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[34.85.207.234]) by smtp.gmail.com with UTF8SMTPSA id 00721157ae682-717c620ec59sm4223147b3.120.2025.07.10.12.39.27 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 10 Jul 2025 12:39:27 -0700 (PDT) Date: Thu, 10 Jul 2025 15:39:27 -0400 From: Willem de Bruijn To: Eric Dumazet , "David S . Miller" , Jakub Kicinski , Paolo Abeni , Neal Cardwell Cc: Simon Horman , Kuniyuki Iwashima , Willem de Bruijn , netdev@vger.kernel.org, eric.dumazet@gmail.com, Eric Dumazet Message-ID: <687016ef6a2c_114eb42941d@willemb.c.googlers.com.notmuch> In-Reply-To: <20250710155641.3028726-1-edumazet@google.com> References: <20250710155641.3028726-1-edumazet@google.com> Subject: Re: [PATCH net-next] selftests/net: packetdrill: add --mss option to three tests Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit 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 Reviewed-by: Willem de Bruijn 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 */