From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Miller Subject: Re: [PATCH 2/6] xen-netfront: reduce gso_max_size to account for ethernet header Date: Mon, 25 Mar 2013 14:39:23 -0400 (EDT) Message-ID: <20130325.143923.762657816602656826.davem@davemloft.net> References: <1364209702-12437-3-git-send-email-wei.liu2@citrix.com> <20130325.121809.2023305728304341952.davem@davemloft.net> <20130325183206.GA7004@zion.uk.xensource.com> Mime-Version: 1.0 Content-Type: Text/Plain; charset=us-ascii Content-Transfer-Encoding: 7bit Cc: 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: wei.liu2@citrix.com Return-path: Received: from shards.monkeyblade.net ([149.20.54.216]:55114 "EHLO shards.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932611Ab3CYSj1 (ORCPT ); Mon, 25 Mar 2013 14:39:27 -0400 In-Reply-To: <20130325183206.GA7004@zion.uk.xensource.com> Sender: netdev-owner@vger.kernel.org List-ID: From: Wei Liu Date: Mon, 25 Mar 2013 18:32:06 +0000 > On Mon, Mar 25, 2013 at 04:18:09PM +0000, David Miller wrote: >> From: Wei Liu >> Date: Mon, 25 Mar 2013 11:08:18 +0000 >> >> > The maximum packet including ethernet header that can be handled by netfront / >> > netback wire format is 65535. Reduce gso_max_size accordingly. >> > >> > Drop skb and print warning when skb->len > 65535. This can 1) save the effort >> > to send malformed packet to netback, 2) help spotting misconfiguration of >> > netfront in the future. >> > >> > Signed-off-by: Wei Liu >> >> 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); > > OK. But I see similar fix for a physical nic (commit b7e5887e0e414b), am > I missing something here? > > And the symptom is that if I don't reserve headroom I see skb->len = > 65538. Can you shed some light on this? See Eric's reply. If a GRO frame is forwarded we don't make the GSO size checks on the send size as we should.