[PATCH net-next 4/5] net/tun: implement ndo_set_rx_headroom

[Paolo Abeni]

ndo_set_rx_headroom controls the align value used by tun devices to
allocate skbs on frame reception.
When the xmit device adds a large encapsulation, this avoids an skb
head reallocation on forwarding.

The measured improvement when forwarding towards a vxlan dev with
frame size below the egress device MTU is around 6% when tunneling over
ipv6.

In case of ipv4 tunnels there is no improvement, since the tun
device default alignment provides enough headroom to avoid the skb
head reallocation, at least on hosts with 64 bytes cacheline.

Signed-off-by: Paolo Abeni <pab...@redhat.com>
---
 drivers/net/tun.c | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 88bb8cc..5812693 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -187,6 +187,7 @@ struct tun_struct {
 #define TUN_USER_FEATURES (NETIF_F_HW_CSUM|NETIF_F_TSO_ECN|NETIF_F_TSO| \
                          NETIF_F_TSO6|NETIF_F_UFO)
 
+       int                     align;
        int                     vnet_hdr_sz;
        int                     sndbuf;
        struct tap_filter       txflt;
@@ -934,6 +935,17 @@ static void tun_poll_controller(struct net_device *dev)
        return;
 }
 #endif
+
+static void tun_set_headroom(struct net_device *dev, int new_hr)
+{
+       struct tun_struct *tun = netdev_priv(dev);
+
+       if (new_hr < NET_SKB_PAD)
+               new_hr = NET_SKB_PAD;
+
+       tun->align = new_hr;
+}
+
 static const struct net_device_ops tun_netdev_ops = {
        .ndo_uninit             = tun_net_uninit,
        .ndo_open               = tun_net_open,
@@ -945,6 +957,7 @@ static const struct net_device_ops tun_netdev_ops = {
 #ifdef CONFIG_NET_POLL_CONTROLLER
        .ndo_poll_controller    = tun_poll_controller,
 #endif
+       .ndo_set_rx_headroom    = tun_set_headroom,
 };
 
 static const struct net_device_ops tap_netdev_ops = {
@@ -962,6 +975,7 @@ static const struct net_device_ops tap_netdev_ops = {
        .ndo_poll_controller    = tun_poll_controller,
 #endif
        .ndo_features_check     = passthru_features_check,
+       .ndo_set_rx_headroom    = tun_set_headroom,
 };
 
 static void tun_flow_init(struct tun_struct *tun)
@@ -1086,7 +1100,7 @@ static ssize_t tun_get_user(struct tun_struct *tun, 
struct tun_file *tfile,
        struct tun_pi pi = { 0, cpu_to_be16(ETH_P_IP) };
        struct sk_buff *skb;
        size_t total_len = iov_iter_count(from);
-       size_t len = total_len, align = NET_SKB_PAD, linear;
+       size_t len = total_len, align = tun->align, linear;
        struct virtio_net_hdr gso = { 0 };
        int good_linear;
        int copylen;
-- 
1.8.3.1