* [PATCH 1/2] vhost_user: Offer VIRTIO_NET_F_GUEST_CSUM
2026-04-03 8:02 [PATCH 0/2] vhost-user, dhcp: Fix iPXE network boot over vhost-user Laurent Vivier
@ 2026-04-03 8:02 ` Laurent Vivier
2026-04-03 8:02 ` [PATCH 2/2] dhcp: Add --dhcp-boot option to set boot filename in DHCP replies Laurent Vivier
1 sibling, 0 replies; 3+ messages in thread
From: Laurent Vivier @ 2026-04-03 8:02 UTC (permalink / raw)
To: passt-dev; +Cc: Laurent Vivier
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
^ permalink raw reply [flat|nested] 3+ messages in thread* [PATCH 2/2] dhcp: Add --dhcp-boot option to set boot filename in DHCP replies
2026-04-03 8:02 [PATCH 0/2] vhost-user, dhcp: Fix iPXE network boot over vhost-user Laurent Vivier
2026-04-03 8:02 ` [PATCH 1/2] vhost_user: Offer VIRTIO_NET_F_GUEST_CSUM Laurent Vivier
@ 2026-04-03 8:02 ` Laurent Vivier
1 sibling, 0 replies; 3+ messages in thread
From: Laurent Vivier @ 2026-04-03 8:02 UTC (permalink / raw)
To: passt-dev; +Cc: Laurent Vivier
Add a --dhcp-boot option that populates the 'file' field in DHCP reply
messages with the given filename.
Using --dhcp-boot together with --no-dhcp is rejected at startup.
Signed-off-by: Laurent Vivier <lvivier@redhat.com>
---
conf.c | 10 ++++++++++
dhcp.c | 5 ++++-
passt.1 | 6 ++++++
passt.h | 1 +
4 files changed, 21 insertions(+), 1 deletion(-)
diff --git a/conf.c b/conf.c
index ae37bf96dac4..2cf1bc7420d3 100644
--- a/conf.c
+++ b/conf.c
@@ -984,6 +984,7 @@ static void usage(const char *name, FILE *f, int status)
" --no-udp Disable UDP protocol handler\n"
" --no-icmp Disable ICMP/ICMPv6 protocol handler\n"
" --no-dhcp Disable DHCP server\n"
+ " --dhcp-boot FILE DHCP boot filename option\n"
" --no-ndp Disable NDP responses\n"
" --no-dhcpv6 Disable DHCPv6 server\n"
" --no-ra Disable router advertisements\n"
@@ -1537,6 +1538,7 @@ void conf(struct ctx *c, int argc, char **argv)
{"migrate-exit", no_argument, NULL, 29 },
{"migrate-no-linger", no_argument, NULL, 30 },
{"stats", required_argument, NULL, 31 },
+ {"dhcp-boot", required_argument, NULL, 32 },
{ 0 },
};
const char *optstring = "+dqfel:hs:F:I:p:P:m:a:n:M:g:i:o:D:S:H:461t:u:T:U:";
@@ -1775,6 +1777,11 @@ void conf(struct ctx *c, int argc, char **argv)
die("Can't display statistics if not running in foreground");
c->stats = strtol(optarg, NULL, 0);
break;
+ case 32:
+ if (snprintf_check(c->dhcp_boot, sizeof(c->dhcp_boot),
+ "%s", optarg))
+ die("Invalid DHCP bootfile name: %s", optarg);
+ break;
case 'd':
c->debug = 1;
c->quiet = 0;
@@ -2206,6 +2213,9 @@ void conf(struct ctx *c, int argc, char **argv)
c->no_dhcpv6 = 1;
}
+ if (c->no_dhcp && c->dhcp_boot[0])
+ die("--dhcp-boot cannot be used with --no-dhcp");
+
get_dns(c);
if (!*c->pasta_ifn) {
diff --git a/dhcp.c b/dhcp.c
index 1ff8cba9f93d..7fdb6e051dec 100644
--- a/dhcp.c
+++ b/dhcp.c
@@ -344,6 +344,9 @@ int dhcp(const struct ctx *c, struct iov_tail *data)
m->op != BOOTREQUEST)
return -1;
+ static_assert(sizeof(reply.file) == sizeof(c->dhcp_boot),
+ "dhcp_boot must have the same size as reply.file");
+
reply.op = BOOTREPLY;
reply.htype = m->htype;
reply.hlen = m->hlen;
@@ -357,7 +360,7 @@ int dhcp(const struct ctx *c, struct iov_tail *data)
reply.giaddr = m->giaddr;
memcpy(&reply.chaddr, m->chaddr, sizeof(reply.chaddr));
memset(&reply.sname, 0, sizeof(reply.sname));
- memset(&reply.file, 0, sizeof(reply.file));
+ memcpy(&reply.file, c->dhcp_boot, sizeof(reply.file));
reply.magic = m->magic;
for (i = 0; i < ARRAY_SIZE(opts); i++)
diff --git a/passt.1 b/passt.1
index 13e8df9de9f3..a6ac2884bf3f 100644
--- a/passt.1
+++ b/passt.1
@@ -342,6 +342,12 @@ Disable the DHCP server. DHCP client requests coming from guest or target
namespace will be silently dropped. Implied if there is no gateway on the
selected IPv4 default route.
+.TP
+.BR \-\-dhcp-boot " " \fIfile
+Set the boot filename in DHCP replies to \fIfile\fR. This populates the
+\fIfile\fR field in BOOTP/DHCP reply messages, which can be used for
+network booting (PXE). Cannot be used with \fB--no-dhcp\fR.
+
.TP
.BR \-\-no-ndp
Disable Neighbor Discovery. NDP messages coming from guest or target
diff --git a/passt.h b/passt.h
index 62b8dcdf0a41..4044a4c07ea3 100644
--- a/passt.h
+++ b/passt.h
@@ -255,6 +255,7 @@ struct ctx {
char hostname[PASST_MAXDNAME];
char fqdn[PASST_MAXDNAME];
+ char dhcp_boot[128];
int ifi6;
struct ip6_ctx ip6;
--
2.53.0
^ permalink raw reply [flat|nested] 3+ messages in thread