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diff --git a/share/man/man4/epair.4 b/share/man/man4/epair.4
--- a/share/man/man4/epair.4
+++ b/share/man/man4/epair.4
@@ -25,7 +25,7 @@
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
-.Dd March 18, 2015
+.Dd July 28, 2025
.Dt EPAIR 4
.Os
.Sh NAME
@@ -96,6 +96,24 @@
can have a
.Xr vlan 4
configured on top of it.
+.Pp
+The
+.Nm
+supports
+TXCSUM and TXCSUM6 (for TCP and UDP) as well as RXCSUM and RXCSUM6,
+but only by forwarding the order to compute the checksum or the information
+that the checksum has been validated.
+Thus, when using an
+.Nm
+interface, a sender can offload checksum computation
+to a physical interface, and a receiver can skip validating the checksum if a
+physical interface did it.
+Note that, in case the packet does not leave the host, the checksum is
+unnecessary and will be ignored if offloaded.
+The TXCSUM and TXCSUM6 capabilities are synchronized between the
+.Nm
+interface pair (i.e., enabling/disabling the capability on one end
+enables/disables it on the other end).
.Sh SEE ALSO
.Xr ioctl 2 ,
.Xr altq 4 ,
diff --git a/share/man/man9/mbuf.9 b/share/man/man9/mbuf.9
--- a/share/man/man9/mbuf.9
+++ b/share/man/man9/mbuf.9
@@ -22,7 +22,7 @@
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
-.Dd December 28, 2023
+.Dd July 28, 2025
.Dt MBUF 9
.Os
.\"
@@ -1203,6 +1203,17 @@
calculated over any valid packet will be
.Li 0xFFFF
as long as the original checksum field is included.
+.Pp
+Note that the flag
+.Dv CSUM_TCP ,
+.Dv CSUM_UDP ,
+or
+.Dv CSUM_SCTP
+can appear on input if a packet sent by the local host with checksum
+offloading switched to the input path (e.g., due to a virtual interface
+such as tap or epair).
+The TCP, UDP, or SCTP checksum is still incorrect but will be ignored because
+the packet has not been on the wire.
.Sh STRESS TESTING
When running a kernel compiled with the option
.Dv MBUF_STRESS_TEST ,
diff --git a/sys/net/if_epair.c b/sys/net/if_epair.c
--- a/sys/net/if_epair.c
+++ b/sys/net/if_epair.c
@@ -66,9 +66,9 @@
#include <net/if_var.h>
#include <net/if_clone.h>
#include <net/if_media.h>
-#include <net/if_var.h>
#include <net/if_private.h>
#include <net/if_types.h>
+#include <net/if_vlan_var.h>
#include <net/netisr.h>
#ifdef RSS
#include <net/rss_config.h>
@@ -96,6 +96,7 @@
#define EPAIR_LOCK_DESTROY() mtx_destroy(&epair_n_index_mtx)
#define EPAIR_LOCK() mtx_lock(&epair_n_index_mtx)
#define EPAIR_UNLOCK() mtx_unlock(&epair_n_index_mtx)
+#define EPAIR_LOCK_ASSERT() mtx_assert(&epair_n_index_mtx, MA_OWNED);
struct epair_softc;
struct epair_queue {
@@ -425,6 +426,23 @@
imr->ifm_active = IFM_ETHER | IFM_10G_T | IFM_FDX;
}
+/*
+ * To be called under EPAIR_LOCK. Update ifp->if_hwassist according to the
+ * current value of ifp->if_capenable.
+ */
+static void
+epair_caps_changed(struct ifnet *ifp)
+{
+ uint64_t hwassist = 0;
+
+ EPAIR_LOCK_ASSERT();
+ if (ifp->if_capenable & IFCAP_TXCSUM)
+ hwassist |= CSUM_IP_TCP | CSUM_IP_UDP;
+ if (ifp->if_capenable & IFCAP_TXCSUM_IPV6)
+ hwassist |= CSUM_IP6_TCP | CSUM_IP6_UDP;
+ ifp->if_hwassist = hwassist;
+}
+
static int
epair_ioctl(struct ifnet *ifp, u_long cmd, caddr_t data)
{
@@ -452,6 +470,33 @@
error = 0;
break;
+ case SIOCGIFCAP:
+ ifr->ifr_reqcap = ifp->if_capabilities;
+ ifr->ifr_curcap = ifp->if_capenable;
+ error = 0;
+ break;
+ case SIOCSIFCAP:
+ sc = ifp->if_softc;
+ EPAIR_LOCK();
+ ifp->if_capenable = ifr->ifr_reqcap & ifp->if_capabilities;
+ epair_caps_changed(ifp);
+ /*
+ * If IFCAP_TXCSUM(_IPV6) has been changed, change it on the
+ * other epair interface as well.
