Re: [PATCH net-next] selftests/net: packetdrill: add --mss option to three tests

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

Eric Dumazet wrote:
> Three tests are cooking GSO packets but do not provide
> gso_size information to the kernel, triggering this message:
> 
> TCP: tun0: Driver has suspect GRO implementation, TCP performance may be compromised.
> 
> Add --mss option to avoid this warning.
> 
> Signed-off-by: Eric Dumazet <edumazet@google.com>

Reviewed-by: Willem de Bruijn <willemb@google.com>

This is because of this in packetdrill linux_tun_write, I suppose:

        if (packet->tcp && packet->mss) {
                if (packet->ipv4)
                        gso.gso_type = VIRTIO_NET_HDR_GSO_TCPV4;
                else
                        gso.gso_type = VIRTIO_NET_HDR_GSO_TCPV6;
                gso.gso_size = packet->mss;
        }

Which seems to imply that setting mss means that all generated packets
use GRO. Which tracks with the commit that introduced it: commit
58a7865c47e3 ("net-test: packetdrill: add GRO support to packetdrill").
Which includes

	+++ b/gtests/net/packetdrill/config.h
	@@ -73,6 +73,7 @@ struct config {
		u32 speed;                      /* speed reported by tun driver;
						 * may require special tun driver
						 */
	+       int mss;                        /* gso_size for GRO packets to tun device */