From: David Gibson <david@gibson.dropbear.id.au>
To: Laurent Vivier <lvivier@redhat.com>
Cc: passt-dev@passt.top
Subject: Re: [PATCH v10 3/3] udp: Pass iov_tail to udp_update_hdr4()/udp_update_hdr6()
Date: Wed, 20 May 2026 11:30:22 +1000 [thread overview]
Message-ID: <ag0OrrwICcBHJkSf@zatzit> (raw)
In-Reply-To: <20260519155613.3127607-4-lvivier@redhat.com>
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On Tue, May 19, 2026 at 05:56:13PM +0200, Laurent Vivier wrote:
> Change udp_update_hdr4() and udp_update_hdr6() to take an iov_tail
> pointing at the UDP frame instead of a contiguous udp_payload_t buffer
> and explicit data length. This lets vhost-user pass scatter-gather
> virtqueue buffers directly without an intermediate copy.
>
> The UDP header is built into a local struct udphdr and written back with
> IOV_PUSH_HEADER(). On the tap side, udp_tap_prepare() wraps the
> existing udp_payload_t in a two-element iov to match the new interface.
>
> Signed-off-by: Laurent Vivier <lvivier@redhat.com>
> ---
> iov.c | 1 -
> udp.c | 74 +++++++++++++++++-----------------
> udp_internal.h | 6 ++-
> udp_vu.c | 106 ++++++++++++++++++-------------------------------
> 4 files changed, 78 insertions(+), 109 deletions(-)
Lovely :).
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Couple of cosmetic nits below, follow up at your discretion.
>
> diff --git a/iov.c b/iov.c
> index 6a5d7d35b67f..9248ba95a9f2 100644
> --- a/iov.c
> +++ b/iov.c
> @@ -367,7 +367,6 @@ void *iov_peek_header_(struct iov_tail *tail, void *v, size_t len, size_t align)
> *
> * Return: number of bytes written
> */
> -/* cppcheck-suppress unusedFunction */
> size_t iov_push_header_(struct iov_tail *tail, const void *v, size_t len)
> {
> size_t l;
> diff --git a/udp.c b/udp.c
> index 66dc7766868c..cfc7b2439881 100644
> --- a/udp.c
> +++ b/udp.c
> @@ -255,20 +255,22 @@ static void udp_iov_init(const struct ctx *c)
> /**
> * udp_update_hdr4() - Update headers for one IPv4 datagram
> * @ip4h: Pre-filled IPv4 header (except for tot_len and saddr)
> - * @bp: Pointer to udp_payload_t to update
> + * @uh: UDP header to fill
> + * @payload: UDP payload
> * @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)
> + * Return: size of datagram (UDP header + data)
> */
> -size_t udp_update_hdr4(struct iphdr *ip4h, struct udp_payload_t *bp,
> +size_t udp_update_hdr4(struct iphdr *ip4h, struct udphdr *uh,
> + struct iov_tail *payload,
> const struct flowside *toside, size_t dlen,
> 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 = dlen + sizeof(*uh);
> size_t l3len = l4len + sizeof(*ip4h);
>
> assert(src && dst);
> @@ -278,19 +280,13 @@ 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, dlen);
> - }
> + 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, dlen);
>
> return l4len;
> }
> @@ -299,18 +295,20 @@ size_t udp_update_hdr4(struct iphdr *ip4h, struct udp_payload_t *bp,
> * udp_update_hdr6() - Update headers for one IPv6 datagram
> * @ip6h: Pre-filled IPv6 header (except for payload_len and
> * addresses)
> - * @bp: Pointer to udp_payload_t to update
> + * @uh: UDP header to fill
> + * @payload: UDP payload
> * @toside: Flowside for destination side
> * @dlen: Length of UDP payload
> * @no_udp_csum: Do not set UDP checksum
> *
> - * Return: size of IPv6 payload (UDP header + data)
> + * Return: size of datagram (UDP header + data)
> */
> -size_t udp_update_hdr6(struct ipv6hdr *ip6h, struct udp_payload_t *bp,
> +size_t udp_update_hdr6(struct ipv6hdr *ip6h, struct udphdr *uh,
> + struct iov_tail *payload,
> const struct flowside *toside, size_t dlen,
> bool no_udp_csum)
> {
> - uint16_t l4len = dlen + sizeof(bp->uh);
> + uint16_t l4len = dlen + sizeof(*uh);
>
> ip6h->payload_len = htons(l4len);
> ip6h->daddr = toside->eaddr.a6;
> @@ -319,23 +317,19 @@ 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 = htons(l4len);
> +
> 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,
> - dlen);
> + csum_udp6(uh, &toside->oaddr.a6, &toside->eaddr.a6,
> + payload, dlen);
> }
>
> return l4len;
> @@ -372,15 +366,20 @@ static void udp_tap_prepare(const struct mmsghdr *mmh,
> bool no_udp_csum)
> {
> struct iovec (*tap_iov)[UDP_NUM_IOVS] = &udp_l2_iov[idx];
> + struct udphdr *uh = (*tap_iov)[UDP_IOV_PAYLOAD].iov_base;
> + struct iov_tail payload = IOV_TAIL(&(*tap_iov)[UDP_IOV_PAYLOAD], 1,
> + sizeof(*uh));
> 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];
> size_t l4len, l2len;
>
> + l4len = sizeof(struct udphdr) + mmh[idx].msg_len;
Nit: sizeof(*uh).
