The VLAN_HWTAGGING diff is in snapshots for a while, I saw no
additional feedback.  How about adding TSO to igc(4) next?

I ran thorough tests on bluhms' setup:
http://bluhm.genua.de/netlink/results/2024-04-30T00%3A37%3A41Z/netlink.html
and additional tests on my home router using pppoe(4) and vlan(4).

ok?

Index: dev/pci/if_igc.c
===================================================================
RCS file: /cvs/src/sys/dev/pci/if_igc.c,v
diff -u -p -r1.20 if_igc.c
--- dev/pci/if_igc.c	12 Apr 2024 19:27:43 -0000	1.20
+++ dev/pci/if_igc.c	1 May 2024 16:16:29 -0000
@@ -44,10 +44,14 @@
 
 #include <net/if.h>
 #include <net/if_media.h>
+#include <net/route.h>
 #include <net/toeplitz.h>
 
 #include <netinet/in.h>
 #include <netinet/if_ether.h>
+#include <netinet/tcp.h>
+#include <netinet/tcp_timer.h>
+#include <netinet/tcp_var.h>
 
 #if NBPFILTER > 0
 #include <net/bpf.h>
@@ -796,6 +800,7 @@ igc_setup_interface(struct igc_softc *sc
 	ifp->if_capabilities |= IFCAP_CSUM_IPv4;
 	ifp->if_capabilities |= IFCAP_CSUM_TCPv4 | IFCAP_CSUM_UDPv4;
 	ifp->if_capabilities |= IFCAP_CSUM_TCPv6 | IFCAP_CSUM_UDPv6;
+	ifp->if_capabilities |= IFCAP_TSOv4 | IFCAP_TSOv6;
 
 	/* Initialize ifmedia structures. */
 	ifmedia_init(&sc->media, IFM_IMASK, igc_media_change, igc_media_status);
@@ -993,8 +998,6 @@ igc_start(struct ifqueue *ifq)
 			continue;
 		}
 
-		olinfo_status = m->m_pkthdr.len << IGC_ADVTXD_PAYLEN_SHIFT;
-
 		bus_dmamap_sync(txr->txdma.dma_tag, map, 0,
 		    map->dm_mapsize, BUS_DMASYNC_PREWRITE);
 
@@ -1007,6 +1010,9 @@ igc_start(struct ifqueue *ifq)
 			prod++;
 			prod &= mask;
 			free--;
+		} else {
+			olinfo_status =
+			    m->m_pkthdr.len << IGC_ADVTXD_PAYLEN_SHIFT;
 		}
 
 		for (i = 0; i < map->dm_nsegs; i++) {
@@ -2025,12 +2031,15 @@ igc_tx_ctx_setup(struct tx_ring *txr, st
 {
 	struct ether_extracted ext;
 	struct igc_adv_tx_context_desc *txdesc;
+	uint32_t mss_l4len_idx = 0;
 	uint32_t type_tucmd_mlhl = 0;
 	uint32_t vlan_macip_lens = 0;
 	int off = 0;
 
-	ether_extract_headers(mp, &ext);
-	vlan_macip_lens |= (sizeof(*ext.eh) << IGC_ADVTXD_MACLEN_SHIFT);
+	if (mp->m_pkthdr.csum_flags & M_TCP_TSO)
+		*olinfo_status = 0;
+	else
+		*olinfo_status = mp->m_pkthdr.len << IGC_ADVTXD_PAYLEN_SHIFT;
 
 	/*
 	 * In advanced descriptors the vlan tag must
@@ -2046,6 +2055,10 @@ igc_tx_ctx_setup(struct tx_ring *txr, st
 	}
 #endif
 
+	ether_extract_headers(mp, &ext);
+
+	vlan_macip_lens |= (sizeof(*ext.eh) << IGC_ADVTXD_MACLEN_SHIFT);
+
 	if (ext.ip4) {
 		type_tucmd_mlhl |= IGC_ADVTXD_TUCMD_IPV4;
 		if (ISSET(mp->m_pkthdr.csum_flags, M_IPV4_CSUM_OUT)) {
@@ -2056,6 +2069,9 @@ igc_tx_ctx_setup(struct tx_ring *txr, st
 	} else if (ext.ip6) {
 		type_tucmd_mlhl |= IGC_ADVTXD_TUCMD_IPV6;
 #endif
+	} else {
+		if (mp->m_pkthdr.csum_flags & M_TCP_TSO)
+			tcpstat_inc(tcps_outbadtso);
 	}
 
 	vlan_macip_lens |= ext.iphlen;
@@ -2075,6 +2091,29 @@ igc_tx_ctx_setup(struct tx_ring *txr, st
 		}
 	}
 
+	if (mp->m_pkthdr.csum_flags & M_TCP_TSO) {
+		if (ext.tcp) {
+			uint32_t hdrlen, thlen, paylen, outlen;
+
+			thlen = ext.tcphlen;
+
+			outlen = mp->m_pkthdr.ph_mss;
+			mss_l4len_idx |= outlen << IGC_ADVTXD_MSS_SHIFT;
+			mss_l4len_idx |= thlen << IGC_ADVTXD_L4LEN_SHIFT;
+
+			hdrlen = sizeof(*ext.eh) + ext.iphlen + thlen;
+			paylen = mp->m_pkthdr.len - hdrlen;
+			*olinfo_status |= paylen << IGC_ADVTXD_PAYLEN_SHIFT;
+
+			*cmd_type_len |= IGC_ADVTXD_DCMD_TSE;
+			off = 1;
+
+			tcpstat_add(tcps_outpkttso,
+			    (paylen + outlen - 1) / outlen);
+		} else
+			tcpstat_inc(tcps_outbadtso);
+	}
+
 	if (off == 0)
 		return 0;
 
@@ -2085,7 +2124,7 @@ igc_tx_ctx_setup(struct tx_ring *txr, st
 	htolem32(&txdesc->vlan_macip_lens, vlan_macip_lens);
 	htolem32(&txdesc->type_tucmd_mlhl, type_tucmd_mlhl);
 	htolem32(&txdesc->seqnum_seed, 0);
-	htolem32(&txdesc->mss_l4len_idx, 0);
+	htolem32(&txdesc->mss_l4len_idx, mss_l4len_idx);
 
 	return 1;
 }