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

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Message-ID: <687016ef6a2c_114eb42941d@willemb.c.googlers.com.notmuch>
Date: Thu, 10 Jul 2025 15:39:27 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Eric Dumazet <edumazet@...gle.com>, 
 "David S . Miller" <davem@...emloft.net>, 
 Jakub Kicinski <kuba@...nel.org>, 
 Paolo Abeni <pabeni@...hat.com>, 
 Neal Cardwell <ncardwell@...gle.com>
Cc: Simon Horman <horms@...nel.org>, 
 Kuniyuki Iwashima <kuniyu@...gle.com>, 
 Willem de Bruijn <willemb@...gle.com>, 
 netdev@...r.kernel.org, 
 eric.dumazet@...il.com, 
 Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH net-next] selftests/net: packetdrill: add --mss option to
 three tests

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@...gle.com>

Reviewed-by: Willem de Bruijn <willemb@...gle.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 */