[ofa-general] Re: 0-length RDMA Read

[Steve Wise]

This looks like a bug.  The lib assumes an SGE entry will be provided.

A workaround for now is to set num_sge to 1 and initialize the sge entry to:

sge.addr 0
sge.lkey 2
sge.length 0
rkey 2
remote_addr 0

I'll fix this in libcxgb3 to allow num_sge == 0 to mean 0B read.  Also, 
right now you need to specify non-zero (and yet still valid possible) 
lkey and rkey values.  I'll fix this too so if length is 0 or num_sge is 
0, then the library will create a valid 0B read request for you ignoring 
the other fields.  I opened bug 1496 for this: 
https://bugs.openfabrics.org/show_bug.cgi?id=1496

Lemme know if you want a new libcxgb3 tarball with the fix.

By the way, as of ofed-1.3.1 and 2.6.27 kernels, iw_cxgb3 supports a 
mode where it handles this client-must-send-first issue for you.  There 
is a module option called peer2peer.  Set it to 1 and all subsequent 
connections will handle this by doing a 0B read from the client.

'echo 1 > /sys/module/iw_cxgb3/parameters/peer2peer'  will do the trick...

Steve.


Philip Frey1 wrote:
>
> Hi,
>
> since for iWARP, that after the MPA connection establishment, the
> MPA initiator must send the first FPDU, I wanted to do that using a 
> 0-length
> RDMA Read. When using the T3 Chelsio RNIC, I end up with a segmentation
> fault from the libcxgb3 (function t3b_post_send).
>
> I was trying to post a 0-lenght WR for RDMA Read like this:
>
> struct ibv_send_wr        wr;
>
> wr.wr_id = 1;
> wr.next = NULL;
> wr.sg_list = NULL;
> wr.num_sge = 0;
> wr.wr.rdma.remote_addr = 0;
> wr.wr.rdma.rkey = 0;
> wr.opcode = IBV_WR_RDMA_READ;
> wr.send_flags = IBV_SEND_SIGNALED;
>
> ibv_post_send(qp, &wr, &bad_wr);
>
> Question1: Are 0-length RDMA Reads supported at all by the T3?
> Question2: If they are, how do I have to write a correct send WR?
>
> Many thanks for your advice,
>  Philip
>
>
>
> -- 
>   Philip Frey                  
>   IBM Zurich Research Laboratory
>   Saumerstrasse 4                                   |  Phone: +41 44 
> 724 8613
>   CH-8803 Rueschlikon/Switzerland  |  Email: phf at zurich.ibm.com