Re: [PATCH 04/12] udp_vu: Move virtqueue management from udp_vu_sock_recv() to its caller

[David Gibson <david@gibson.dropbear.id.au>]

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On Fri, Feb 27, 2026 at 03:03:22PM +0100, Laurent Vivier wrote:
> udp_vu_sock_recv() currently mixes two concerns: receiving data from the
> socket and managing virtqueue buffers (collecting, rewinding, releasing).
> This makes the function harder to reason about and couples socket I/O
> with virtqueue state.
> 
> Move all virtqueue operations, vu_collect(), vu_init_elem(),
> vu_queue_rewind(), and the queue-readiness check, into
> udp_vu_sock_to_tap(), which is the only caller.  This turns
> udp_vu_sock_recv() into a pure socket receive function that simply reads
> into the provided iov_tail and adjusts its length.
> 
> Signed-off-by: Laurent Vivier <lvivier@redhat.com>

Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

> ---
>  udp_vu.c | 79 ++++++++++++++++++++++++++------------------------------
>  1 file changed, 37 insertions(+), 42 deletions(-)
> 
> diff --git a/udp_vu.c b/udp_vu.c
> index 8f4d0aedac10..aefcab0b86c2 100644
> --- a/udp_vu.c
> +++ b/udp_vu.c
> @@ -58,75 +58,41 @@ static size_t udp_vu_hdrlen(bool v6)
>  
>  /**
>   * udp_vu_sock_recv() - Receive datagrams from socket into vhost-user buffers
> - * @c:		Execution context
>   * @data:	IO vector tail for the frame (modified on output)
> - * @vq:		virtqueue to use to receive data
>   * @s:		Socket to receive from
>   * @v6:		Set for IPv6 connections
>   *
> - * Return: size of received data, 0 if the datagram
> - *         was discarded because the virtqueue is not ready, -1 on error
> + * Return: size of received data, -1 on error
>   */
> -static ssize_t udp_vu_sock_recv(const struct ctx *c, struct iov_tail *data,
> -				struct vu_virtq *vq, int s, bool v6)
> +static ssize_t udp_vu_sock_recv(struct iov_tail *data, int s, bool v6)
>  {
> -	const struct vu_dev *vdev = c->vdev;
> -	struct msghdr msg  = { 0 };
> +	struct iovec msg_iov[data->cnt];
> +	struct msghdr msg = { 0 };
>  	struct iov_tail payload;
>  	size_t hdrlen;
>  	ssize_t dlen;
> -	int iov_cnt;
> -
> -	ASSERT(!c->no_udp);
> -
> -	if (!vu_queue_enabled(vq) || !vu_queue_started(vq)) {
> -		debug("Got UDP packet, but RX virtqueue not usable yet");
> -
> -		if (recvmsg(s, &msg, MSG_DONTWAIT) < 0)
> -			debug_perror("Failed to discard datagram");
> -
> -		data->cnt = 0;
> -		return 0;
> -	}
>  
>  	/* compute L2 header length */
>  	hdrlen = udp_vu_hdrlen(v6);
>  
> -	vu_init_elem(elem, (struct iovec *)data->iov, data->cnt);
> -
> -	iov_cnt = vu_collect(vdev, vq, elem, ARRAY_SIZE(elem),
> -			     IP_MAX_MTU + ETH_HLEN + VNET_HLEN, NULL);
> -	if (iov_cnt == 0)
> -		return -1;
> -
> -	data->cnt = iov_cnt;
> -
>  	/* reserve space for the headers */
>  	ASSERT(iov_tail_size(data) >= MAX(hdrlen, ETH_ZLEN + VNET_HLEN));
>  
>  	payload = *data;
>  	iov_drop_header(&payload, hdrlen);
>  
> -	struct iovec msg_iov[payload.cnt];
>  	msg.msg_iov = msg_iov;
>  	msg.msg_iovlen = iov_tail_clone(msg.msg_iov, payload.cnt, &payload);
>  
>  	/* read data from the socket */
>  	dlen = recvmsg(s, &msg, 0);
> -	if (dlen < 0) {
> -		vu_queue_rewind(vq, iov_cnt);
> +	if (dlen < 0)
>  		return -1;
> -	}
>  
>  	iov_tail_truncate(data, MAX(dlen + hdrlen, ETH_ZLEN + VNET_HLEN));
>  	iov_tail_zero_end(data, dlen + hdrlen);
>  	iov_tail_truncate(data, dlen + hdrlen);
>  
> -	vu_set_vnethdr(vdev, data->iov[0].iov_base, data->cnt);
> -
> -	/* release unused buffers */
> -	vu_queue_rewind(vq, iov_cnt - data->cnt);
> -
>  	return dlen;
>  }
>  
> @@ -216,16 +182,45 @@ void udp_vu_sock_to_tap(const struct ctx *c, int s, int n, flow_sidx_t tosidx)
>  	struct iov_tail data;
>  	int i;
>  
> +	ASSERT(!c->no_udp);
> +
> +	if (!vu_queue_enabled(vq) || !vu_queue_started(vq)) {
> +		struct msghdr msg = { 0 };
> +
> +		debug("Got UDP packet, but RX virtqueue not usable yet");
> +
> +		for (i = 0; i < n; i++) {
> +			if (recvmsg(s, &msg, MSG_DONTWAIT) < 0)
> +				debug_perror("Failed to discard datagram");
> +		}
> +
> +		return;
> +	}
> +
>  	for (i = 0; i < n; i++) {
>  		ssize_t dlen;
> +		int elem_cnt;
>  
> -		data = IOV_TAIL(iov_vu, VIRTQUEUE_MAX_SIZE, 0);
> +		vu_init_elem(elem, iov_vu, ARRAY_SIZE(elem));
>  
> -		dlen = udp_vu_sock_recv(c, &data, vq, s, v6);
> -		if (dlen < 0)
> +		elem_cnt = vu_collect(vdev, vq, elem, ARRAY_SIZE(elem),
> +				      IP_MAX_MTU + ETH_HLEN + VNET_HLEN, NULL);
> +		if (elem_cnt == 0)
>  			break;
>  
> +		data = IOV_TAIL(iov_vu, elem_cnt, 0);
> +
> +		dlen = udp_vu_sock_recv(&data, s, v6);
> +		if (dlen < 0) {
> +			vu_queue_rewind(vq, elem_cnt);
> +			continue;
> +		}
> +
> +		/* release unused buffers */
> +		vu_queue_rewind(vq, elem_cnt - data.cnt);
> +
>  		if (data.cnt > 0) {
> +			vu_set_vnethdr(vdev, data.iov[0].iov_base, data.cnt);
>  			udp_vu_prepare(c, &data, toside, dlen);
>  			if (*c->pcap) {
>  				udp_vu_csum(toside, &data);
> -- 
> 2.53.0
> 

-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
http://www.ozlabs.org/~dgibson

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