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Date:   Thu, 28 Sep 2023 05:16:10 +0200
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Subject: Re: [PATCH net-next v4 09/18] net/smc: introduce SMC-D loopback
 device
To:     Alexandra Winter <wintera@linux.ibm.com>,
        dust.li@linux.alibaba.com, Wen Gu <guwen@linux.alibaba.com>,
        kgraul@linux.ibm.com, wenjia@linux.ibm.com, davem@davemloft.net,
        edumazet@google.com, kuba@kernel.org, pabeni@redhat.com
Cc:     schnelle@linux.ibm.com, gbayer@linux.ibm.com, pasic@linux.ibm.com,
        alibuda@linux.alibaba.com, tonylu@linux.alibaba.com,
        linux-s390@vger.kernel.org, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org
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From:   Jan Karcher <jaka@linux.ibm.com>
Organization: IBM - Network Linux on Z
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On 26/09/2023 09:24, Alexandra Winter wrote:
> 
> 
> On 25.09.23 17:18, Dust Li wrote:
>>> Hello Wen Gu,
>>>
>>> thank you for adding the Kconfig, so the distributions can decide when to offer this feature.
>>>
>>> I propose you add some kind of runtime switch as well. Not every user who loads the SMC module
>>> may want to exploit smcd-loopback. Especially in native environements without containers.
>>>
>>> If no RoCE interfaces or no ISM interfaces exist, the respective handling is skipped in SMC.
>>> If loopback is always created unconditionally, there is no way to opt-out.
>> Hi Sandy,
>>
>> After talking to Wen Gu offline, I think the real issue here might be
>> we don't have an abstract layer in SMC, something like net/core/dev.c
>>
>> Without this, we cannot do:
>>
>> 1. Enable/disable those devices dynamically
>>     Currently, If we want to disable a SMC-R device to communicate with
>>     others, we need to refer to 'ip link set dev xxx down' to disable the
>>     netdevice, then Infiniband subsystem will notify SMC that the state of
>>     the IB device has changed. We cannot explicitly choose not to use some
>>     specific IB/RoCE devices without disable totally.
>>     If the loopback device need to support enable/disable itself, I
>>     think it might be better to enable this feature for all SMC devices.
>>
>> 2. Do statistics per device
>>     Now, we have to relay on IB/RoCE devices' hardware statistics to see
>>     how many packets/bytes we have sent through this device.
>>
>> Both the above issues get worse when the IB/RoCE device is shared by SMC
>> and userspace RDMA applications. If SMC-R and userspace RDMA applications
>> run at the same time, we can't enable the device to run userspace RDMA
>> applications while block it from running SMC. For statistics, we cannot
>> tell how many packets/bytes were sent by SMC and how many were sent by
>> userspace RDMA applications.
>>
>> So I think those are better to support in the SMC layer.
>>
>> Best regards!
>> Dust
> 
> Thank you very much for your considerations. I also think a generic handling
> of these requirements in the smc layer would be best. Especially, if we want
> to add virtio-ism support soon. There we will face the same issues again.
> Let's hear what others think about this.
> 
> 

Thanks you Sandy for bringing it up and Dust Li & Wen Gu for your thoughts.
I agree that such a runtime switch is needed and also that this generic 
handling would be good in the smc layer.