+ */
+ if ((ifp->if_capenable ^ sc->oifp->if_capenable) &
+ (IFCAP_TXCSUM | IFCAP_TXCSUM_IPV6)) {
+ sc->oifp->if_capenable &=
+ ~(IFCAP_TXCSUM | IFCAP_TXCSUM_IPV6);
+ sc->oifp->if_capenable |= ifp->if_capenable &
+ (IFCAP_TXCSUM | IFCAP_TXCSUM_IPV6);
+ epair_caps_changed(sc->oifp);
+ }
+ EPAIR_UNLOCK();
+ VLAN_CAPABILITIES(ifp);
+ error = 0;
+ break;
+
default:
/* Let the common ethernet handler process this. */
error = ether_ioctl(ifp, cmd, data);
@@ -549,8 +594,13 @@
ifp->if_dname = epairname;
ifp->if_dunit = unit;
ifp->if_flags = IFF_BROADCAST | IFF_SIMPLEX | IFF_MULTICAST;
- ifp->if_capabilities = IFCAP_VLAN_MTU;
- ifp->if_capenable = IFCAP_VLAN_MTU;
+ EPAIR_LOCK();
+ ifp->if_capabilities = IFCAP_VLAN_MTU | IFCAP_TXCSUM |
+ IFCAP_TXCSUM_IPV6 | IFCAP_RXCSUM | IFCAP_RXCSUM_IPV6;
+ ifp->if_capenable = IFCAP_VLAN_MTU | IFCAP_TXCSUM |
+ IFCAP_TXCSUM_IPV6 | IFCAP_RXCSUM | IFCAP_RXCSUM_IPV6;
+ epair_caps_changed(ifp);
+ EPAIR_UNLOCK();
ifp->if_transmit = epair_transmit;
ifp->if_qflush = epair_qflush;
ifp->if_start = epair_start;
diff --git a/sys/netinet/ip_fastfwd.c b/sys/netinet/ip_fastfwd.c
--- a/sys/netinet/ip_fastfwd.c
+++ b/sys/netinet/ip_fastfwd.c
@@ -69,6 +69,7 @@
#include <sys/cdefs.h>
#include "opt_ipstealth.h"
+#include "opt_sctp.h"
#include <sys/param.h>
#include <sys/systm.h>
@@ -102,6 +103,10 @@
#include <machine/in_cksum.h>
+#if defined(SCTP) || defined(SCTP_SUPPORT)
+#include <netinet/sctp_crc32.h>
+#endif
+
#define V_ipsendredirects VNET(ipsendredirects)
static struct mbuf *
@@ -460,6 +465,23 @@
} else
gw = (const struct sockaddr *)dst;
+ /*
+ * If TCP/UDP header still needs a valid checksum and interface will not
+ * calculate it for us, do it here.
+ */
+ if (__predict_false(m->m_pkthdr.csum_flags & CSUM_DELAY_DATA &
+ ~nh->nh_ifp->if_hwassist)) {
+ in_delayed_cksum(m);
+ m->m_pkthdr.csum_flags &= ~CSUM_DELAY_DATA;
+ }
+#if defined(SCTP) || defined(SCTP_SUPPORT)
+ if (__predict_false(m->m_pkthdr.csum_flags & CSUM_IP_SCTP &
+ ~nh->nh_ifp->if_hwassist)) {
+ sctp_delayed_cksum(m, (uint32_t)(ip->ip_hl << 2));
+ m->m_pkthdr.csum_flags &= ~CSUM_IP_SCTP;
+ }
+#endif
+
/* Handle redirect case. */
redest.s_addr = 0;
if (V_ipsendredirects && osrc.s_addr == ip->ip_src.s_addr &&
diff --git a/sys/netinet/sctp_input.c b/sys/netinet/sctp_input.c
--- a/sys/netinet/sctp_input.c
+++ b/sys/netinet/sctp_input.c
@@ -5780,7 +5780,11 @@
goto out;
}
ecn_bits = ip->ip_tos;
- if (m->m_pkthdr.csum_flags & CSUM_SCTP_VALID) {
+ if (m->m_pkthdr.csum_flags & (CSUM_SCTP_VALID | CSUM_IP_SCTP)) {
+ /*
+ * Packet with CSUM_IP_SCTP were sent from local host using
+ * checksum offloading. Checksum not required.
