On Fri, Feb 27, 2026 at 03:03:24PM +0100, Laurent Vivier wrote:
> Change udp_update_hdr4() and udp_update_hdr6() to take a separate
> struct udphdr pointer and an iov_tail for the payload, instead of a
> struct udp_payload_t pointer and an explicit data length.
> 
> This decouples the header update functions from the udp_payload_t memory
> layout, which assumes all headers and data sit in a single contiguous
> buffer.  The vhost-user path uses virtqueue-provided scatter-gather
> buffers where this assumption does not hold; passing an iov_tail lets
> both the tap path and the vhost-user path share the same functions
> without casting through layout-specific helpers.
> 
> Signed-off-by: Laurent Vivier <lvivier@redhat.com>

Concept looks good, a couple of nits below.

> ---
>  udp.c          | 65 ++++++++++++++++++++++----------------------------
>  udp_internal.h | 10 ++++----
>  udp_vu.c       | 14 +++++++++--
>  3 files changed, 47 insertions(+), 42 deletions(-)
> 
> diff --git a/udp.c b/udp.c
> index 19adda065f44..52e93e357a94 100644
> --- a/udp.c
> +++ b/udp.c
> @@ -257,18 +257,17 @@ static void udp_iov_init(const struct ctx *c)
>   * @ip4h:		Pre-filled IPv4 header (except for tot_len and saddr)
>   * @bp:			Pointer to udp_payload_t to update

Needs to be replaced with @payload description.

>   * @toside:		Flowside for destination side
> - * @dlen:		Length of UDP payload
>   * @no_udp_csum:	Do not set UDP checksum
>   *
>   * Return: size of IPv4 payload (UDP header + data)
>   */
> -size_t udp_update_hdr4(struct iphdr *ip4h, struct udp_payload_t *bp,
> -		       const struct flowside *toside, size_t dlen,
> -		       bool no_udp_csum)
> +size_t udp_update_hdr4(struct iphdr *ip4h, struct udphdr *uh,
> +		       struct iov_tail *payload,
> +		       const struct flowside *toside, bool no_udp_csum)
>  {
>  	const struct in_addr *src = inany_v4(&toside->oaddr);
>  	const struct in_addr *dst = inany_v4(&toside->eaddr);
> -	size_t l4len = dlen + sizeof(bp->uh);
> +	size_t l4len = iov_tail_size(payload) + sizeof(*uh);
>  	size_t l3len = l4len + sizeof(*ip4h);
>  
>  	ASSERT(src && dst);
> @@ -278,19 +277,13 @@ size_t udp_update_hdr4(struct iphdr *ip4h, struct udp_payload_t *bp,
>  	ip4h->saddr = src->s_addr;
>  	ip4h->check = csum_ip4_header(l3len, IPPROTO_UDP, *src, *dst);
>  
> -	bp->uh.source = htons(toside->oport);
> -	bp->uh.dest = htons(toside->eport);
> -	bp->uh.len = htons(l4len);
> -	if (no_udp_csum) {
> -		bp->uh.check = 0;
> -	} else {
> -		const struct iovec iov = {
> -			.iov_base = bp->data,
> -			.iov_len = dlen
> -		};
> -		struct iov_tail data = IOV_TAIL(&iov, 1, 0);
> -		csum_udp4(&bp->uh, *src, *dst, &data);
> -	}
> +	uh->source = htons(toside->oport);
> +	uh->dest = htons(toside->eport);
> +	uh->len = htons(l4len);
> +	if (no_udp_csum)
> +		uh->check = 0;
> +	else
> +		csum_udp4(uh, *src, *dst, payload);
>  
>  	return l4len;
>  }
> @@ -306,11 +299,11 @@ size_t udp_update_hdr4(struct iphdr *ip4h, struct udp_payload_t *bp,
>   *

This comment also needs updates for the changed parameters.

