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

From: Gal Pressman <galpress@amazon.com>
To: Leon Romanovsky <leon@kernel.org>
Cc: Jason Gunthorpe <jgg@ziepe.ca>,
	Doug Ledford <dledford@redhat.com>, <linux-rdma@vger.kernel.org>,
	Alexander Matushevsky <matua@amazon.com>,
	"Firas JahJah" <firasj@amazon.com>,
	Yossi Leybovich <sleybo@amazon.com>
Subject: Re: [PATCH for-next] RDMA/efa: Move provider specific attributes to ucontext allocation response
Date: Tue, 16 Jun 2020 20:44:37 +0300	[thread overview]
Message-ID: <f6773480-5954-b58b-21f0-f5ee4ec7238b@amazon.com> (raw)
In-Reply-To: <20200616093835.GB2383158@unreal>

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?

>> Whether it's done through query device/ucontext response, both happen for each
>> new context call. With this patch, we gather all needed data in one system call
>> instead of two.
> 
> Is it important in control path to have one call?

Not a huge difference, better one than two though.

>>> What is wrong with calling one query_device before allocating any
>>> ucontext? What are you trying to achieve and what will it give?
>>
>> How can you call query device without allocating a context?
> 
> Forget about my comment above, it was my over-thinking.
> 
> I had in mind some scheme that first ucontext will cache the all device
> related data and share it with other ucontexts.
> 
> Thanks
> 