> + (*tap_iov)[UDP_IOV_PAYLOAD].iov_len = l4len;
> +
> 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);
> + udp_update_hdr6(&bm->ip6h, uh, &payload, toside,
> + mmh[idx].msg_len, no_udp_csum);
>
> l2len = MAX(l4len + sizeof(bm->ip6h) + ETH_HLEN, ETH_ZLEN);
> tap_hdr_update(&bm->taph, l2len);
> @@ -388,8 +387,8 @@ 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);
> + udp_update_hdr4(&bm->ip4h, uh, &payload, toside,
> + mmh[idx].msg_len, no_udp_csum);
>
> l2len = MAX(l4len + sizeof(bm->ip4h) + ETH_HLEN, ETH_ZLEN);
> tap_hdr_update(&bm->taph, l2len);
> @@ -397,7 +396,6 @@ static void udp_tap_prepare(const struct mmsghdr *mmh,
> eh->h_proto = htons_constant(ETH_P_IP);
> (*tap_iov)[UDP_IOV_IP] = IOV_OF_LVALUE(bm->ip4h);
> }
> - (*tap_iov)[UDP_IOV_PAYLOAD].iov_len = l4len;
>
> udp_tap_pad(*tap_iov);
> }
> diff --git a/udp_internal.h b/udp_internal.h
> index 64e457748324..361cc7495a01 100644
> --- a/udp_internal.h
> +++ b/udp_internal.h
> @@ -25,10 +25,12 @@ struct udp_payload_t {
> } __attribute__ ((packed, aligned(__alignof__(unsigned int))));
> #endif
>
> -size_t udp_update_hdr4(struct iphdr *ip4h, struct udp_payload_t *bp,
> +size_t udp_update_hdr4(struct iphdr *ip4h, struct udphdr *uh,
> + struct iov_tail *payload,
> const struct flowside *toside, size_t dlen,
> bool no_udp_csum);
> -size_t udp_update_hdr6(struct ipv6hdr *ip6h, struct udp_payload_t *bp,
> +size_t udp_update_hdr6(struct ipv6hdr *ip6h, struct udphdr *uh,
> + struct iov_tail *payload,
> const struct flowside *toside, size_t dlen,
> bool no_udp_csum);
> void udp_sock_fwd(const struct ctx *c, int s, int rule_hint,
> diff --git a/udp_vu.c b/udp_vu.c
> index 74bf79d57969..888367ef4e16 100644
> --- a/udp_vu.c
> +++ b/udp_vu.c
> @@ -55,40 +55,33 @@ static size_t udp_vu_hdrlen(bool v6)
>
> /**
> * udp_vu_sock_recv() - Receive datagrams from socket into vhost-user buffers
> - * @iov: IO vector for the frame (in/out)
> - * @cnt: Number of available entries in @iov (input)
> - * Number of used entries in @iov to store the datagram (output)
> + * @payload: UDP payload
Nit: "Buffer(s) for UDP payload" maybe?
