netstack/fdbased: add generic segmentation offload (GSO) support

The linux packet socket can handle GSO packets, so we can segment packets to
64K instead of the MTU which is usually 1500.

Here are numbers for the nginx-1m test:
runsc:		579330.01 [Kbytes/sec] received
runsc-gso:	1794121.66 [Kbytes/sec] received
runc:		2122139.06 [Kbytes/sec] received

and for tcp_benchmark:

$ tcp_benchmark  --duration 15   --ideal
[  4]  0.0-15.0 sec  86647 MBytes  48456 Mbits/sec

$ tcp_benchmark --client --duration 15   --ideal
[  4]  0.0-15.0 sec  2173 MBytes  1214 Mbits/sec

$ tcp_benchmark --client --duration 15   --ideal --gso 65536
[  4]  0.0-15.0 sec  19357 MBytes  10825 Mbits/sec

PiperOrigin-RevId: 240809103
Change-Id: I2637f104db28b5d4c64e1e766c610162a195775a

1 files changed, 13 insertions, 8 deletions

diff --git a/pkg/tcpip/transport/tcp/connect.go b/pkg/tcpip/transport/tcp/connect.go
index c4353718e..056e0b09a 100644
--- a/pkg/tcpip/transport/tcp/connect.go
+++ b/pkg/tcpip/transport/tcp/connect.go
@@ -557,14 +557,14 @@ func sendSynTCP(r *stack.Route, id stack.TransportEndpointID, flags byte, seq, a
 	}
 
 	options := makeSynOptions(opts)
-	err := sendTCP(r, id, buffer.VectorisedView{}, r.DefaultTTL(), flags, seq, ack, rcvWnd, options)
+	err := sendTCP(r, id, buffer.VectorisedView{}, r.DefaultTTL(), flags, seq, ack, rcvWnd, options, nil)
 	putOptions(options)
 	return err
 }
 
 // sendTCP sends a TCP segment with the provided options via the provided
 // network endpoint and under the provided identity.
-func sendTCP(r *stack.Route, id stack.TransportEndpointID, data buffer.VectorisedView, ttl uint8, flags byte, seq, ack seqnum.Value, rcvWnd seqnum.Size, opts []byte) *tcpip.Error {
+func sendTCP(r *stack.Route, id stack.TransportEndpointID, data buffer.VectorisedView, ttl uint8, flags byte, seq, ack seqnum.Value, rcvWnd seqnum.Size, opts []byte, gso *stack.GSO) *tcpip.Error {
 	optLen := len(opts)
 	// Allocate a buffer for the TCP header.
 	hdr := buffer.NewPrependable(header.TCPMinimumSize + int(r.MaxHeaderLength()) + optLen)
@@ -586,12 +586,17 @@ func sendTCP(r *stack.Route, id stack.TransportEndpointID, data buffer.Vectorise
 	})
 	copy(tcp[header.TCPMinimumSize:], opts)
 
+	length := uint16(hdr.UsedLength() + data.Size())
+	xsum := r.PseudoHeaderChecksum(ProtocolNumber, length)
 	// Only calculate the checksum if offloading isn't supported.
-	if r.Capabilities()&stack.CapabilityChecksumOffload == 0 {
-		length := uint16(hdr.UsedLength() + data.Size())
-		xsum := r.PseudoHeaderChecksum(ProtocolNumber, length)
+	if gso != nil && gso.NeedsCsum {
+		// This is called CHECKSUM_PARTIAL in the Linux kernel. We
+		// calculate a checksum of the pseudo-header and save it in the
+		// TCP header, then the kernel calculate a checksum of the
+		// header and data and get the right sum of the TCP packet.
+		tcp.SetChecksum(xsum)
+	} else if r.Capabilities()&stack.CapabilityChecksumOffload == 0 {
 		xsum = header.ChecksumVV(data, xsum)
-
 		tcp.SetChecksum(^tcp.CalculateChecksum(xsum))
 	}
 
@@ -600,7 +605,7 @@ func sendTCP(r *stack.Route, id stack.TransportEndpointID, data buffer.Vectorise
 		r.Stats().TCP.ResetsSent.Increment()
 	}
 
-	return r.WritePacket(hdr, data, ProtocolNumber, ttl)
+	return r.WritePacket(gso, hdr, data, ProtocolNumber, ttl)
 }
 
 // makeOptions makes an options slice.
@@ -649,7 +654,7 @@ func (e *endpoint) sendRaw(data buffer.VectorisedView, flags byte, seq, ack seqn
 		sackBlocks = e.sack.Blocks[:e.sack.NumBlocks]
 	}
 	options := e.makeOptions(sackBlocks)
-	err := sendTCP(&e.route, e.id, data, e.route.DefaultTTL(), flags, seq, ack, rcvWnd, options)
+	err := sendTCP(&e.route, e.id, data, e.route.DefaultTTL(), flags, seq, ack, rcvWnd, options, e.gso)
 	putOptions(options)
 	return err
 }