[PATCH v2 1/2] vhost_user: Offer VIRTIO_NET_F_GUEST_CSUM

Laurent Vivier
2026-04-16T16:21:46Z

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 <lvivier@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
---
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 7f7e43860b10..3da14a4942d6 100644
--- a/tcp_vu.c
+++ b/tcp_vu.c
@@ -124,6 +124,7 @@ 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];
int elem_cnt, dup_elem_cnt = 0;
struct tcp_syn_opts opts;
@@ -135,6 +136,9 @@ 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,
@@ -173,7 +177,7 @@ 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);

@@ -519,7 +523,7 @@ 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 8cf50ca1c38f..54d824b5530c 100644
--- a/udp_vu.c
+++ b/udp_vu.c
@@ -233,12 +233,13 @@ 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);
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, dlen);
+ 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