[PATCH 1/2] vhost_user: Offer VIRTIO_NET_F_GUEST_CSUM

[Laurent Vivier <lvivier@redhat.com>]

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>
---
 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