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no allocs

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This commit is contained in:
JackDoan
2026-04-17 10:29:46 -05:00
parent 9d59cba7e1
commit 5241bf6d16
2 changed files with 59 additions and 13 deletions
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42
overlay/tun_linux.go
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@@ -10,6 +10,7 @@ import (
"net"
"net/netip"
"os"
"runtime"
"strings"
"sync"
"sync/atomic"
@@ -44,8 +45,14 @@ type tunFile struct {
segBuf []byte // backing store for segmented output
pending [][]byte // segments waiting to be drained by Read
pendingIdx int
writeIovs [2]unix.Iovec // preallocated iovecs for vnetHdr writes; iovs[0] is fixed to zeroVnetHdr
}
// zeroVnetHdr is the 10-byte virtio_net_hdr we prepend to every TUN write when
// IFF_VNET_HDR is active. All-zero signals "no GSO, no checksum offload"; the
// kernel accepts the packet as-is.
var zeroVnetHdr [virtioNetHdrLen]byte
// newFriend makes a tunFile for a MultiQueueReader that copies the shutdown eventfd from the parent tun
func (r *tunFile) newFriend(fd int) (*tunFile, error) {
if err := unix.SetNonblock(fd, true); err != nil {
@@ -67,6 +74,8 @@ func (r *tunFile) newFriend(fd int) (*tunFile, error) {
if r.vnetHdr {
out.readBuf = make([]byte, tunReadBufSize)
out.segBuf = make([]byte, tunSegBufSize)
out.writeIovs[0].Base = &zeroVnetHdr[0]
out.writeIovs[0].SetLen(virtioNetHdrLen)
}
return out, nil
}
@@ -98,6 +107,8 @@ func newTunFd(fd int, vnetHdr bool) (*tunFile, error) {
if vnetHdr {
out.readBuf = make([]byte, tunReadBufSize)
out.segBuf = make([]byte, tunSegBufSize)
out.writeIovs[0].Base = &zeroVnetHdr[0]
out.writeIovs[0].SetLen(virtioNetHdrLen)
}
return out, nil
@@ -203,10 +214,6 @@ func (r *tunFile) Read(buf []byte) (int, error) {
}
}
// zeroVnetHdr is the prefix we prepend to every write when IFF_VNET_HDR is
// active and we have no offload info to convey.
var zeroVnetHdr [virtioNetHdrLen]byte
func (r *tunFile) Write(buf []byte) (int, error) {
if !r.vnetHdr {
for {
@@ -225,25 +232,34 @@ func (r *tunFile) Write(buf []byte) (int, error) {
}
}
iovs := [][]byte{zeroVnetHdr[:], buf}
if len(buf) == 0 {
return 0, nil
}
// Point the payload iovec at the caller's buffer. iovs[0] is pre-wired
// to zeroVnetHdr during tunFile construction so we don't rebuild it here.
r.writeIovs[1].Base = &buf[0]
r.writeIovs[1].SetLen(len(buf))
iovPtr := uintptr(unsafe.Pointer(&r.writeIovs[0]))
for {
n, err := unix.Writev(r.fd, iovs)
if err == nil {
if n < virtioNetHdrLen {
n, _, errno := unix.Syscall(unix.SYS_WRITEV, uintptr(r.fd), iovPtr, 2)
if errno == 0 {
runtime.KeepAlive(buf)
if int(n) < virtioNetHdrLen {
return 0, io.ErrShortWrite
}
return n - virtioNetHdrLen, nil
return int(n) - virtioNetHdrLen, nil
}
if err == unix.EAGAIN {
if err = r.blockOnWrite(); err != nil {
if errno == unix.EAGAIN {
if err := r.blockOnWrite(); err != nil {
return 0, err
}
continue
}
if err == unix.EINTR {
if errno == unix.EINTR {
continue
}
return 0, err
runtime.KeepAlive(buf)
return 0, errno
}
}