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

[Doug Ledford <dledford-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

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On Mon, 2015-06-01 at 10:43 -0600, Jason Gunthorpe wrote:
> On Mon, Jun 01, 2015 at 07:25:04AM -0400, Doug Ledford wrote:
> 
> > attempted abstraction of ibverbs.  Passing in the wc struct allows the
> > driver to internally allocate a wc struct with extra private elements
> > and pass that back to the user, when the user passes it back to
> > ibv_get_timestamp the elements are there in the private portion of the
> > struct.
> 
> wc structures are allocated by the caller, there is no option for the
> driver to create private elements.

You're right, the data would have to be housed somewhere in the driver
private completion structs (for example, in the CQE the card posts to
memory that the driver then massages into a WC).

> AFAIK, Christoph's use case is essentially the only meaningful use
> case for this feature, generalizing too much may destroy the
> performance that is valuable here.

I'm not convinced of that.  Steve has already spoke up about the
timestamps available in cxgb4.  Those are very different and yet still
highly valuable to someone investigating performance of their RDMA
application.

-- 
Doug Ledford <dledford-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
              GPG KeyID: 0E572FDD


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