Re: [PATCH for-next V2 0/9] Add completion timestamping support

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

On Thu, Jun 04, 2015 at 04:17:35PM +0000, Hefty, Sean wrote:
> > No, if they followed (I didn't check yes) the extension scheme then the
> > poll call is
> > 
> >  struct ibv_wc_ex wcs[num_wcs]
> >  ibv_poll_wc_ex(&wcs,num_wcs,sizeof(wcs[0]));
> > 
> > And the drivers decide what to do based on the 3rd argument, which is
> > essentially the ABI version.
> 
> As a couple of options:
> 
> You could add a 'completion format' field to the cq attribute
> structure, which would allow poll to cast to a specific structure.

Subtle, and tricky to use.. But sure..

> If we wanted to go with Doug's earlier idea, you could add a
> 'provider context' field to the wc.  This would allow calling back
> into the provider to retrieve more data.  Though, this concept would
> be racy if other completions can overwrite whatever data was trying
> to be retrieved.

Right, in the model we have where poll advances the HW completion
queue, that is not straightforward at all.

Can you share what the libfabric change would look like to add a
timestamp field to the completion? That might be informative,
libfabric is also consuming the kernel UAPI.

Jason
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