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Message-ID: <44ea7d83-4fa7-427b-9d54-678f05fd09e9@linux.alibaba.com>
Date: Fri, 12 Apr 2024 10:02:37 +0800
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Subject: Re: [RFC PATCH net-next v5 04/11] net/smc: implement some unsupported
 operations of loopback-ism
To: Alexandra Winter <wintera@linux.ibm.com>,
 Niklas Schnelle <schnelle@linux.ibm.com>, Gerd Bayer <gbayer@linux.ibm.com>,
 twinkler@linux.ibm.com, hca@linux.ibm.com, gor@linux.ibm.com,
 agordeev@linux.ibm.com, davem@davemloft.net, edumazet@google.com,
 kuba@kernel.org, pabeni@redhat.com, wenjia@linux.ibm.com, jaka@linux.ibm.com
Cc: borntraeger@linux.ibm.com, svens@linux.ibm.com,
 alibuda@linux.alibaba.com, tonylu@linux.alibaba.com,
 linux-kernel@vger.kernel.org, linux-s390@vger.kernel.org,
 netdev@vger.kernel.org
References: <20240324135522.108564-1-guwen@linux.alibaba.com>
 <20240324135522.108564-5-guwen@linux.alibaba.com>
 <3122eece5b484abcf8d23f85d6c18c36f0b939ff.camel@linux.ibm.com>
 <1db6ccab-b49f-45d2-a93c-05b0f79371a3@linux.alibaba.com>
 <3b3ff37643e9030ec1246e67720683a2cf5660e5.camel@linux.ibm.com>
 <7a0fc481-658e-4c99-add7-ccbd5f9dce1e@linux.alibaba.com>
 <7291dd1b2d16fd9bbd90988ac5bcc3a46d17e3f4.camel@linux.ibm.com>
 <46e8e227-8058-4062-a9db-6b9c774f63cc@linux.alibaba.com>
 <12ae995f-4af4-4c6b-9130-04672d157293@linux.ibm.com>
From: Wen Gu <guwen@linux.alibaba.com>
In-Reply-To: <12ae995f-4af4-4c6b-9130-04672d157293@linux.ibm.com>
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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.