netdev - Re: [PATCH net-next v4 09/18] net/smc: introduce SMC-D loopback device

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Message-ID: <c4ba2015-d951-451a-f96c-2946bfb9611c@linux.ibm.com>
Date: Mon, 25 Sep 2023 15:29:08 +0200
From: Alexandra Winter <wintera@...ux.ibm.com>
To: Wen Gu <guwen@...ux.alibaba.com>, kgraul@...ux.ibm.com,
        wenjia@...ux.ibm.com, jaka@...ux.ibm.com, davem@...emloft.net,
        edumazet@...gle.com, kuba@...nel.org, pabeni@...hat.com
Cc: schnelle@...ux.ibm.com, gbayer@...ux.ibm.com, pasic@...ux.ibm.com,
        alibuda@...ux.alibaba.com, tonylu@...ux.alibaba.com,
        dust.li@...ux.alibaba.com, linux-s390@...r.kernel.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next v4 09/18] net/smc: introduce SMC-D loopback
 device



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@...ux.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).