From 038c51e324695b65c154bc0eee30bd19a7423ad2 Mon Sep 17 00:00:00 2001 From: Laurent Vivier Date: Thu, 16 Apr 2026 18:21:39 +0200 Subject: vhost_user: Offer VIRTIO_NET_F_GUEST_CSUM According to the virtio-net specification, when the VIRTIO_NET_F_GUEST_CSUM is negotiated, the device can set VIRTIO_NET_HDR_F_DATA_VALID in the virtio-net header to indicate that packet checksums have been validated, allowing the guest to skip verification. Without this feature, the device must provide fully checksummed packets. The vhost-user TCP and UDP paths were unconditionally skipping checksum computation, regardless of whether GUEST_CSUM was negotiated. This went undetected with Linux guests because Linux's virtio-net driver honours VIRTIO_NET_HDR_F_DATA_VALID regardless of whether VIRTIO_NET_F_GUEST_CSUM was negotiated, marking such packets as CHECKSUM_UNNECESSARY and skipping verification. iPXE, however, does not negotiate GUEST_CSUM, ignores the DATA_VALID flag entirely, and always verifies checksums. This caused TCP connections to fail: the SYN-ACK had a zero TCP checksum, iPXE rejected it, and the connection timed out in SYN_RCVD. Adding --pcap happened to mask the bug, because the pcap code path forces checksum computation to ensure correct captures. Offer VIRTIO_NET_F_GUEST_CSUM in the device features, and only skip checksum computation when the guest has actually negotiated it. When GUEST_CSUM is not negotiated, always compute valid checksums as required by the specification. We keep setting VIRTIO_NET_HDR_F_DATA_VALID unconditionally in VU_HEADER: when GUEST_CSUM is negotiated, the flag lets the guest skip checksum verification; when it is not, the spec says the guest should ignore the flags field, so setting it is harmless. Signed-off-by: Laurent Vivier Reviewed-by: David Gibson [sbrivio: Resolved conflicts, in particular with commit dec66c02b5e4 ("udp: Pass iov_tail to udp_update_hdr4()/udp_update_hdr6()")] Signed-off-by: Stefano Brivio --- tcp_vu.c | 8 ++++++-- udp_vu.c | 7 +++++-- vhost_user.c | 1 + 3 files changed, 12 insertions(+), 4 deletions(-) diff --git a/tcp_vu.c b/tcp_vu.c index 3063376..7e2a7db 100644 --- a/tcp_vu.c +++ b/tcp_vu.c @@ -129,6 +129,7 @@ int tcp_vu_send_flag(const struct ctx *c, struct tcp_tap_conn *conn, int flags) struct vu_virtq_element flags_elem[2]; struct iov_tail payload, l2frame; int elem_cnt, dup_elem_cnt = 0; + uint32_t csum_flags = IP4_CSUM; struct iovec flags_iov[64]; struct tcp_syn_opts opts; struct tcphdr th = { 0 }; @@ -138,6 +139,9 @@ int tcp_vu_send_flag(const struct ctx *c, struct tcp_tap_conn *conn, int flags) uint32_t seq; int ret; + if (*c->pcap || !vu_has_feature(vdev, VIRTIO_NET_F_GUEST_CSUM)) + csum_flags |= TCP_CSUM; + hdrlen = tcp_vu_hdrlen(CONN_V6(conn)); elem_cnt = vu_collect(vdev, vq, &flags_elem[0], 1, @@ -174,7 +178,7 @@ int tcp_vu_send_flag(const struct ctx *c, struct tcp_tap_conn *conn, int flags) iov_from_buf(payload.iov, payload.cnt, payload.off, &opts, optlen); tcp_fill_headers(c, conn, &eh, CONN_V4(conn) ? &ip4h : NULL, CONN_V6(conn) ? &ip6h : NULL, &th, &payload, - optlen, IP4_CSUM | (*c->pcap ? TCP_CSUM : 0), seq); + optlen, csum_flags, seq); vu_pad(flags_elem[0].in_sg, iov_cnt, hdrlen + optlen); @@ -520,7 +524,7 @@ int tcp_vu_data_from_sock(const struct ctx *c, struct tcp_tap_conn *conn) hdrlen = tcp_vu_hdrlen(v6); check = IP4_CSUM; - if (*c->pcap) + if (*c->pcap || !vu_has_feature(vdev, VIRTIO_NET_F_GUEST_CSUM)) check |= TCP_CSUM; for (i = 0, previous_dlen = -1; i < frame_cnt; i++) { struct iovec *iov = &iov_vu[frame[i].idx_iovec]; diff --git a/udp_vu.c b/udp_vu.c index 4cfe573..e4fb105 100644 --- a/udp_vu.c +++ b/udp_vu.c @@ -103,6 +103,7 @@ static void udp_vu_prepare(const struct ctx *c, struct iov_tail *data, bool ipv4 = inany_v4(&toside->eaddr) && inany_v4(&toside->oaddr); struct ethhdr eh; struct udphdr uh; + bool no_csum; /* ethernet header */ memcpy(eh.h_dest, c->guest_mac, sizeof(eh.h_dest)); @@ -114,17 +115,19 @@ static void udp_vu_prepare(const struct ctx *c, struct iov_tail *data, eh.h_proto = htons(ETH_P_IPV6); IOV_PUSH_HEADER(data, eh); + no_csum = vu_has_feature(c->vdev, VIRTIO_NET_F_GUEST_CSUM) && !*c->pcap; + /* initialize header */ if (ipv4) { struct iphdr iph = (struct iphdr)L2_BUF_IP4_INIT(IPPROTO_UDP); - udp_update_hdr4(&iph, &uh, payload, toside, dlen, !*c->pcap); + udp_update_hdr4(&iph, &uh, payload, toside, dlen, no_csum); IOV_PUSH_HEADER(data, iph); } else { struct ipv6hdr ip6h = (struct ipv6hdr)L2_BUF_IP6_INIT(IPPROTO_UDP); - udp_update_hdr6(&ip6h, &uh, payload, toside, dlen, !*c->pcap); + udp_update_hdr6(&ip6h, &uh, payload, toside, dlen, no_csum); IOV_PUSH_HEADER(data, ip6h); } diff --git a/vhost_user.c b/vhost_user.c index f062bad..a1259c2 100644 --- a/vhost_user.c +++ b/vhost_user.c @@ -322,6 +322,7 @@ static bool vu_get_features_exec(struct vu_dev *vdev, { uint64_t features = 1ULL << VIRTIO_F_VERSION_1 | + 1ULL << VIRTIO_NET_F_GUEST_CSUM | 1ULL << VIRTIO_NET_F_MRG_RXBUF | 1ULL << VHOST_F_LOG_ALL | 1ULL << VHOST_USER_F_PROTOCOL_FEATURES; -- cgit v1.2.3