Re: [PATCH for-next 13/16] IB/{hfi1, ipoib, rdma}: Broadcast ping sent packets which exceeded mtu size

[Jason Gunthorpe <jgg@ziepe.ca>]

On Fri, Feb 21, 2020 at 02:40:28PM -0500, Dennis Dalessandro wrote:
> On 2/18/2020 7:42 PM, Jason Gunthorpe wrote:
> > On Mon, Feb 10, 2020 at 08:19:44AM -0500, Dennis Dalessandro wrote:
> > > From: Gary Leshner <Gary.S.Leshner@intel.com>
> > > 
> > > When in connected mode ipoib sent broadcast pings which exceeded the mtu
> > > size for broadcast addresses.
> > > 
> > > Add an mtu attribute to the rdma_netdev structure which ipoib sets to its
> > > mcast mtu size.
> > > 
> > > The RDMA netdev uses this value to determine if the skb length is too long
> > > for the mtu specified and if it is, drops the packet and logs an error
> > > about the errant packet.
> > 
> > I'm confused by this comment, connected mode is not able to use
> > rdma_netdev, for various technical reason, I thought?
> > 
> > Is this somehow running a rdma_netdev concurrently with connected
> > mode? How?
> 
> No, not concurrently. When ipoib is in connected mode, a broadcast request,
> something like:
> 
> ping -s 2017 -i 0.001 -c 10 -M do -I ib0 -b 192.168.0.255
> 
> will be sent down from user space to ipoib. At an mcast_mtu of 2048, the max
> payload size is 2016 (2048 - 28 - 4). If AIP is not being used then the
> datagram send function (ipoib_send()) does a check and drops the packet.
> 
> However when AIP is enabled ipoib_send is of course not used and we land in
> rn->send function. Which needs to do the same check.

You just contradicted yourself: the first sentence was 'not
concurrently' and here you say we have connected mode turned on and
yet a packet is delivered to AIP, so what do you mean?

What I mean is if you can do connected mode you don't have a
rdma_netdev and you can't do AIP.

How are things in connected mode and a rdma_netdev is available?

Jason