Re: [PATCH v7 1/3] udp_vu: Allow virtqueue elements with multiple iovec entries

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

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On Fri, Apr 03, 2026 at 06:42:39PM +0200, Laurent Vivier wrote:
> The previous code assumed a 1:1 mapping between virtqueue elements and
> iovec entries (enforced by an assert).  Drop that assumption to allow
> elements that span multiple iovecs: track elem_used separately by
> walking the element list against the iov count returned after padding.
> This also fixes vu_queue_rewind() and vu_flush() to use the element
> count rather than the iov count.
> 
> Use iov_tail_clone() in udp_vu_sock_recv() to handle header offset,
> replacing the manual base/len adjustment and restore pattern.
> 
> Signed-off-by: Laurent Vivier <lvivier@redhat.com>

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

> ---
>  udp_vu.c | 31 ++++++++++++++++---------------
>  1 file changed, 16 insertions(+), 15 deletions(-)
> 
> diff --git a/udp_vu.c b/udp_vu.c
> index 30af64034516..16086e6c0c03 100644
> --- a/udp_vu.c
> +++ b/udp_vu.c
> @@ -64,30 +64,25 @@ static size_t udp_vu_hdrlen(bool v6)
>   */
>  static ssize_t udp_vu_sock_recv(struct iovec *iov, size_t *cnt, int s, bool v6)
>  {
> +	struct iovec msg_iov[*cnt];
>  	struct msghdr msg  = { 0 };
> +	struct iov_tail payload;
>  	size_t hdrlen, iov_used;
>  	ssize_t dlen;
>  
>  	/* compute L2 header length */
>  	hdrlen = udp_vu_hdrlen(v6);
>  
> -	/* reserve space for the headers */
> -	assert(iov[0].iov_len >= MAX(hdrlen, ETH_ZLEN + VNET_HLEN));
> -	iov[0].iov_base = (char *)iov[0].iov_base + hdrlen;
> -	iov[0].iov_len -= hdrlen;
> +	payload = IOV_TAIL(iov, *cnt, hdrlen);
>  
> -	/* read data from the socket */
> -	msg.msg_iov = iov;
> -	msg.msg_iovlen = *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)
>  		return -1;
>  
> -	/* restore the pointer to the headers address */
> -	iov[0].iov_base = (char *)iov[0].iov_base - hdrlen;
> -	iov[0].iov_len += hdrlen;
> -
>  	iov_used = iov_skip_bytes(iov, *cnt,
>  				  MAX(dlen + hdrlen, VNET_HLEN + ETH_ZLEN),
>  				  NULL);
> @@ -205,7 +200,7 @@ void udp_vu_sock_to_tap(const struct ctx *c, int s, int n, flow_sidx_t tosidx)
>  	}
>  
>  	for (i = 0; i < n; i++) {
> -		unsigned elem_cnt, elem_used;
> +		unsigned elem_cnt, elem_used, j, k;
>  		size_t iov_cnt;
>  		ssize_t dlen;
>  
> @@ -215,15 +210,21 @@ void udp_vu_sock_to_tap(const struct ctx *c, int s, int n, flow_sidx_t tosidx)
>  		if (elem_cnt == 0)
>  			break;
>  
> -		assert((size_t)elem_cnt == iov_cnt);	/* one iovec per element */
> -
>  		dlen = udp_vu_sock_recv(iov_vu, &iov_cnt, s, v6);
>  		if (dlen < 0) {
>  			vu_queue_rewind(vq, elem_cnt);
>  			break;
>  		}
>  
> -		elem_used = iov_cnt; /* one iovec per element */
> +		elem_used = 0;
> +		for (j = 0, k = 0; k < iov_cnt && j < elem_cnt; j++) {
> +			size_t iov_still_needed = iov_cnt - k;
> +
> +			if (elem[j].in_num > iov_still_needed)
> +				elem[j].in_num = iov_still_needed;
> +			k += elem[j].in_num;
> +			elem_used++;
> +		}
>  
>  		/* release unused buffers */
>  		vu_queue_rewind(vq, elem_cnt - elem_used);
> -- 
> 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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