linux-kernel - Re: Enabling peer to peer device transactions for PCIe devices

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Date:   Wed, 23 Nov 2016 12:12:15 -0700
From:   Jason Gunthorpe <jgunthorpe@...idianresearch.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Bart Van Assche <bart.vanassche@...disk.com>,
        Logan Gunthorpe <logang@...tatee.com>,
        Serguei Sagalovitch <serguei.sagalovitch@....com>,
        "Deucher, Alexander" <Alexander.Deucher@....com>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...1.01.org>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "Kuehling, Felix" <Felix.Kuehling@....com>,
        "Bridgman, John" <John.Bridgman@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "Koenig, Christian" <Christian.Koenig@....com>,
        "Sander, Ben" <ben.sander@....com>,
        "Suthikulpanit, Suravee" <Suravee.Suthikulpanit@....com>,
        "Blinzer, Paul" <Paul.Blinzer@....com>,
        "Linux-media@...r.kernel.org" <Linux-media@...r.kernel.org>
Subject: Re: Enabling peer to peer device transactions for PCIe devices

On Wed, Nov 23, 2016 at 10:40:47AM -0800, Dan Williams wrote:

> I don't think that was designed for the case where the backing memory
> is a special/static physical address range rather than anonymous
> "System RAM", right?

The hardware doesn't care where the memory is. ODP is just a generic
mechanism to provide demand-fault behavior for a mirrored page table.

ODP has the same issue as everything else, it needs to translate a
page table entry into a DMA address, and we have no API to do that
when the page table points to peer-peer memory.

Jason