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

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Date: Thu, 4 Apr 2024 21:12:16 +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 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.

> 
> 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!

>>>