linux-kernel - Re: [PATCH 06/12] IB/core: Add optional PCI P2P flag to rdma_rw_ctx

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Date:   Mon, 8 Jan 2018 11:29:45 -0700
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Logan Gunthorpe <logang@...tatee.com>
Cc:     Christoph Hellwig <hch@....de>, linux-kernel@...r.kernel.org,
        linux-pci@...r.kernel.org, linux-nvme@...ts.infradead.org,
        linux-rdma@...r.kernel.org, linux-nvdimm@...ts.01.org,
        linux-block@...r.kernel.org, Stephen Bates <sbates@...thlin.com>,
        Jens Axboe <axboe@...nel.dk>,
        Keith Busch <keith.busch@...el.com>,
        Sagi Grimberg <sagi@...mberg.me>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Max Gurtovoy <maxg@...lanox.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: Re: [PATCH 06/12] IB/core: Add optional PCI P2P flag to
 rdma_rw_ctx_[init|destroy]()

On Mon, Jan 08, 2018 at 11:17:38AM -0700, Logan Gunthorpe wrote:
 
> >>If at all it should be in the dma_map* wrappers, but for that we'd need
> >>a good identifier.  And it still would not solve the whole fake dma
> >>ops issue.
> >
> >Very long term the IOMMUs under the ops will need to care about this,
> >so the wrapper is not an optimal place to put it - but I wouldn't
> >object if it gets it out of RDMA :)
> 
> Well, creating the extra op doesn't really change anything to the RDMA patch
> in this series.

Not fundamentally, but it lets us solve the bugs the patch introduces
with hfi/etc

> The point is, for the time being, we need to track whether we are
> doing a P2P or normal mapping using a side channel as we don't have
> a good way of tracking it in the SGL at this time.

Well, that is disappointing for now, but I'm OK with the flag in the
RW interface - as long as we all sort of agree it is not desirable and
the SG should self-identify in an ideal someday future world..

Jason