RE: [PATCH v3 05/15] xprtrdma: Remove last ib_reg_phys_mr() call site

["Steve Wise" <swise@opengridcomputing.com>]

> -----Original Message-----
> From: linux-rdma-owner@vger.kernel.org [mailto:linux-rdma-owner@vger.kernel.org] On Behalf Of Tom Talpey
> Sent: Monday, July 20, 2015 7:15 PM
> To: Steve Wise; 'Jason Gunthorpe'
> Cc: 'Chuck Lever'; linux-rdma@vger.kernel.org; linux-nfs@vger.kernel.org
> Subject: Re: [PATCH v3 05/15] xprtrdma: Remove last ib_reg_phys_mr() call site
> 
> On 7/20/2015 3:41 PM, Steve Wise wrote:
> >
> >
> >> -----Original Message-----
> >> From: Tom Talpey [mailto:tom@talpey.com]
> >> Sent: Monday, July 20, 2015 5:04 PM
> >> To: Steve Wise; 'Jason Gunthorpe'
> >> Cc: 'Chuck Lever'; linux-rdma@vger.kernel.org; linux-nfs@vger.kernel.org
> >> Subject: Re: [PATCH v3 05/15] xprtrdma: Remove last ib_reg_phys_mr() call site
> >>
> >> On 7/20/2015 2:16 PM, Steve Wise wrote:
> >>>
> >>>
> >>>> -----Original Message-----
> >>>> From: linux-nfs-owner@vger.kernel.org [mailto:linux-nfs-owner@vger.kernel.org] On Behalf Of Jason Gunthorpe
> >>>> Sent: Monday, July 20, 2015 4:06 PM
> >>>> To: Tom Talpey; Steve Wise
> >>>> Cc: Chuck Lever; linux-rdma@vger.kernel.org; linux-nfs@vger.kernel.org
> >>>> Subject: Re: [PATCH v3 05/15] xprtrdma: Remove last ib_reg_phys_mr() call site
> >>>>
> >>>> On Mon, Jul 20, 2015 at 01:34:16PM -0700, Tom Talpey wrote:
> >>>>> On 7/20/2015 12:03 PM, Chuck Lever wrote:
> >>>>>> All HCA providers have an ib_get_dma_mr() verb. Thus
> >>>>>> rpcrdma_ia_open() will either grab the device's local_dma_key if one
> >>>>>> is available, or it will call ib_get_dma_mr() which is a 100%
> >>>>>> guaranteed fallback.
> >>>>>
> >>>>> I recall that in the past, some providers did not support mapping
> >>>>> all of the machine's potential physical memory with a single dma_mr.
> >>>>> If an rnic did/does not support 44-ish bits of length per region,
> >>>>> for example.
> >>>>
> >>>> Looks like you are right, but the standard in kernel is to require
> >>>> ib_get_dma_mr, if the HCA can't do that, then it cannot be used on a
> >>>> big memory machine with kernel ULPs.
> >>>>
> >>>> Looking deeper, both amso1100 and cxgb3 seem limited to 32 bits of
> >>>> physical memory, and silently break all kernel ULPs if they are used
> >>>> on a modern machine with > 4G.
> >>>>
> >>>> Is that right Steve?
> >>>>
> >>>
> >>> Yes.
> >>>
> >>>> Based on that, should we remove the cxgb3 driver as well? Or at least
> >>>> can you fix it up to at least fail get_dma_mr if there is too much
> >>>> ram?
> >>>>
> >>>
> >>> I would like to keep cxgb3 around.  I can add code to fail if the memory is > 32b.  Do you know how I get the amount of
> > available
> >>> ram?
> >>
> >> A) are you sure it's an unsigned length, i.e. is it really 31 bits?
> >>
> >
> > yes.
> >
> >> B) why bother to check? Are machines with <4GB interesting, and worth
> >> supporting a special optimization?
> >
> > No, but cxgb3 is still interesting to user applications, and perhaps NFSRDMA using FRMRs.
> 
> I'm obviously not making myself clear. I am suggesting that cxgb3 fail
> the ib_get_dma_mr() verb, regardless of installed memory.
> 
> I am not suggesting it fail to load, or fail other memreg requests. It
> should work normally in all other respects.

Even with its limitation, doesn't it have utility for someone using cxgb3 in an embedded 32b environment?