rx_lock);
> +
> +	/* Have some record to process, return it's length. */
> +	if (vvs->seq_state.user_read_seq_len)
> +		goto out;
> +
> +	/* Fetch all orphaned 'RW' packets and send credit update. */
> +	bytes_dropped = virtio_transport_drop_until_seq_begin(vvs);
> +
> +	if (list_empty(&vvs->rx_queue))
> +		goto out;
> +
> +	pkt = list_first_entry(&vvs->rx_queue, struct virtio_vsock_pkt, list);
> +
> +	vvs->seq_state.user_read_copied = 0;
> +
> +	seq_hdr = (struct virtio_vsock_seq_hdr *)pkt->buf;
> +	vvs->seq_state.user_read_seq_len = le32_to_cpu(seq_hdr->msg_len);
> +	vvs->seq_state.curr_rx_msg_id = le32_to_cpu(seq_hdr->msg_id);
> +	virtio_transport_dec_rx_pkt(vvs, pkt);
> +	virtio_transport_remove_pkt(pkt);
> +out:
> +	spin_unlock_bh(&vvs->rx_lock);
> +
> +	if (bytes_dropped)
> +		virtio_transport_send_credit_update(vsk);
> +
> +	return vvs->seq_state.user_read_seq_len;
> +}
> +
> static int virtio_transport_seqpacket_do_dequeue(struct vsock_sock *vsk,
> 						 struct msghdr *msg,
> 						 bool *msg_ready)
> @@ -522,6 +578,11 @@ virtio_transport_seqpacket_dequeue(struct vsock_sock *vsk,
> 	if (flags & MSG_PEEK)
> 		return -EOPNOTSUPP;
>
> +	*msg_len = virtio_transport_seqpacket_seq_get_len(vsk);
> +
> +	if (*msg_len == 0)
> +		return -EAGAIN;
> +

Okay, I see now, I think you can move this patch before the previous one 
or merge them in a single patch, it is better to review and to bisect.

As mentioned, I think we can return msg_len if 
virtio_transport_seqpacket_do_dequeue() does not fail, otherwise the 
error.

I mean something like this:

static ssize_t virtio_transport_seqpacket_do_dequeue(...)
{
	size_t msg_len;
	ssize_t ret;

	msg_len = virtio_transport_seqpacket_seq_get_len(vsk);
	if (msg_len == 0)
		return -EAGAIN;

	ret = virtio_transport_seqpacket_do_dequeue(vsk, msg, msg_ready);
	if (ret < 0)
		return ret;

	return msg_len;
}

> 	return virtio_transport_seqpacket_do_dequeue(vsk, msg, msg_ready);
> }
> EXPORT_SYMBOL_GPL(virtio_transport_seqpacket_dequeue);
> -- 2.25.1