netstack/tcp: software segmentation offload

Right now, we send each tcp packet separately, we call one system
call per-packet. This patch allows to generate multiple tcp packets
and send them by sendmmsg.

The arguable part of this CL is a way how to handle multiple headers.
This CL adds the next field to the Prepandable buffer.

Nginx test results:

Server Software:        nginx/1.15.9
Server Hostname:        10.138.0.2
Server Port:            8080

Document Path:          /10m.txt
Document Length:        10485760 bytes

w/o gso:
Concurrency Level:      5
Time taken for tests:   5.491 seconds
Complete requests:      100
Failed requests:        0
Total transferred:      1048600200 bytes
HTML transferred:       1048576000 bytes
Requests per second:    18.21 [#/sec] (mean)
Time per request:       274.525 [ms] (mean)
Time per request:       54.905 [ms] (mean, across all concurrent requests)
Transfer rate:          186508.03 [Kbytes/sec] received

sw-gso:

Concurrency Level:      5
Time taken for tests:   3.852 seconds
Complete requests:      100
Failed requests:        0
Total transferred:      1048600200 bytes
HTML transferred:       1048576000 bytes
Requests per second:    25.96 [#/sec] (mean)
Time per request:       192.576 [ms] (mean)
Time per request:       38.515 [ms] (mean, across all concurrent requests)
Transfer rate:          265874.92 [Kbytes/sec] received

w/o gso:
$ ./tcp_benchmark --client --duration 15  --ideal
[SUM]  0.0-15.1 sec  2.20 GBytes  1.25 Gbits/sec

software gso:
$ tcp_benchmark --client --duration 15  --ideal --gso $((1<<16)) --swgso
[SUM]  0.0-15.1 sec  3.99 GBytes  2.26 Gbits/sec

PiperOrigin-RevId: 276112677

1 files changed, 39 insertions, 11 deletions

diff --git a/pkg/tcpip/header/checksum.go b/pkg/tcpip/header/checksum.go
index 39a4d69be..9749c7f4d 100644
--- a/pkg/tcpip/header/checksum.go
+++ b/pkg/tcpip/header/checksum.go
@@ -23,11 +23,17 @@ import (
 	"gvisor.dev/gvisor/pkg/tcpip/buffer"
 )
 
-func calculateChecksum(buf []byte, initial uint32) uint16 {
+func calculateChecksum(buf []byte, odd bool, initial uint32) (uint16, bool) {
 	v := initial
 
+	if odd {
+		v += uint32(buf[0])
+		buf = buf[1:]
+	}
+
 	l := len(buf)
-	if l&1 != 0 {
+	odd = l&1 != 0
+	if odd {
 		l--
 		v += uint32(buf[l]) << 8
 	}
@@ -36,7 +42,7 @@ func calculateChecksum(buf []byte, initial uint32) uint16 {
 		v += (uint32(buf[i]) << 8) + uint32(buf[i+1])
 	}
 
-	return ChecksumCombine(uint16(v), uint16(v>>16))
+	return ChecksumCombine(uint16(v), uint16(v>>16)), odd
 }
 
 // Checksum calculates the checksum (as defined in RFC 1071) of the bytes in the
@@ -44,7 +50,8 @@ func calculateChecksum(buf []byte, initial uint32) uint16 {
 //
 // The initial checksum must have been computed on an even number of bytes.
 func Checksum(buf []byte, initial uint16) uint16 {
-	return calculateChecksum(buf, uint32(initial))
+	s, _ := calculateChecksum(buf, false, uint32(initial))
+	return s
 }
 
 // ChecksumVV calculates the checksum (as defined in RFC 1071) of the bytes in
@@ -52,19 +59,40 @@ func Checksum(buf []byte, initial uint16) uint16 {
 //
 // The initial checksum must have been computed on an even number of bytes.
 func ChecksumVV(vv buffer.VectorisedView, initial uint16) uint16 {
-	var odd bool
+	return ChecksumVVWithOffset(vv, initial, 0, vv.Size())
+}
+
+// ChecksumVVWithOffset calculates the checksum (as defined in RFC 1071) of the
+// bytes in the given VectorizedView.
+//
+// The initial checksum must have been computed on an even number of bytes.
+func ChecksumVVWithOffset(vv buffer.VectorisedView, initial uint16, off int, size int) uint16 {
+	odd := false
 	sum := initial
 	for _, v := range vv.Views() {
 		if len(v) == 0 {
 			continue
 		}
-		s := uint32(sum)
-		if odd {
-			s += uint32(v[0])
-			v = v[1:]
+
+		if off >= len(v) {
+			off -= len(v)
+			continue
+		}
+		v = v[off:]
+
+		l := len(v)
+		if l > size {
+			l = size
+		}
+		v = v[:l]
+
+		sum, odd = calculateChecksum(v, odd, uint32(sum))
+
+		size -= len(v)
+		if size == 0 {
+			break
 		}
-		odd = len(v)&1 != 0
-		sum = calculateChecksum(v, s)
+		off = 0
 	}
 	return sum
 }