netdev - Re: [RFC PATCH net-next v5 04/11] net/smc: implement some unsupported operations of loopback-ism

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Message-ID: <46e8e227-8058-4062-a9db-6b9c774f63cc@linux.alibaba.com>
Date: Tue, 9 Apr 2024 09:44:44 +0800
From: Wen Gu <guwen@...ux.alibaba.com>
To: Niklas Schnelle <schnelle@...ux.ibm.com>,
 Gerd Bayer <gbayer@...ux.ibm.com>, wintera@...ux.ibm.com,
 twinkler@...ux.ibm.com, hca@...ux.ibm.com, gor@...ux.ibm.com,
 agordeev@...ux.ibm.com, davem@...emloft.net, edumazet@...gle.com,
 kuba@...nel.org, pabeni@...hat.com, wenjia@...ux.ibm.com, jaka@...ux.ibm.com
Cc: borntraeger@...ux.ibm.com, svens@...ux.ibm.com,
 alibuda@...ux.alibaba.com, tonylu@...ux.alibaba.com,
 linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org,
 netdev@...r.kernel.org
Subject: Re: [RFC PATCH net-next v5 04/11] net/smc: implement some unsupported
 operations of loopback-ism



On 2024/4/4 23:15, Niklas Schnelle wrote:
> On Thu, 2024-04-04 at 21:12 +0800, Wen Gu wrote:
>>
>> On 2024/4/4 19:42, Niklas Schnelle wrote:
>>> On Thu, 2024-04-04 at 17:32 +0800, Wen Gu wrote:
>>>>
>>>> On 2024/4/4 00:25, Gerd Bayer wrote:
>>>>> On Sun, 2024-03-24 at 21:55 +0800, Wen Gu wrote:
>>>>>> This implements some operations that loopback-ism does not support
>>>>>> currently:
>>>>>>     - vlan operations, since there is no strong use-case for it.
>>>>>>     - signal_event operations, since there is no event to be processed
>>>>>> by the loopback-ism device.
>>>>>
>>>>> Hi Wen,
>>>>>
>>>>> I wonder if the these operations that are not supported by loopback-ism
>>>>> should rather be marked "optional" in the struct smcd_ops, and the
>>>>> calling code should call these only when they are implemented.
>>>>>
>>>>> Of course this would mean more changes to net/smc/smc_core.c - but
>>>>> loopback-ism could omit these "boiler-plate" functions.
>>>>>
>>>>
>>>> Hi Gerd.
>>>>
>>>> Thank you for the thoughts! I agree that checks like 'if(smcd->ops->xxx)'
>>>> can avoid the device driver from implementing unsupported operations. But I
>>>> am afraid that which operations need to be defined as 'optional' may differ
>>>> from different device perspectives (e.g. for loopback-ism they are vlan-related
>>>> opts and signal_event). So I perfer to simply let the smc protocol assume
>>>> that all operations have been implemented, and let drivers to decide which
>>>> ones are unsupported in implementation. What do you think?
>>>>
>>>> Thanks!
>>>>
>>>
>>> I agree with Gerd, in my opinion it is better to document ops as
>>> optional and then allow their function pointers to be NULL and check
>>> for that. Acting like they are supported and then they turn out to be
>>> nops to me seems to contradict the principle of least surprises. I also
>>> think we can find a subset of mandatory ops without which SMC-D is
>>> impossible and then everything else should be optional.
>>
>> I see. If we all agree to classify smcd_ops into mandatory and optional ones,
>> I'll add a patch to mark the optional ops and check if they are implemented.
> 
> Keep in mind I don't speak for the SMC maintainers but that does sound
> reasonable to me.
> 

Hi Wenjia and Jan, do you have any comments on this and [1]? Thanks!

[1] https://lore.kernel.org/netdev/60b4aec0b4bf4474d651b653c86c280dafc4518a.camel@linux.ibm.com/

>>
>>>
>>> As a first guess I think the following options may be mandatory:
>>>
>>> * query_remote_gid()
>>> * register_dmb()/unregister_dmb()
>>> * move_data()
>>>     For this one could argue that either move_data() or
>>>     attach_dmb()/detach_dmb() is required though personally I would
>>>     prefer to always have move_data() as a fallback and simple API
>>> * supports_v2()
>>> * get_local_gid()
>>> * get_chid()
>>> * get_dev()
>> I agree with this classification. Just one point, maybe we can take
>> supports_v2() as an optional ops, like support_dmb_nocopy()? e.g. if
>> it is not implemented, we treat it as an ISMv1.
>>
>> Thanks!
> 
> Interpreting a NULL supports_v2() as not supporting v2 sounds
> reasonable to me.