From mboxrd@z Thu Jan 1 00:00:00 1970 From: Eric Dumazet Subject: Re: [PATCH 2/6] xen-netfront: reduce gso_max_size to account for ethernet header Date: Mon, 25 Mar 2013 09:54:32 -0700 Message-ID: <1364230472.29473.21.camel@edumazet-glaptop> References: <1364209702-12437-1-git-send-email-wei.liu2@citrix.com> <1364209702-12437-3-git-send-email-wei.liu2@citrix.com> <20130325.121809.2023305728304341952.davem@davemloft.net> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit Cc: wei.liu2@citrix.com, xen-devel@lists.xen.org, netdev@vger.kernel.org, ian.campbell@citrix.com, annie.li@oracle.com, konrad.wilk@oracle.com, david.vrabel@citrix.com To: David Miller Return-path: Received: from mail-ie0-f179.google.com ([209.85.223.179]:43046 "EHLO mail-ie0-f179.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758086Ab3CYQyf (ORCPT ); Mon, 25 Mar 2013 12:54:35 -0400 Received: by mail-ie0-f179.google.com with SMTP id k11so7685351iea.10 for ; Mon, 25 Mar 2013 09:54:35 -0700 (PDT) In-Reply-To: <20130325.121809.2023305728304341952.davem@davemloft.net> Sender: netdev-owner@vger.kernel.org List-ID: On Mon, 2013-03-25 at 12:18 -0400, David Miller wrote: > > This is effectively the default already, you don't need to change this > value explicitly. > > ->gso_max_size is set by default to 65536 and then TCP performs this > calculation: > > xmit_size_goal = ((sk->sk_gso_max_size - 1) - > inet_csk(sk)->icsk_af_ops->net_header_len - > inet_csk(sk)->icsk_ext_hdr_len - > tp->tcp_header_len); > > thereby making it adhere to your limits just fine. For locally generated TCP traffic this is the case. However, GRO can build packets up to 65535 bytes, not including the Ethernet header. For such packets, it seems xen-netfront needs a segmentation. And we might have other providers as well (UFO for example ?), but I have not checked.