Re: some questions about restrictions in SMC-R v2's implementation

[Wenjia Zhang <wenjia@linux.ibm.com>]

On 07.05.24 07:54, Guangguan Wang wrote:
> Hi, Wenjia and Jan,
> 
> When testing SMC-R v2, I found some scenarios where SMC-R v2 should be worked, but due to some restrictions in SMC-R v2's implementation,
> fallback happened. I want to know why these restrictions exist and what would happen if these restrictions were removed.
> 
> The first is in the function smc_ib_determine_gid_rcu, where restricts the subnet matching between smcrv2->saddr and the RDMA related netdev.
> codes here:
> static int smc_ib_determine_gid_rcu(...)
> {
>      ...
>          in_dev_for_each_ifa_rcu(ifa, in_dev) {
>              if (!inet_ifa_match(smcrv2->saddr, ifa))
>                  continue;
>              subnet_match = true;
>              break;
>          }
>          if (!subnet_match)
>              goto out;
>      ...
> out:
>      return -ENODEV;
> }
> In my testing environment, either server or client, exists two netdevs, eth0 in netnamespace1 and eth0 in netnamespace2. For the sake of clarity
> in the following text, we will refer to eth0 in netnamespace1 as eth1, and eth0 in netnamespace2 as eth2. The eth1's ip is 192.168.0.3/32 and the
> eth2's ip is 192.168.0.4/24. The netmask of eth1 must be 32 due to some reasons. The eth1 is a RDMA related netdev, which means the adaptor of eth1
> has RDMA function. The eth2 has been associated to the eth1's RDMA device using smc_pnet. When testing connection in netnamespace2(using eth2 for
> SMC-R connection), we got fallback connection, rsn is 0x03010000, due to the above subnet matching restriction. But in this scenario, I think
> SMC-R should work.
> In my another testing environment, either server or client, exists two netdevs, eth0 in netnamespace1 and eth1 in netnamespace1. The eth0's ip is
> 192.168.0.3/24 and the eth1's ip is 192.168.1.4/24. The eth0 is a RDMA related netdev, which means the adaptor of eth0 has RDMA function. The eth1 has
> been associated to the eth0's RDMA device using smc_pnet. When testing SMC-R connection through eth1, we got fallback connection, rsn is 0x03010000,
> due to the above subnet matching restriction. In my environment, eth0 and eth1 have the same network connectivity even though they have different
> subnet. I think SMC-R should work in this scenario.
> 
> The other is in the function smc_connect_rdma_v2_prepare, where restricts the symmetric configuration of routing between client and server. codes here:
> static int smc_connect_rdma_v2_prepare(...)
> {
>      ...
>      if (fce->v2_direct) {
>          memcpy(ini->smcrv2.nexthop_mac, &aclc->r0.lcl.mac, ETH_ALEN);
>          ini->smcrv2.uses_gateway = false;
>      } else {
>          if (smc_ib_find_route(net, smc->clcsock->sk->sk_rcv_saddr,
>                smc_ib_gid_to_ipv4(aclc->r0.lcl.gid),
>                ini->smcrv2.nexthop_mac,
>                &ini->smcrv2.uses_gateway))
>              return SMC_CLC_DECL_NOROUTE;
>          if (!ini->smcrv2.uses_gateway) {
>              /* mismatch: peer claims indirect, but its direct */
>              return SMC_CLC_DECL_NOINDIRECT;
>          }
>      }
>      ...
> }
> In my testing environment, server's ip is 192.168.0.3/24, client's ip 192.168.0.4/24, regarding how many netdev in server or client. Server has special
> route setting due to some other reasons, which results in indirect route from 192.168.0.3/24 to 192.168.0.4/24. Thus, when CLC handshake, client will
> get fce->v2_direct==false, but client has no special routing setting and will find direct route from 192.168.0.4/24 to 192.168.0.3/24. Due to the above
> symmetric configuration of routing restriction, we got fallback connection, rsn is 0x030f0000. But I think SMC-R should work in this scenario.
> And more, why check the symmetric configuration of routing only when server is indirect route?
> 
> Waiting for your reply.
> 
> Thanks,
> Guangguan Wang
> 
Hi Guangguan,

Thank you for the questions. We also asked ourselves the same questions 
a while ago, and also did some research on it. Unfortunately, it was not 
yet done and I had to delay it because of my vacation last month. Now 
it's time to pick it up again ;) I'll come back to you as soon as I can 
give a very certain answer.

Thanks,
Wenjia