[openib-general] sharing qp between user and kernel

[Pete Wyckoff]

rdreier at cisco.com wrote on Wed, 07 Feb 2007 15:50 -0800:
>     Pete> Before I dig into this anymore, do you expect this to work?
>     Pete> Are there fundamental problems with QP sharing between user
>     Pete> and kernel?  It would sure be nice not to have to stick the
>     Pete> connection management aspects into the kernel.
> 
> No, I wouldn't expect this to work.  At first glance at least, yes,
> there are fundamental problems.  Sharing a QP between user and
> kernelspace, where userspace is doing full kernel bypass (as eg mthca
> does -- there are NO system calls when doing post work request, poll
> CQ and request CQ notification operations), seems like a huge
> problem.  I don't see any way that the kernel can keep a consistent
> view of the QP state unless userspace has to call into the kernel for
> every operation, which would kill performance.

My hope was not to allow full QP sharing between user and kernel,
but just a limited interface to "send this kernel data now".  It
requires that the kernel register some physical memory, and submit
the work requests.  Perhaps the kernel can invoke the equivalent of
the userspace post function instead of trying to use the kernel API
for sending.

Thanks to all for the comments.  We'll keep working with non-bypass
devices to see if the approach offers any advantages first.

		-- Pete