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Message-ID: <c4ba2015-d951-451a-f96c-2946bfb9611c@linux.ibm.com>
Date:   Mon, 25 Sep 2023 15:29:08 +0200
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Subject: Re: [PATCH net-next v4 09/18] net/smc: introduce SMC-D loopback
 device
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From:   Alexandra Winter <wintera@linux.ibm.com>
To:     Wen Gu <guwen@linux.alibaba.com>, kgraul@linux.ibm.com,
        wenjia@linux.ibm.com, jaka@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,
        dust.li@linux.alibaba.com, linux-s390@vger.kernel.org,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org
References: <1695568613-125057-1-git-send-email-guwen@linux.alibaba.com>
 <1695568613-125057-10-git-send-email-guwen@linux.alibaba.com>
 <3febdf3e-e213-7acf-7dd4-75d177676c3e@linux.ibm.com>
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On 25.09.23 13:50, Alexandra Winter wrote:
> 
> 
> On 24.09.23 17:16, Wen Gu wrote:
>> This patch introduces a kind of loopback device for SMC-D. The device
>> is created when SMC module is loaded and destroyed when the SMC module
>> is unloaded. The loopback device is a kernel device used only by the
>> SMC module and is not restricted by net namespace, so it can be used
>> for local inter-process or inter-container communication.
>>
>> Signed-off-by: Wen Gu <guwen@linux.alibaba.com>
>> ---
>>  net/smc/Kconfig        |  13 ++++
>>  net/smc/Makefile       |   2 +-
>>  net/smc/af_smc.c       |  12 +++-
>>  net/smc/smc_loopback.c | 165 +++++++++++++++++++++++++++++++++++++++++++++++++
>>  net/smc/smc_loopback.h |  33 ++++++++++
>>  5 files changed, 223 insertions(+), 2 deletions(-)
>>  create mode 100644 net/smc/smc_loopback.c
>>  create mode 100644 net/smc/smc_loopback.h
> 
> 
> 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.
> 

Another thing came to my mind:

When loopback is created and registered when the SMC module is loaded, it will implicitly always have highest priority, right?
That should be stated somewhere.
Also, if you create a runtime switch this will change, so then you need to decide about priority of loopback vs ISM device (and other future smcd-devices).