rtnetlink and many VFs

[Ben Hutchings <bhutchings@solarflare.com>]

My colleagues have been working on SR-IOV support for sfc.  The hardware
supports up to 127 VFs per port.

If we configure all 127 VFs through the net device, an RTM_GETLINK dump
will need to include messages describing them, with a total size of:

127 * (sizeof(struct ifla_vf_mac) + sizeof(struct ifla_vf_vlan) +
       sizeof(struct ifla_vf_tx_rate) + protocol overhead)
> 7112

These messages are nested within the message describing the device as a
whole, so they cannot be split.  The maximum size of an outgoing netlink
message, based on NLMSG_GOODSIZE, seems to be min(PAGE_SIZE, 8192).  So
when PAGE_SIZE = 4096 it is simply impossible to dump information about
such a device!

I think it needs to be made possible to grow a netlink skb during
generation of the first message.  Userspace may still be unable to
receive the large message but at least it has a chance.

Ben.

-- 
Ben Hutchings, Senior Software Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.