Luigi Rizzo [off-list ref] writes:

Add a netdevice flag to control skb linearization in generic xdp mode.

The attribute can be modified through
	/sys/class/net/<DEVICE>/xdpgeneric_linearize
The default is 1 (on)

Motivation: xdp expects linear skbs with some minimum headroom, and
generic xdp calls skb_linearize() if needed. The linearization is
expensive, and may be unnecessary e.g. when the xdp program does
not need access to the whole payload.
This sysfs entry allows users to opt out of linearization on a
per-device basis (linearization is still performed on cloned skbs).

On a kernel instrumented to grab timestamps around the linearization
code in netif_receive_generic_xdp, and heavy netperf traffic with 1500b
mtu, I see the following times (nanoseconds/pkt)

The receiver generally sees larger packets so the difference is more
significant.

ns/pkt                   RECEIVER                 SENDER

                    p50     p90     p99       p50   p90    p99

LINEARIZATION:    600ns  1090ns  4900ns     149ns 249ns  460ns
NO LINEARIZATION:  40ns    59ns    90ns      40ns  50ns  100ns

v1 --> v2 : added Documentation
v2 --> v3 : adjusted for skb_cloned
v3 --> v4 : renamed to xdpgeneric_linearize, documentation

Signed-off-by: Luigi Rizzo <redacted>

Acked-by: Toke Høiland-Jørgensen <redacted>