> + * @cnt: Number of used entries in @payload to store the datagram (output)
> * Unchanged on failure
> * @s: Socket to receive from
> - * @v6: Set for IPv6 connections
> *
> * Return: size of received data, -1 on error
> */
> -static ssize_t udp_vu_sock_recv(struct iovec *iov, size_t *cnt, int s, bool v6)
> +static ssize_t udp_vu_sock_recv(struct iov_tail *payload, size_t *cnt, int s)
> {
> struct iovec msg_iov[VIRTQUEUE_MAX_SIZE];
> struct msghdr msg = { 0 };
> - struct iov_tail payload;
> - size_t hdrlen, iov_used;
> + size_t iov_used;
> ssize_t dlen;
>
> - /* compute L2 header length */
> - hdrlen = udp_vu_hdrlen(v6);
> -
> - payload = IOV_TAIL(iov, *cnt, hdrlen);
> -
> msg.msg_iov = msg_iov;
> - msg.msg_iovlen = iov_tail_clone(msg.msg_iov, payload.cnt, &payload);
> + msg.msg_iovlen = iov_tail_clone(msg.msg_iov, ARRAY_SIZE(msg_iov),
> + payload);
>
> /* read data from the socket */
> dlen = recvmsg(s, &msg, 0);
> if (dlen < 0)
> return -1;
>
> - iov_used = iov_skip_bytes(iov, *cnt,
> - MAX(dlen + hdrlen, VNET_HLEN + ETH_ZLEN),
> - NULL);
> - if (iov_used < *cnt)
> + iov_used = iov_skip_bytes(payload->iov, payload->cnt,
> + MAX(dlen + payload->off,
> + VNET_HLEN + ETH_ZLEN), NULL);
> + if (iov_used < payload->cnt)
> iov_used++;
> *cnt = iov_used; /* one iovec per element */
>
> @@ -98,69 +91,44 @@ static ssize_t udp_vu_sock_recv(struct iovec *iov, size_t *cnt, int s, bool v6)
> /**
> * udp_vu_prepare() - Prepare the packet header
> * @c: Execution context
> - * @iov: IO vector for the frame (including vnet header)
> + * @data: IO vector tail for the L2 frame, on return points to the L4 header
> + * @payload: UDP payload
> * @toside: Address information for one side of the flow
> * @dlen: Packet data length
> */
> -static void udp_vu_prepare(const struct ctx *c, const struct iovec *iov,
> - const struct flowside *toside, ssize_t dlen)
> +static void udp_vu_prepare(const struct ctx *c, struct iov_tail *data,
> + struct iov_tail *payload,
> + const struct flowside *toside, size_t dlen)
> {
> - struct ethhdr *eh;
> + bool ipv4 = inany_v4(&toside->eaddr) && inany_v4(&toside->oaddr);
> + struct ethhdr eh;
> + struct udphdr uh;
>
> /* ethernet header */
> - eh = vu_eth(iov[0].iov_base);
> + memcpy(eh.h_dest, c->guest_mac, sizeof(eh.h_dest));
> + memcpy(eh.h_source, c->our_tap_mac, sizeof(eh.h_source));
>
> - memcpy(eh->h_dest, c->guest_mac, sizeof(eh->h_dest));
> - memcpy(eh->h_source, c->our_tap_mac, sizeof(eh->h_source));
> + if (ipv4)
> + eh.h_proto = htons(ETH_P_IP);
> + else
> + eh.h_proto = htons(ETH_P_IPV6);
> + IOV_PUSH_HEADER(data, eh);
>
> /* initialize header */
> - 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);
> -
> - eh->h_proto = htons(ETH_P_IP);
> + if (ipv4) {
> + struct iphdr iph = (struct iphdr)L2_BUF_IP4_INIT(IPPROTO_UDP);
>
> - *iph = (struct iphdr)L2_BUF_IP4_INIT(IPPROTO_UDP);
> + udp_update_hdr4(&iph, &uh, payload, toside, dlen, !*c->pcap);
>
> - udp_update_hdr4(iph, bp, toside, dlen, true);
> + IOV_PUSH_HEADER(data, iph);
> } else {
> - struct ipv6hdr *ip6h = vu_ip(iov[0].iov_base);
