IPv6 UFO for VMs

[Ben Hutchings]

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 <***@stressinduktion.org>
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.

Ben.

[Hannes Frederic Sowa]

Post by Ben Hutchings
There are several ways that VMs can take advantage of UFO and get the
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;
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.
commit 916e4cf46d0204806c062c8c6c4d1f633852c5b6
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