From mboxrd@z Thu Jan 1 00:00:00 1970 From: Hannes Frederic Sowa Subject: Re: IPv6 UFO for VMs Date: Wed, 22 Oct 2014 11:35:41 +0200 Message-ID: <1413970541.2337.16.camel@localhost> References: <1413935045.5994.2.camel@decadent.org.uk> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit Cc: netdev , virtualization@lists.linux-foundation.org To: Ben Hutchings Return-path: Received: from out1-smtp.messagingengine.com ([66.111.4.25]:43125 "EHLO out1-smtp.messagingengine.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754333AbaJVJfn (ORCPT ); Wed, 22 Oct 2014 05:35:43 -0400 Received: from compute6.internal (compute6.nyi.internal [10.202.2.46]) by gateway2.nyi.internal (Postfix) with ESMTP id AC72B20A3B for ; Wed, 22 Oct 2014 05:35:42 -0400 (EDT) In-Reply-To: <1413935045.5994.2.camel@decadent.org.uk> Sender: netdev-owner@vger.kernel.org List-ID: On Mi, 2014-10-22 at 00:44 +0100, Ben Hutchings wrote: > There are several ways that VMs can take advantage of UFO and get the > host to do fragmentation for them: > > drivers/net/macvtap.c: gso_type = SKB_GSO_UDP; > drivers/net/tun.c: skb_shinfo(skb)->gso_type = SKB_GSO_UDP; > drivers/net/virtio_net.c: skb_shinfo(skb)->gso_type = SKB_GSO_UDP; > > Our implementation of UFO for IPv6 does: > > fptr = (struct frag_hdr *)(skb_network_header(skb) + unfrag_ip6hlen); > fptr->nexthdr = nexthdr; > fptr->reserved = 0; > fptr->identification = skb_shinfo(skb)->ip6_frag_id; > > which assumes ip6_frag_id has been set. That's only true if the local > stack constructed the skb; otherwise it appears we get zero. > > This seems to be a regression as a result of: > > commit 916e4cf46d0204806c062c8c6c4d1f633852c5b6 > Author: Hannes Frederic Sowa > Date: Fri Feb 21 02:55:35 2014 +0100 > > ipv6: reuse ip6_frag_id from ip6_ufo_append_data > > However, that change seems reasonable - we *shouldn't* be choosing IDs > for any other stack. Any paravirt net driver that can use IPv6 UFO > needs to have some way of passing a fragmentation ID to put in > skb_shared_info::ip6_frag_id. Do we really gain a lot of performance by enabling UFO on those devices or would it make sense to just drop support? It only helps fragmenting large UDP packets, so I don't think it is worth it. Otherwise I agree with Ben, we need to pass a fragmentation id from the host over to the system segmenting the gso frame. Fragmentation ids must be generated by the end system. Hmm... Bye, Hannes