> - struct udp_payload_t *bp = vu_payloadv6(iov[0].iov_base);
> + struct ipv6hdr ip6h = (struct ipv6hdr)L2_BUF_IP6_INIT(IPPROTO_UDP);
>
> - eh->h_proto = htons(ETH_P_IPV6);
> + udp_update_hdr6(&ip6h, &uh, payload, toside, dlen, !*c->pcap);
>
> - *ip6h = (struct ipv6hdr)L2_BUF_IP6_INIT(IPPROTO_UDP);
> -
> - udp_update_hdr6(ip6h, bp, toside, dlen, true);
> - }
> -}
> -
> -/**
> - * udp_vu_csum() - Calculate and set checksum for a UDP packet
> - * @toside: Address information for one side of the flow
> - * @iov: IO vector for the frame
> - * @cnt: Number of IO vector entries
> - * @dlen: Data length
> - */
> -static void udp_vu_csum(const struct flowside *toside, const struct iovec *iov,
> - size_t cnt, size_t dlen)
> -{
> - const struct in_addr *src4 = inany_v4(&toside->oaddr);
> - const struct in_addr *dst4 = inany_v4(&toside->eaddr);
> - char *base = iov[0].iov_base;
> - struct udp_payload_t *bp;
> - struct iov_tail data;
> -
> - if (src4 && dst4) {
> - bp = vu_payloadv4(base);
> - data = IOV_TAIL(iov, cnt, (char *)&bp->data - base);
> - csum_udp4(&bp->uh, *src4, *dst4, &data, dlen);
> - } else {
> - bp = vu_payloadv6(base);
> - data = IOV_TAIL(iov, cnt, (char *)&bp->data - base);
> - csum_udp6(&bp->uh, &toside->oaddr.a6, &toside->eaddr.a6, &data,
> - dlen);
> + IOV_PUSH_HEADER(data, ip6h);
> }
> + IOV_PUSH_HEADER(data, uh);
> }
>
> /**
> @@ -198,6 +166,7 @@ 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, j, k;
> + struct iov_tail payload;
> size_t iov_cnt;
> ssize_t dlen;
>
> @@ -207,7 +176,8 @@ void udp_vu_sock_to_tap(const struct ctx *c, int s, int n, flow_sidx_t tosidx)
> if (elem_cnt == 0)
> break;
>
> - dlen = udp_vu_sock_recv(iov_vu, &iov_cnt, s, v6);
> + payload = IOV_TAIL(iov_vu, iov_cnt, hdrlen);
> + dlen = udp_vu_sock_recv(&payload, &iov_cnt, s);
> if (dlen < 0) {
> vu_queue_rewind(vq, elem_cnt);
> break;
> @@ -227,9 +197,9 @@ void udp_vu_sock_to_tap(const struct ctx *c, int s, int n, flow_sidx_t tosidx)
> vu_queue_rewind(vq, elem_cnt - elem_used);
>
> if (iov_cnt > 0) {
> - udp_vu_prepare(c, iov_vu, toside, dlen);
> + struct iov_tail data = IOV_TAIL(iov_vu, iov_cnt, VNET_HLEN);
> + udp_vu_prepare(c, &data, &payload, toside, dlen);
> if (*c->pcap) {
> - udp_vu_csum(toside, iov_vu, iov_cnt, dlen);
> pcap_iov(iov_vu, iov_cnt, VNET_HLEN,
> hdrlen + dlen - VNET_HLEN);
> }
> --
> 2.54.0
>
--
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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prev parent reply other threads:[~2026-05-20 1:30 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-05-19 15:56 [PATCH v10 0/3] vhost-user,udp: Handle multiple iovec entries per virtqueue element Laurent Vivier
2026-05-19 15:56 ` [PATCH v10 1/3] udp_vu: Allow virtqueue elements with multiple iovec entries Laurent Vivier
2026-05-19 15:56 ` [PATCH v10 2/3] iov: Introduce IOV_PUSH_HEADER() macro Laurent Vivier
2026-05-19 15:56 ` [PATCH v10 3/3] udp: Pass iov_tail to udp_update_hdr4()/udp_update_hdr6() Laurent Vivier
2026-05-20 1:30 ` David Gibson [this message]
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