+ */
SCTP_STAT_INCR(sctps_recvhwcrc);
compute_crc = 0;
} else {
diff --git a/sys/netinet/tcp_input.c b/sys/netinet/tcp_input.c
--- a/sys/netinet/tcp_input.c
+++ b/sys/netinet/tcp_input.c
@@ -649,6 +649,12 @@
th->th_sum = in6_cksum_pseudo(ip6, tlen,
IPPROTO_TCP, m->m_pkthdr.csum_data);
th->th_sum ^= 0xffff;
+ } else if (m->m_pkthdr.csum_flags & CSUM_IP6_TCP) {
+ /*
+ * Packet from local host (maybe from a VM).
+ * Checksum not required.
+ */
+ th->th_sum = 0;
} else
th->th_sum = in6_cksum(m, IPPROTO_TCP, off0, tlen);
if (th->th_sum) {
@@ -709,6 +715,12 @@
htonl(m->m_pkthdr.csum_data + tlen +
IPPROTO_TCP));
th->th_sum ^= 0xffff;
+ } else if (m->m_pkthdr.csum_flags & CSUM_IP_TCP) {
+ /*
+ * Packet from local host (maybe from a VM).
+ * Checksum not required.
+ */
+ th->th_sum = 0;
} else {
struct ipovly *ipov = (struct ipovly *)ip;
diff --git a/sys/netinet/udp_usrreq.c b/sys/netinet/udp_usrreq.c
--- a/sys/netinet/udp_usrreq.c
+++ b/sys/netinet/udp_usrreq.c
@@ -555,6 +555,12 @@
ip->ip_dst.s_addr, htonl((u_short)len +
m->m_pkthdr.csum_data + proto));
uh_sum ^= 0xffff;
+ } else if (m->m_pkthdr.csum_flags & CSUM_IP_UDP) {
+ /*
+ * Packet from local host (maybe from a VM).
+ * Checksum not required.
+ */
+ uh_sum = 0;
} else {
char b[offsetof(struct ipovly, ih_src)];
struct ipovly *ipov = (struct ipovly *)ip;
diff --git a/sys/netinet6/ip6_fastfwd.c b/sys/netinet6/ip6_fastfwd.c
--- a/sys/netinet6/ip6_fastfwd.c
+++ b/sys/netinet6/ip6_fastfwd.c
@@ -27,6 +27,7 @@
#include <sys/cdefs.h>
#include "opt_inet6.h"
#include "opt_ipstealth.h"
+#include "opt_sctp.h"
#include <sys/param.h>
#include <sys/systm.h>
@@ -54,6 +55,10 @@
#include <netinet6/ip6_var.h>
#include <netinet6/nd6.h>
+#if defined(SCTP) || defined(SCTP_SUPPORT)
+#include <netinet/sctp_crc32.h>
+#endif
+
static int
ip6_findroute(struct nhop_object **pnh, const struct sockaddr_in6 *dst,
struct mbuf *m)
@@ -277,6 +282,29 @@
ip6->ip6_hlim -= IPV6_HLIMDEC;
}
+ /*
+ * If TCP/UDP header still needs a valid checksum and interface will not
+ * calculate it for us, do it here.
+ */
+ if (__predict_false(m->m_pkthdr.csum_flags & CSUM_DELAY_DATA_IPV6 &
+ ~nh->nh_ifp->if_hwassist)) {
+ int offset = ip6_lasthdr(m, 0, IPPROTO_IPV6, NULL);
+
+ if (offset < sizeof(struct ip6_hdr) || offset > m->m_pkthdr.len)
+ goto drop;
+ in6_delayed_cksum(m, m->m_pkthdr.len - offset, offset);
+ m->m_pkthdr.csum_flags &= ~CSUM_DELAY_DATA_IPV6;
+ }
+#if defined(SCTP) || defined(SCTP_SUPPORT)
+ if (__predict_false(m->m_pkthdr.csum_flags & CSUM_IP6_SCTP &
+ ~nh->nh_ifp->if_hwassist)) {
+ int offset = ip6_lasthdr(m, 0, IPPROTO_IPV6, NULL);
+
+ sctp_delayed_cksum(m, offset);
+ m->m_pkthdr.csum_flags &= ~CSUM_IP6_SCTP;
+ }
+#endif
+
m_clrprotoflags(m); /* Avoid confusing lower layers. */
IP_PROBE(send, NULL, NULL, ip6, nh->nh_ifp, NULL, ip6);
diff --git a/sys/netinet6/ip6_forward.c b/sys/netinet6/ip6_forward.c
--- a/sys/netinet6/ip6_forward.c
+++ b/sys/netinet6/ip6_forward.c
@@ -75,6 +75,10 @@
#include <netipsec/ipsec_support.h>
+#if defined(SCTP) || defined(SCTP_SUPPORT)
+#include <netinet/sctp_crc32.h>
+#endif
+
/*
* Forward a packet. If some error occurs return the sender
* an icmp packet. Note we can't always generate a meaningful
@@ -389,6 +393,29 @@
goto bad;
}
+ /*
+ * If TCP/UDP header still needs a valid checksum and interface will not
+ * calculate it for us, do it here.
