netdev - Re: [net-next 1/2] xen-netback: add module parameter to disable ctrl-ring

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Date:   Sun, 21 Mar 2021 17:31:08 +0100
From:   "Hsu, Chiahao" <andyhsu@...zon.com>
To:     Leon Romanovsky <leon@...nel.org>
CC:     Andrew Lunn <andrew@...n.ch>, <netdev@...r.kernel.org>,
        <wei.liu@...nel.org>, <paul@....org>, <davem@...emloft.net>,
        <kuba@...nel.org>, <xen-devel@...ts.xenproject.org>
Subject: Re: [net-next 1/2] xen-netback: add module parameter to disable ctrl-ring



Leon Romanovsky 於 2021/3/17 18:22 寫道:
> CAUTION: This email originated from outside of the organization. Do not click links or open attachments unless you can confirm the sender and know the content is safe.
>
>
>
> On Tue, Mar 16, 2021 at 04:22:21PM +0100, Hsu, Chiahao wrote:
>>
>> Leon Romanovsky 於 2021/3/14 11:04 寫道:
>>> CAUTION: This email originated from outside of the organization. Do not click links or open attachments unless you can confirm the sender and know the content is safe.
>>>
>>>
>>>
>>> On Fri, Mar 12, 2021 at 09:36:59PM +0100, Andrew Lunn wrote:
>>>> On Fri, Mar 12, 2021 at 04:18:02PM +0100, Hsu, Chiahao wrote:
>>>>> Andrew Lunn 於 2021/3/12 15:52 寫道:
>>>>>> CAUTION: This email originated from outside of the organization. Do not click links or open attachments unless you can confirm the sender and know the content is safe.
>>>>>>
>>>>>>
>>>>>>
>>>>>> On Thu, Mar 11, 2021 at 10:59:44PM +0000, ChiaHao Hsu wrote:
>>>>>>> In order to support live migration of guests between kernels
>>>>>>> that do and do not support 'feature-ctrl-ring', we add a
>>>>>>> module parameter that allows the feature to be disabled
>>>>>>> at run time, instead of using hardcode value.
>>>>>>> The default value is enable.
>>>>>> Hi ChiaHao
>>>>>>
>>>>>> There is a general dislike for module parameters. What other mechanisms
>>>>>> have you looked at? Would an ethtool private flag work?
>>>>>>
>>>>>>         Andrew
>>>>> Hi Andrew,
>>>>>
>>>>> I can survey other mechanisms, however before I start doing that,
>>>>>
>>>>> could you share more details about what the problem is with using module
>>>>> parameters? thanks.
>>>> It is not very user friendly. No two kernel modules use the same
>>>> module parameters. Often you see the same name, but different
>>>> meaning. There is poor documentation, you often need to read the
>>>> kernel sources it figure out what it does, etc.
>>> +1, It is also global parameter to whole system/devices that use this
>>> module, which is rarely what users want.
>>>
>>> Thanks
>> Hi,
>> I think I would say the current implementation(modparams) isappropriate
>> after reviewing it again.
>>
>> We are talking about 'feature leveling', a way to support live migrationof
>> guest
>> between kernels that do and do not support the features. So we want to
>> refrain
>> fromadding the features if guest would be migrated to the kernel which does
>> not support the feature. Everythingshould be done (in probe function) before
>> frontend connects, and this is why ethtool is not appropriate for this.
> It wouldn't be a surprise to you that feature discovery is not supposed
> to be done through module parameters. Instead of asking from user to
> randomly disable some feature, the system is expected to be backward
> compatible and robust enough to query the list of supported/needed
> features.
>
> Thanks
>
>> Thanks
>>
>>
Typically there should be one VM running netback on each host,
and having control over what interfaces or features it exposes is also 
important for stability.
How about we create a 'feature flags' modparam, each bits is specified 
for different new features?