Refactor software GSO code.

Software GSO implementation currently has a complicated code path with
implicit assumptions that all packets to WritePackets carry same Data
and it does this to avoid allocations on the path etc. But this makes it
hard to reuse the WritePackets API.

This change breaks all such assumptions by introducing a new Vectorised
View API ReadToVV which can be used to cleanly split a VV into multiple
independent VVs. Further this change also makes packet buffers linkable
to form an intrusive list. This allows us to get rid of the array of
packet buffers that are passed in the WritePackets API call and replace
it with a list of packet buffers.

While this code does introduce some more allocations in the benchmarks
it doesn't cause any degradation.

Updates #231

PiperOrigin-RevId: 304731742

2 files changed, 26 insertions, 27 deletions

diff --git a/pkg/tcpip/transport/tcp/connect.go b/pkg/tcpip/transport/tcp/connect.go
index 3239a5911..2ca3fb809 100644
--- a/pkg/tcpip/transport/tcp/connect.go
+++ b/pkg/tcpip/transport/tcp/connect.go
@@ -756,8 +756,7 @@ func (e *endpoint) sendTCP(r *stack.Route, tf tcpFields, data buffer.VectorisedV
 func buildTCPHdr(r *stack.Route, tf tcpFields, pkt *stack.PacketBuffer, gso *stack.GSO) {
 	optLen := len(tf.opts)
 	hdr := &pkt.Header
-	packetSize := pkt.DataSize
-	off := pkt.DataOffset
+	packetSize := pkt.Data.Size()
 	// Initialize the header.
 	tcp := header.TCP(hdr.Prepend(header.TCPMinimumSize + optLen))
 	pkt.TransportHeader = buffer.View(tcp)
@@ -782,12 +781,18 @@ func buildTCPHdr(r *stack.Route, tf tcpFields, pkt *stack.PacketBuffer, gso *sta
 		// header and data and get the right sum of the TCP packet.
 		tcp.SetChecksum(xsum)
 	} else if r.Capabilities()&stack.CapabilityTXChecksumOffload == 0 {
-		xsum = header.ChecksumVVWithOffset(pkt.Data, xsum, off, packetSize)
+		xsum = header.ChecksumVV(pkt.Data, xsum)
 		tcp.SetChecksum(^tcp.CalculateChecksum(xsum))
 	}
 }
 
 func sendTCPBatch(r *stack.Route, tf tcpFields, data buffer.VectorisedView, gso *stack.GSO, owner tcpip.PacketOwner) *tcpip.Error {
+	// We need to shallow clone the VectorisedView here as ReadToView will
+	// split the VectorisedView and Trim underlying views as it splits. Not
+	// doing the clone here will cause the underlying views of data itself
+	// to be altered.
+	data = data.Clone(nil)
+
 	optLen := len(tf.opts)
 	if tf.rcvWnd > 0xffff {
 		tf.rcvWnd = 0xffff
@@ -796,31 +801,25 @@ func sendTCPBatch(r *stack.Route, tf tcpFields, data buffer.VectorisedView, gso
 	mss := int(gso.MSS)
 	n := (data.Size() + mss - 1) / mss
 
-	// Allocate one big slice for all the headers.
-	hdrSize := header.TCPMinimumSize + int(r.MaxHeaderLength()) + optLen
-	buf := make([]byte, n*hdrSize)
-	pkts := make([]stack.PacketBuffer, n)
-	for i := range pkts {
-		pkts[i].Header = buffer.NewEmptyPrependableFromView(buf[i*hdrSize:][:hdrSize])
-	}
-
 	size := data.Size()
-	off := 0
+	hdrSize := header.TCPMinimumSize + int(r.MaxHeaderLength()) + optLen
+	var pkts stack.PacketBufferList
 	for i := 0; i < n; i++ {
 		packetSize := mss
 		if packetSize > size {
 			packetSize = size
 		}
 		size -= packetSize
-		pkts[i].DataOffset = off
-		pkts[i].DataSize = packetSize
-		pkts[i].Data = data
-		pkts[i].Hash = tf.txHash
-		pkts[i].Owner = owner
-		buildTCPHdr(r, tf, &pkts[i], gso)
-		off += packetSize
+		var pkt stack.PacketBuffer
+		pkt.Header = buffer.NewPrependable(hdrSize)
+		pkt.Hash = tf.txHash
+		pkt.Owner = owner
+		data.ReadToVV(&pkt.Data, packetSize)
+		buildTCPHdr(r, tf, &pkt, gso)
 		tf.seq = tf.seq.Add(seqnum.Size(packetSize))
+		pkts.PushBack(&pkt)
 	}
+
 	if tf.ttl == 0 {
 		tf.ttl = r.DefaultTTL()
 	}
@@ -845,12 +844,10 @@ func sendTCP(r *stack.Route, tf tcpFields, data buffer.VectorisedView, gso *stac
 	}
 
 	pkt := stack.PacketBuffer{
-		Header:     buffer.NewPrependable(header.TCPMinimumSize + int(r.MaxHeaderLength()) + optLen),
-		DataOffset: 0,
-		DataSize:   data.Size(),
-		Data:       data,
-		Hash:       tf.txHash,
-		Owner:      owner,
+		Header: buffer.NewPrependable(header.TCPMinimumSize + int(r.MaxHeaderLength()) + optLen),
+		Data:   data,
+		Hash:   tf.txHash,
+		Owner:  owner,
 	}
 	buildTCPHdr(r, tf, &pkt, gso)
 
diff --git a/pkg/tcpip/transport/tcp/segment.go b/pkg/tcpip/transport/tcp/segment.go
index e6fe7985d..40461fd31 100644
--- a/pkg/tcpip/transport/tcp/segment.go
+++ b/pkg/tcpip/transport/tcp/segment.go
@@ -77,9 +77,11 @@ func newSegmentFromView(r *stack.Route, id stack.TransportEndpointID, v buffer.V
 		id:     id,
 		route:  r.Clone(),
 	}
-	s.views[0] = v
-	s.data = buffer.NewVectorisedView(len(v), s.views[:1])
 	s.rcvdTime = time.Now()
+	if len(v) != 0 {
+		s.views[0] = v
+		s.data = buffer.NewVectorisedView(len(v), s.views[:1])
+	}
 	return s
 }