Re: [PATCH v3 04/10] udp_vu: Move virtqueue management from udp_vu_sock_recv() to its caller

[Jon Maloy <jmaloy@redhat.com>]

On 2026-04-16 11:57, 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(), vu_set_vnethdr(), 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 array and adjusts its length.
> 
> Signed-off-by: Laurent Vivier <lvivier@redhat.com>
> Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

Reviewed-by: Jon Maloy <jmaloy@redhat.com>

see below.

> ---
>   udp_vu.c | 98 +++++++++++++++++++++++++++++---------------------------
>   1 file changed, 50 insertions(+), 48 deletions(-)
> 
> diff --git a/udp_vu.c b/udp_vu.c
> index f8629af58ab5..bd9fd5abb971 100644
> --- a/udp_vu.c
[...]
> +
> +	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++) {
> +		unsigned elem_cnt, elem_used;

unsigned int is standard in our code, I think.
/jon
> +		size_t iov_cnt;
>   		ssize_t dlen;
> -		int iov_used;
>   
> -		iov_used = udp_vu_sock_recv(c, vq, s, v6, &dlen);
> -		if (iov_used < 0)
> +		elem_cnt = vu_collect(vdev, vq, elem, ARRAY_SIZE(elem),
> +				      iov_vu, ARRAY_SIZE(iov_vu), &iov_cnt,
> +				      IP_MAX_MTU + ETH_HLEN + VNET_HLEN, NULL);
> +		if (elem_cnt == 0)
> +			break;
> +
> +		assert((size_t)elem_cnt == iov_cnt);	/* one iovec per element */
> +
> +		dlen = udp_vu_sock_recv(s, v6, &iov_cnt);
> +		if (dlen < 0) {
> +			vu_queue_rewind(vq, iov_cnt);
>   			break;
> +		}
> +
> +		elem_used = iov_cnt; /* one iovec per element */
> +
> +		/* release unused buffers */
> +		vu_queue_rewind(vq, elem_cnt - elem_used);
>   
> -		if (iov_used > 0) {
> +		if (iov_cnt > 0) {
>   			udp_vu_prepare(c, toside, dlen);
>   			if (*c->pcap) {
> -				udp_vu_csum(toside, iov_used);
> -				pcap_iov(iov_vu, iov_used, VNET_HLEN);
> +				udp_vu_csum(toside, iov_cnt);
> +				pcap_iov(iov_vu, iov_cnt, VNET_HLEN);
>   			}
> -			vu_flush(vdev, vq, elem, iov_used);
> +			vu_flush(vdev, vq, elem, iov_cnt);
>   			vu_queue_notify(vdev, vq);
>   		}
>   	}