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

[Wen Gu <guwen@linux.alibaba.com>]

On 2024/4/11 19:12, Alexandra Winter wrote:
> 
> 
> On 09.04.24 03:44, Wen Gu wrote:
>>
>>
>> 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.
>>
> 
> Let me add my thoughts to the discussion:
> For the vlan operations and signal_event operations that loopback-ism does
> not support:
> I like the idea to set the ops to NULL and make sure the caller checks that
> and can live with it. That is readable and efficient.
> 
> I don't think there is a need to discuss a strategy now, which ops could be
> optional in the future. This is all inside the kernel. loopback-ism is even
> inside the smc module. Such comments in the code get outdated very easily.
> 
> I would propose to mark those as optional struct smcd_ops, where all callers can
> handle a NULL pointer and still be productive.
> Future support of other devices for SMC-D can update that.
> 
> 

Hi Sandy, just to confirm if I understand you correctly.

You are proposing that don't draw a conclusion about the classification now,
but supplementally mark which one become a optional operation in struct smcd_ops
during the introduction of new devices for SMC-D.