slightly nicer contract?

overlay/batch/tcp_coalesce.go

@@ -397,7 +397,7 @@ func (c *TCPCoalescer) flushSlot(s *coalesceSlot) error {
 	tcsum := s.ipHdrLen + 16
 	binary.BigEndian.PutUint16(hdr[tcsum:tcsum+2], foldOnceNoInvert(psum))
 
-	return c.gsoW.WriteGSO(hdr, s.payIovs, uint16(s.gsoSize), uint16(s.ipHdrLen))
+	return c.gsoW.WriteGSO(hdr[:s.ipHdrLen], hdr[s.ipHdrLen:], s.payIovs)
 }
 
 // headersMatch compares two IP+TCP header prefixes for byte-for-byte

overlay/tio/tio.go

@@ -55,6 +55,13 @@ type Queue interface {
 // hdr's TCP checksum field must already hold the pseudo-header partial
 // sum (single-fold, not inverted), per virtio NEEDS_CSUM semantics.
 type GSOWriter interface {
-	WriteGSO(hdr []byte, pays [][]byte, gsoSize uint16, csumStart uint16) error
+	// WriteGSO emits a TCP TSO superpacket in a single writev. hdr is the
+	// IPv4/IPv6 + TCP header prefix (already finalized — total length, IP csum,
+	// and TCP pseudo-header partial set by the caller). pays are payload
+	// fragments whose concatenation forms the full coalesced payload; each
+	// slice is read-only and must stay valid until return.
+	// every segment in pays except possibly the last is exactly the same size.
+	// csumStart is the byte offset where the TCP header begins within hdr.
+	WriteGSO(hdr []byte, transportHdr []byte, pays [][]byte) error
 	GSOSupported() bool
 }

overlay/tio/tio_gso_linux.go

@@ -291,33 +291,18 @@ func (r *Offload) rawWrite(iovs []unix.Iovec) (int, error) {
 // Write calls.
 func (r *Offload) GSOSupported() bool { return true }
 
-// WriteGSO emits a TCP TSO superpacket in a single writev. hdr is the
-// IPv4/IPv6 + TCP header prefix (already finalized — total length, IP csum,
-// and TCP pseudo-header partial set by the caller). pays are payload
-// fragments whose concatenation forms the full coalesced payload; each
-// slice is read-only and must stay valid until return. gsoSize is the MSS;
-// every segment except possibly the last is exactly gsoSize bytes.
-// csumStart is the byte offset where the TCP header begins within hdr.
-func (r *Offload) WriteGSO(hdr []byte, pays [][]byte, gsoSize uint16, csumStart uint16) error {
-	if len(hdr) == 0 || len(pays) == 0 {
+func (r *Offload) WriteGSO(hdr []byte, transportHdr []byte, pays [][]byte) error {
+	if len(hdr) == 0 || len(pays) == 0 || len(transportHdr) == 0 {
 		return nil
 	}
-
-	// Build the virtio_net_hdr. When pays total to <= gsoSize the kernel
-	// would produce a single segment; keep NEEDS_CSUM semantics but skip
-	// the GSO type so the kernel doesn't spuriously mark this as TSO.
 	vhdr := VirtioNetHdr{
 		Flags:      unix.VIRTIO_NET_HDR_F_NEEDS_CSUM,
-		HdrLen:     uint16(len(hdr)),
+		HdrLen:     uint16(len(hdr) + len(transportHdr)),
 		GSOSize:    uint16(len(pays[0])),
-		CsumStart:  csumStart,
+		CsumStart:  uint16(len(hdr)),
 		CsumOffset: 16, // TCP checksum field lives 16 bytes into the TCP header
 	}
-	var totalPay int
-	for _, p := range pays {
-		totalPay += len(p)
-	}
-	if totalPay > int(gsoSize) {
+	if len(pays) > 1 {
 		ipVer := hdr[0] >> 4
 		if ipVer == 6 {
 			vhdr.GSOType = unix.VIRTIO_NET_HDR_GSO_TCPV6
@@ -333,9 +318,9 @@ func (r *Offload) WriteGSO(hdr []byte, pays [][]byte, gsoSize uint16, csumStart
 	}
 	vhdr.encode(r.gsoHdrBuf[:])
 
-	// Build the iovec array: [virtio_hdr, hdr, pays...]. r.gsoIovs[0] is
+	// Build the iovec array: [virtio_hdr, hdr, transportHdr, pays...]. r.gsoIovs[0] is
 	// wired to gsoHdrBuf at construction and never changes.
-	need := 2 + len(pays)
+	need := 3 + len(pays)
 	if cap(r.gsoIovs) < need {
 		grown := make([]unix.Iovec, need)
 		grown[0] = r.gsoIovs[0]
@@ -345,9 +330,11 @@ func (r *Offload) WriteGSO(hdr []byte, pays [][]byte, gsoSize uint16, csumStart
 	}
 	r.gsoIovs[1].Base = &hdr[0]
 	r.gsoIovs[1].SetLen(len(hdr))
+	r.gsoIovs[2].Base = &transportHdr[0]
+	r.gsoIovs[2].SetLen(len(transportHdr))
 	for i, p := range pays {
-		r.gsoIovs[2+i].Base = &p[0]
-		r.gsoIovs[2+i].SetLen(len(p))
+		r.gsoIovs[3+i].Base = &p[0]
+		r.gsoIovs[3+i].SetLen(len(p))
 	}
 
 	_, err := r.rawWrite(r.gsoIovs)