Re: [PATCH V3 1/5] RDMA/core: Transport-independent access flags

Jason Gunthorpe
2015-07-13T20:15:38Z

On Mon, Jul 13, 2015 at 03:36:44PM -0400, Tom Talpey wrote:
> On 7/11/2015 6:25 AM, 'Christoph Hellwig' wrote:
> >I think what we need to support for now is FRMR as the primary target,
> >and FMR as a secondar[y].
>
> FMR is a *very* bad choice, for several reasons.

If an API can transparently support FMR, then I think it can also
transparently support ib_get_phys_mr as an alternative, they look
pretty similar... ?

> Personally, I'd recommend ib_get_phys_mr() over FMR. It at least
> doesn't suffer from issues 1, 2 and 4.

Your comments are right for the rkey case, but for lkey, there is no
security concern with using a FMR, or pooling them. It doesn't look
like any iwarp drivers supports FMR, so they are certainly safe to use
on IB as the lkey.

This is why I am becoming more convinced that treating lkey
and rkey the same is not helpful.

Conversely, it looks like if we could drop ehca and mthca we could
ditch FMR entirely..

Jason