Re: [PATCH for-next] RDMA/efa: Move provider specific attributes to ucontext allocation response

[Gal Pressman <galpress@amazon.com>]

On 17/06/2020 20:49, Gal Pressman wrote:
> On 17/06/2020 18:36, Jason Gunthorpe wrote:
>> On Tue, Jun 16, 2020 at 08:44:37PM +0300, Gal Pressman wrote:
>>> On 16/06/2020 12:38, Leon Romanovsky wrote:
>>>> On Tue, Jun 16, 2020 at 11:53:11AM +0300, Gal Pressman wrote:
>>>>> On 16/06/2020 9:30, Leon Romanovsky wrote:
>>>>>> On Mon, Jun 15, 2020 at 10:59:20AM +0300, Gal Pressman wrote:
>>>>>>> Provider specific attributes which are necessary for the userspace
>>>>>>> functionality should be part of the alloc ucontext response, not query
>>>>>>> device. This way a userspace provider could work without issuing a query
>>>>>>> device verb call. However, the fields will remain in the query device
>>>>>>> ABI in order to maintain backwards compatibility.
>>>>>>
>>>>>> I don't really understand why "should be ..."? Device properties exposed
>>>>>> here are per-device and will be equal to all ucontexts, so instead of
>>>>>> doing one very fast system call, you are "punishing" every ucontext
>>>>>> call.
>>>>>
>>>>> I talked about it with Jason in the past, the query device verb is intended to
>>>>> follow the IBA verb, alloc ucontext should return driver specific data that's
>>>>> required to operate the user space provider.
>>>>> A query device call should not be mandatory to load the provider.
>>>>
>>>> Why? query_device is declared as mandatory verb for any provider, so
>>>> anyway all in-the-tree RDMA drivers will have such verb.
>>>
>>> I don't think the concern here is if the verb exists or not, my understanding is
>>> that query device should be used for IBA query device attributes, not other
>>> provider specific stuff.
>>> Jason, want to chime in with your thoughts?
>>
>> query_device should be used to implement the ibverb query_device and
>> query_device_ex
>>
>> It should only return rdma-core defined common stuff because that is
>> what that verb does - there is no reason to return driver specific
>> things as there is nothing the driver can do with it.
>>
>> The only exception might be some provider specific query_device dv
>> that needs more information.
>>
>> query_device should not be used as some two-part
>> create_context. Information related only to create_context that is not
>> already exposed to query_device should not be added to query_device
>> only for create_context's use.
>>
>> Similarly, information in query_device should not be duplicated into
>> create_context just to save a system call.
> 
> That makes sense.
> To clarify, the "duplicated" fields in this patch are moved to the ucontext
> allocation, where they originally belong as all of them are necessary for the
> provider's functionality.
> Future fields such as these will only be added to alloc_ucontext, not both, so
> there's no duplication.
> 
> Otherwise, future extensions will either have to be added to query_device, which
> is the wrong place, just to be consistent with the existing code. Or we add them
> to the ucontext response, where they belong, and end up with some hybrid
> solution where different fields are gathered from different places (yuck :\).
> 
> We got it wrong the first place but it's a two way door, let's fix it.

Uhh.. We can't really get rid of the query device call as the provider needs the
max_qp attribute in order to allocate the QP table properly.

I still think we should move the fields to keep things clean, but I can drop
this change if you prefer to avoid the churn. The provider will always call
query device on context initialization and gather different fields from
different system calls.

Thoughts?