On Fri, Apr 03, 2026 at 10:02:03AM +0200, Laurent Vivier wrote:
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
--- tcp_vu.c | 8 ++++++-- udp_vu.c | 7 ++++--- vhost_user.c | 1 + 3 files changed, 11 insertions(+), 5 deletions(-)
diff --git a/tcp_vu.c b/tcp_vu.c index 1927b14e0962..49d39e7de201 100644 --- a/tcp_vu.c +++ b/tcp_vu.c @@ -126,6 +126,7 @@ int tcp_vu_send_flag(const struct ctx *c, struct tcp_tap_conn *conn, int flags) struct vu_virtq *vq = &vdev->vq[VHOST_USER_RX_QUEUE]; size_t optlen, hdrlen, iov_cnt, iov_used; struct vu_virtq_element flags_elem[2]; + uint32_t csum_flags = IP4_CSUM; struct iovec flags_iov[64]; struct tcp_syn_opts opts; struct iov_tail payload; @@ -137,6 +138,9 @@ int tcp_vu_send_flag(const struct ctx *c, struct tcp_tap_conn *conn, int flags) int elem_cnt; 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, @@ -175,7 +179,7 @@ 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); vu_flush(vdev, vq, flags_elem, elem_cnt, hdrlen + optlen); @@ -516,7 +520,7 @@ 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 5bc9509a1b98..ed888a2baab3 100644 --- a/udp_vu.c +++ b/udp_vu.c @@ -234,12 +234,13 @@ void udp_vu_sock_to_tap(const struct ctx *c, int s, int n, flow_sidx_t tosidx) if (iov_cnt > 0) { struct iov_tail data = IOV_TAIL(iov_vu, iov_cnt, VNET_HLEN); size_t l4len = udp_vu_prepare(c, &data, toside, dlen); - if (*c->pcap) { + if (!vu_has_feature(vdev, VIRTIO_NET_F_GUEST_CSUM) || + *c->pcap) udp_vu_csum(toside, &data, l4len); + vu_pad(iov_vu, iov_cnt, hdrlen + dlen); + if (*c->pcap) pcap_iov(iov_vu, iov_cnt, VNET_HLEN, hdrlen + dlen - VNET_HLEN); - } - vu_pad(iov_vu, iov_cnt, hdrlen + dlen); vu_flush(vdev, vq, elem, elem_used, hdrlen + dlen); vu_queue_notify(vdev, vq); } diff --git a/vhost_user.c b/vhost_user.c index f062badd3311..a1259c2624c0 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; -- 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