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

[Leon Romanovsky <leon@kernel.org>]

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?

I would like to hear it too. Almost all (if not all) extended
verbs support providing vendor specific data in a response. Why
should query_device() be different here?

>
> >> 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.

My problem here is duplication of functionality and data.

>
> >>> 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
> >