holy crap 2x

overlay/device.go

@@ -30,3 +30,22 @@ type Device interface {
 	SupportsMultiqueue() bool
 	NewMultiQueueReader() (Queue, error)
 }
+
+// GSOWriter is implemented by Queues that can write a TCP TSO superpacket as
+// a single virtio_net_hdr + payload writev, letting the kernel segment on
+// egress. Callers type-assert on it; backends that don't support GSO return
+// false from Supported and all coalescing logic is skipped.
+//
+// pkt must contain the IPv4/IPv6 + TCP header plus the concatenated
+// coalesced payload. hdrLen is the total L3+L4 header length (where the
+// payload starts). csumStart is the byte offset where the TCP header
+// begins (= IP header length). gsoSize is the MSS — every segment except
+// possibly the last must be exactly this many payload bytes. isV6 selects
+// GSO_TCPV4 vs GSO_TCPV6.
+//
+// pkt'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(pkt []byte, gsoSize uint16, isV6 bool, hdrLen, csumStart uint16) error
+	GSOSupported() bool
+}