Re: [dpdk-dev] [RFC] net/null: add empty promiscuous mode functions

[Ferruh Yigit <ferruh.yigit@intel.com>]

On 10/18/2019 2:12 PM, Bruce Richardson wrote:
> On Fri, Oct 18, 2019 at 12:38:53PM +0100, Ferruh Yigit wrote:
>> On 10/18/2019 11:13 AM, Bruce Richardson wrote:
>>> On Thu, Oct 17, 2019 at 04:33:59PM +0100, Ferruh Yigit wrote:
>>>> On 10/17/2019 2:43 PM, Bruce Richardson wrote:
>>>>> On Thu, Oct 17, 2019 at 12:05:56PM +0100, Ferruh Yigit wrote:
>>>>>> On 10/17/2019 11:51 AM, Andrew Rybchenko wrote:
>>>>>>> On 10/17/19 1:47 PM, Ferruh Yigit wrote:
>>>>>>>> On 10/17/2019 11:37 AM, Andrew Rybchenko wrote:
>>>>>>>>> On 10/16/19 9:07 PM, Ferruh Yigit wrote:
>>>>>>>>>> On 10/16/2019 4:46 PM, Ciara Power wrote:
>>>>>>>>>>> Adding promiscuous functions prevents sample applications failing when run
>>>>>>>>>>> on this virtual PMD. The sample applications call promiscuous functions,
>>>>>>>>>>> and fail if this function call returns an error, which occurs when the
>>>>>>>>>>> virtual PMD does not support the promiscuous function being called.
>>>>>>>>>>>
>>>>>>>>>>> This change will be implemented for all virtual PMDs that currently do not
>>>>>>>>>>> have existing promiscuous functions. Multicast functions will also be
>>>>>>>>>>> added for virtual PMDs to prevent sample application breakages here also.
>>>>>>>>>> +Andrew
>>>>>>>>>>
>>>>>>>>>> With the some ethdev APIs returning error code, some sample applications stop
>>>>>>>>>> working with virtual interfaces,
>>>>>>>>>>
>>>>>>>>>> We can,
>>>>>>>>>> 1- update sample applications to ignore the errors
>>>>>>>>>> 2- Add dummy dev_ops support to (almost all) virtual PMDs (what this RFC suggests)
>>>>>>>>>>
>>>>>>>>>> (1) puts us back to before the ethdev APIs updated status, and this may be wrong
>>>>>>>>>> for the physical devices case, so I am for this RFC.
>>>>>>>>>>
>>>>>>>>>> Only perhaps we can have some common empty function and keep assigning that one
>>>>>>>>>> to reduce the dummy code, what do you think?
>>>>>>>>> I don't like the idea to have common empty/dummy functions.
>>>>>>>>> If virtual PMD behaves in accordance with enabled promiscuous mode,
>>>>>>>>> it should initialize it properly on init:
>>>>>>>>>        eth_dev->data->promiscuous = 1;
>>>>>>>>> If so, if application requires promiscuous mode, attempt to enable will
>>>>>>>>> do nothing. If application requires non-promiscuous mode, disable will
>>>>>>>>> fail and it is good.
>>>>>>>> It is technically correct that we can't disable promiscuous mode in virtual PMDs
>>>>>>>> but I think mainly we don't really care so it returning error may make the
>>>>>>>> applications fail/exit unnecessarily with virtual PMDs.
>>>>>>>
>>>>>>> If I test virtual PMD promiscuous mode, I would prefer enable/disable
>>>>>>> callback to say me truth.
>>>>>>>
>>>>>>> If application really does not care, it should be in the application code.
>>>>>>
>>>>>> Application can't change this because they may be caring return result for the
>>>>>> physical devices.
>>>>>>
>>>>>> Up until this release these missing dev_ops in virtual PMDs were silently
>>>>>> ignored, now APIs are more strict on this (which is good) but to get close the
>>>>>> previous behavior for virtual PMDs we need to relax on these features (like
>>>>>> saying success on promiscuous disable although it didn't).
>>>>>>
>>>>> The other variable here is how often an app is going to request promiscuous
>>>>> disabling? Given that most ports generally come up in that state anyway,
>>>>> and one needs to request enabling it, surely the disable case is relatively
>>>>> rare? In that case I'd tend to agree with having disabling it returning
>>>>> error for vpmds.
>>>>>
>>>>
>>>> Yes disabling most probably rare, but still it will generate an error and
>>>> application is failing because of ring PMD promiscuous disable doesn't look
>>>> right to me.
>>>
>>> Well, if an app needs promiscuous mode disabled then having it fail is the
>>> right thing to do. If the app doesn't care about promiscuous mode failing,
>>> why is it checking the return value at all?
>>>
>>>>
>>>> Perhaps application should differentiate between -ENOTSUP error and operation
>>>> fail error, but that looks to me adding unnecessary complexity to the app.
>>>>
>>> Again, does the app care or not? It's probably still better to return
>>> correct info to the app in all cases, and then let the app decide how best
>>> to handle it.
>>
>> My thinking is, application "cares" about the ethdev API return values, but to
>> check/test quickly with null/ring PMD perhaps not really care that much abut
>> promisc/allmuticast support of these PMDs and let's relax these support of
>> virtual PMDs to make life easy.
>>
>> But eventually the main target is to fix sample applications to run with virtual
>> PMDs which has been broken in this release.
>> Both approach works,
>> a) Implement dummy dev_ops to virtual PMDs to report success
>> b) Update ethdev APIs to not call dev_ops if the requested configuration is
>> already satisfied and change virtual PMDs to report promisc & allmulticast
>> enabled by default. (disable still will have same issue)
>>
>> Is the consensus option (b)?
>>
>>
>>
>> btw, the problem exists in high level for the offload support, if the
>> application is requesting a specific offload support it fails to run with the
>> virtual PMDs since virtual PMDs doesn't support any offloads. Indeed I have same
>> suggestion for this case too, relax the virtual PMD by claiming it supports all
>> offloads. Because at least for me when I use those virtual PMDs I don't really
>> would like to test offloads or the procmisc/allmulticast features ...
>>
> I really dislike having the drivers lying. It may work in some cases, but
> eventually you will hit a problem where an app really does need a feature
> and then breaks for the user in mysterious ways when run with a virtual
> PMD. Much better to have the vPMD always report the truth to the app, and
> let the app worry about whether the app can continue on error or not.
> 
> Final option I'd throw out there, is to allow a vdev parameter to tell the
> vpmd it's allowed to lie. That gives an override in case of an app that
> can't handle a non-fatal failure.

vdev devarg can work for the offload case, only issue I can see at first glance
is code duplication among the PMDs but perhaps that can be solved when issue
checked in-dept.