>   * Return: size of IPv6 payload (UDP header + data)
>   */
> -size_t udp_update_hdr6(struct ipv6hdr *ip6h, struct udp_payload_t *bp,
> -		       const struct flowside *toside, size_t dlen,
> -		       bool no_udp_csum)
> +size_t udp_update_hdr6(struct ipv6hdr *ip6h, struct udphdr *uh,
> +		       struct iov_tail *payload,
> +		       const struct flowside *toside, bool no_udp_csum)
>  {
> -	uint16_t l4len = dlen + sizeof(bp->uh);
> +	uint16_t l4len = iov_tail_size(payload) + sizeof(*uh);
>  
>  	ip6h->payload_len = htons(l4len);
>  	ip6h->daddr = toside->eaddr.a6;
> @@ -319,22 +312,17 @@ size_t udp_update_hdr6(struct ipv6hdr *ip6h, struct udp_payload_t *bp,
>  	ip6h->nexthdr = IPPROTO_UDP;
>  	ip6h->hop_limit = 255;
>  
> -	bp->uh.source = htons(toside->oport);
> -	bp->uh.dest = htons(toside->eport);
> -	bp->uh.len = ip6h->payload_len;
> +	uh->source = htons(toside->oport);
> +	uh->dest = htons(toside->eport);
> +	uh->len = ip6h->payload_len;
>  	if (no_udp_csum) {
>  		/* 0 is an invalid checksum for UDP IPv6 and dropped by
>  		 * the kernel stack, even if the checksum is disabled by virtio
>  		 * flags. We need to put any non-zero value here.
>  		 */
> -		bp->uh.check = 0xffff;
> +		uh->check = 0xffff;
>  	} else {
> -		const struct iovec iov = {
> -			.iov_base = bp->data,
> -			.iov_len = dlen
> -		};
> -		struct iov_tail data = IOV_TAIL(&iov, 1, 0);
> -		csum_udp6(&bp->uh, &toside->oaddr.a6, &toside->eaddr.a6, &data);
> +		csum_udp6(uh, &toside->oaddr.a6, &toside->eaddr.a6, payload);
>  	}
>  
>  	return l4len;
> @@ -374,12 +362,17 @@ static void udp_tap_prepare(const struct mmsghdr *mmh,
>  	struct ethhdr *eh = (*tap_iov)[UDP_IOV_ETH].iov_base;
>  	struct udp_payload_t *bp = &udp_payload[idx];
>  	struct udp_meta_t *bm = &udp_meta[idx];
> +	const struct iovec iov = {
> +		.iov_base = bp->data,
> +		.iov_len = mmh[idx].msg_len,
> +	};
> +	struct iov_tail payload = IOV_TAIL(&iov, 1, 0);
>  	size_t l4len, l2len;
>  
>  	eth_update_mac(eh, NULL, tap_omac);
>  	if (!inany_v4(&toside->eaddr) || !inany_v4(&toside->oaddr)) {
> -		l4len = udp_update_hdr6(&bm->ip6h, bp, toside,
> -					mmh[idx].msg_len, no_udp_csum);
> +		l4len = udp_update_hdr6(&bm->ip6h, &bp->uh, &payload, toside,
> +					no_udp_csum);
>  
>  		l2len = MAX(l4len + sizeof(bm->ip6h) + ETH_HLEN, ETH_ZLEN);
>  		tap_hdr_update(&bm->taph, l2len);
> @@ -387,8 +380,8 @@ static void udp_tap_prepare(const struct mmsghdr *mmh,
>  		eh->h_proto = htons_constant(ETH_P_IPV6);
>  		(*tap_iov)[UDP_IOV_IP] = IOV_OF_LVALUE(bm->ip6h);
>  	} else {
> -		l4len = udp_update_hdr4(&bm->ip4h, bp, toside,
> -					mmh[idx].msg_len, no_udp_csum);
> +		l4len = udp_update_hdr4(&bm->ip4h, &bp->uh, &payload, toside,
> +					no_udp_csum);
>  
>  		l2len = MAX(l4len + sizeof(bm->ip4h) + ETH_HLEN, ETH_ZLEN);
>  		tap_hdr_update(&bm->taph, l2len);
> diff --git a/udp_internal.h b/udp_internal.h
> index 0a8fe490b07e..aca831808ab4 100644
> --- a/udp_internal.h
> +++ b/udp_internal.h
> @@ -22,11 +22,13 @@ struct udp_payload_t {
>  } __attribute__ ((packed, aligned(__alignof__(unsigned int))));
>  #endif
>  
> -size_t udp_update_hdr4(struct iphdr *ip4h, struct udp_payload_t *bp,
> -		       const struct flowside *toside, size_t dlen,
> +size_t udp_update_hdr4(struct iphdr *ip4h, struct udphdr *uh,
> +		       struct iov_tail *payload,
> +		       const struct flowside *toside,
>  		       bool no_udp_csum);
> -size_t udp_update_hdr6(struct ipv6hdr *ip6h, struct udp_payload_t *bp,
> -		       const struct flowside *toside, size_t dlen,
> +size_t udp_update_hdr6(struct ipv6hdr *ip6h, struct udphdr *uh,
> +		       struct iov_tail *payload,
> +		       const struct flowside *toside,
>  		       bool no_udp_csum);
>  void udp_sock_fwd(const struct ctx *c, int s, int rule_hint,
>  		  uint8_t frompif, in_port_t port, const struct timespec *now);
> diff --git a/udp_vu.c b/udp_vu.c
> index aefcab0b86c2..dd8904d65a38 100644
> --- a/udp_vu.c
> +++ b/udp_vu.c
> @@ -122,21 +122,31 @@ static size_t udp_vu_prepare(const struct ctx *c, const struct iov_tail *data,
>  	if (inany_v4(&toside->eaddr) && inany_v4(&toside->oaddr)) {
>  		struct iphdr *iph = vu_ip(iov[0].iov_base);
>  		struct udp_payload_t *bp = vu_payloadv4(iov[0].iov_base);
> +		const struct iovec payload_iov = {
> +			.iov_base = bp->data,
> +			.iov_len = dlen,
> +		};
> +		struct iov_tail payload = IOV_TAIL(&payload_iov, 1, 0);
>  
>  		eh->h_proto = htons(ETH_P_IP);
>  
>  		*iph = (struct iphdr)L2_BUF_IP4_INIT(IPPROTO_UDP);
>  
> -		l4len = udp_update_hdr4(iph, bp, toside, dlen, true);
> +		l4len = udp_update_hdr4(iph, &bp->uh, &payload, toside, true);
>  	} else {
>  		struct ipv6hdr *ip6h = vu_ip(iov[0].iov_base);
>  		struct udp_payload_t *bp = vu_payloadv6(iov[0].iov_base);
> +		const struct iovec payload_iov = {
> +			.iov_base = bp->data,
> +			.iov_len = dlen,
> +		};
> +		struct iov_tail payload = IOV_TAIL(&payload_iov, 1, 0);
>  
>  		eh->h_proto = htons(ETH_P_IPV6);
>  
>  		*ip6h = (struct ipv6hdr)L2_BUF_IP6_INIT(IPPROTO_UDP);
>  
> -		l4len = udp_update_hdr6(ip6h, bp, toside, dlen, true);
> +		l4len = udp_update_hdr6(ip6h, &bp->uh, &payload, toside, true);
>  	}
>  
>  	return l4len;
> -- 
> 2.53.0
> 

-- 
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david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
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