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

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Date:   Thu, 25 Mar 2021 11:08:10 +0100
From:   Stefano Garzarella <sgarzare@...hat.com>
To:     Arseny Krasnov <arseny.krasnov@...persky.com>
Cc:     Stefan Hajnoczi <stefanha@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Jorgen Hansen <jhansen@...are.com>,
        Andra Paraschiv <andraprs@...zon.com>,
        Colin Ian King <colin.king@...onical.com>,
        Norbert Slusarek <nslusarek@....net>,
        Alexander Popov <alex.popov@...ux.com>, kvm@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, stsp2@...dex.ru, oxffffaa@...il.com
Subject: Re: [RFC PATCH v7 12/22] virtio/vsock: fetch length for SEQPACKET
 record

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@...persky.com>
>---
> 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
>