[Openib-windows] Added ability to change IPoIB mtu size

[Fabian Tillier]

Hi folks,

On 11/19/06, Anatoly Lisenko <anatolyl at voltaire.com> wrote:
>
> Hi Yossi,
>
> This patch add the ability to change MTU size in IPoIB.
>
> Please review.
>
> Thanks,
>
> Anatoly

It would be nice to have an explanation of what problem patches are
trying to solve.  I assume this is done to enable virtualization over
IPoIB, which requires network packets to be ethernet MTU-sized.

I didn't have a chance to look at the patch, but have these thoughts.
Limiting the MTU reported to the OS works fine for outbound packets,
but care must be exercised for inbound packets.

1. The buffers posted to the receive queue must be the full 2048 byte
IB MTU so that the IPoIB driver can receive any packets sent to it
without hitting a protection error due to overrunning the receive
buffers.
2. If a buffer larger than the configured MTU is received, it must be
reported to the OS somehow.  I don't know what's required to do this,
whether you create a new IP header for the data beyond the first, or
whatnot, but it must be done in such a way that subsequent packets
don't cause issues with sequence numbers.  This doesn't seem like a
simple thing to do right, let alone in a performant way.

Cheers,

- Fab