+ */
+ if (__predict_false(m->m_pkthdr.csum_flags & CSUM_DELAY_DATA_IPV6 &
+ ~nh->nh_ifp->if_hwassist)) {
+ int offset = ip6_lasthdr(m, 0, IPPROTO_IPV6, NULL);
+
+ if (offset < sizeof(struct ip6_hdr) || offset > m->m_pkthdr.len)
+ goto bad;
+ in6_delayed_cksum(m, m->m_pkthdr.len - offset, offset);
+ m->m_pkthdr.csum_flags &= ~CSUM_DELAY_DATA_IPV6;
+ }
+#if defined(SCTP) || defined(SCTP_SUPPORT)
+ if (__predict_false(m->m_pkthdr.csum_flags & CSUM_IP6_SCTP &
+ ~nh->nh_ifp->if_hwassist)) {
+ int offset = ip6_lasthdr(m, 0, IPPROTO_IPV6, NULL);
+
+ sctp_delayed_cksum(m, offset);
+ m->m_pkthdr.csum_flags &= ~CSUM_IP6_SCTP;
+ }
+#endif
+
/* Currently LLE layer stores embedded IPv6 addresses */
if (IN6_IS_SCOPE_LINKLOCAL(&dst.sin6_addr)) {
in6_set_unicast_scopeid(&dst.sin6_addr, dst.sin6_scope_id);
diff --git a/sys/netinet6/sctp6_usrreq.c b/sys/netinet6/sctp6_usrreq.c
--- a/sys/netinet6/sctp6_usrreq.c
+++ b/sys/netinet6/sctp6_usrreq.c
@@ -139,7 +139,11 @@
goto out;
}
ecn_bits = IPV6_TRAFFIC_CLASS(ip6);
- if (m->m_pkthdr.csum_flags & CSUM_SCTP_VALID) {
+ if (m->m_pkthdr.csum_flags & (CSUM_SCTP_VALID | CSUM_IP6_SCTP)) {
+ /*
+ * Packet with CSUM_IP6_SCTP were sent from local host using
+ * checksum offloading. Checksum not required.
+ */
SCTP_STAT_INCR(sctps_recvhwcrc);
compute_crc = 0;
} else {
diff --git a/sys/netinet6/udp6_usrreq.c b/sys/netinet6/udp6_usrreq.c
--- a/sys/netinet6/udp6_usrreq.c
+++ b/sys/netinet6/udp6_usrreq.c
@@ -429,6 +429,12 @@
uh_sum = in6_cksum_pseudo(ip6, ulen, nxt,
m->m_pkthdr.csum_data);
uh_sum ^= 0xffff;
+ } else if (m->m_pkthdr.csum_flags & CSUM_IP6_UDP) {
+ /*
+ * Packet from local host (maybe from a VM).
+ * Checksum not required.
+ */
+ uh_sum = 0;
} else
uh_sum = in6_cksum_partial(m, nxt, off, plen, ulen);
diff --git a/sys/sys/mbuf.h b/sys/sys/mbuf.h
--- a/sys/sys/mbuf.h
+++ b/sys/sys/mbuf.h
@@ -647,10 +647,10 @@
* Outbound flags that are set by upper protocol layers requesting lower
* layers, or ideally the hardware, to perform these offloading tasks.
* For outbound packets this field and its flags can be directly tested
- * against ifnet if_hwassist. Note that the outbound and the inbound flags do
- * not collide right now but they could be allowed to (as long as the flags are
- * scrubbed appropriately when the direction of an mbuf changes). CSUM_BITS
- * would also have to split into CSUM_BITS_TX and CSUM_BITS_RX.
+ * against ifnet if_hwassist. Note that outbound flags CSUM_IP_UDP,
+ * CSUM_IP_TCP, and CSUM_IP_SCTP can appear on an inbound packet if the mbuf
+ * changed the direction. In such a case the checksum is still incorrect but
+ * TCP, UDP, or SCTP ignores that since the packet has not been on the wire.
*
* CSUM_INNER_<x> is the same as CSUM_<x> but it applies to the inner frame.
* The CSUM_ENCAP_<x> bits identify the outer encapsulation.

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