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From: Dan Williams <dan.j.williams@intel.com>
Date: Tue, 18 Apr 2017 14:36:06 -0700
Message-ID: <CAPcyv4g5ifbpukthMXMro8qKdfoXAhftDpiwWWFCLZ4dK8JnnA@mail.gmail.com>
Subject: Re: [RFC 0/8] Copy Offload with Peer-to-Peer PCI Memory
To: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
Cc: Logan Gunthorpe <logang@deltatee.com>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Bjorn Helgaas <helgaas@kernel.org>, Christoph Hellwig <hch@lst.de>,
        Sagi Grimberg <sagi@grimberg.me>,
        "James E.J. Bottomley" <jejb@linux.vnet.ibm.com>,
        "Martin K. Petersen" <martin.petersen@oracle.com>,
        Jens Axboe <axboe@kernel.dk>, Steve Wise <swise@opengridcomputing.com>,
        Stephen Bates <sbates@raithlin.com>, Max Gurtovoy <maxg@mellanox.com>,
        Keith Busch <keith.busch@intel.com>, linux-pci@vger.kernel.org,
        linux-scsi <linux-scsi@vger.kernel.org>,
        linux-nvme@lists.infradead.org, linux-rdma@vger.kernel.org,
        linux-nvdimm <linux-nvdimm@ml01.01.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Jerome Glisse <jglisse@redhat.com>
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On Tue, Apr 18, 2017 at 2:22 PM, Jason Gunthorpe
<jgunthorpe@obsidianresearch.com> wrote:
> On Tue, Apr 18, 2017 at 02:11:33PM -0700, Dan Williams wrote:
>> > I think this opens an even bigger can of worms..
>>
>> No, I don't think it does. You'd only shim when the target page is
>> backed by a device, not host memory, and you can figure this out by a
>> is_zone_device_page()-style lookup.
>
> The bigger can of worms is how do you meaningfully stack dma_ops.

This goes back to my original comment to make this capability a
function of the pci bridge itself. The kernel has an implementation of
a dynamically created bridge device that injects its own dma_ops for
the devices behind the bridge. See vmd_setup_dma_ops() in
drivers/pci/host/vmd.c.

> What does the p2p provider do when it detects a p2p page?

Check to see if the arch requires this offset translation that Ben
brought up and if not provide the physical address as the patches are
doing now.