Re: [PATCH for-next V2 8/9] IB/mlx4: Support extended create_cq and query_device uverbs

[Jason Gunthorpe <jgunthorpe-ePGOBjL8dl3ta4EC/59zMFaTQe2KTcn/@public.gmane.org>]

On Wed, Jun 03, 2015 at 09:58:25PM +0300, Or Gerlitz wrote:
> On Wed, Jun 3, 2015 at 7:31 PM, Jason Gunthorpe
> <jgunthorpe-ePGOBjL8dl3ta4EC/59zMFaTQe2KTcn/@public.gmane.org> wrote:
> > On Wed, Jun 03, 2015 at 11:57:12AM +0300, Matan Barak wrote:
> >> That's a general comment regarding the extension mechanism.
> >
> > Yes, but it is also a specific comment about patch #4 which adds,
> > ib_uverbs_ex_create_cq.
> >
> > Based on the implementation of create_cq, it is pretty clear that
> > every driver supports ib_uverbs_ex_create_cq, so patch #4 should just
> > force the flag in the device register function so it is globally enabled.
> 
> But the other drivers currently do not support any CQ creation flag
> and hence no extended functionality, I don't see the point signaling
> towards user-space that the verb is supported, please elaborate.

They support the base functionality, the flags = 0 case.

There is no reason to block access to the base functionality via the
extended api. That just creates hassles for userspace.

If userspace detects the extended API is present, it can just
switch unconditionally all usage to that API. This is how most new
kernel syscalls are introduced (glibc does this transparently).

Detecting what flags a driver supports (if any) is any entirely
different and orthogonal issue to introducing comp_mask/etc.

Jason
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