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From: Dan Williams <dan.j.williams@intel.com>
Date: Tue, 18 Apr 2017 12:48:35 -0700
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Subject: Re: [RFC 0/8] Copy Offload with Peer-to-Peer PCI Memory
To: Logan Gunthorpe <logang@deltatee.com>
Cc: Jason Gunthorpe <jgunthorpe@obsidianresearch.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>,
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        "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 12:35 PM, Logan Gunthorpe <logang@deltatee.com> wrote:
>
>
> On 18/04/17 01:01 PM, Jason Gunthorpe wrote:
>> Ultimately every dma_ops will need special code to support P2P with
>> the special hardware that ops is controlling, so it makes some sense
>> to start by pushing the check down there in the first place. This
>> advice is partially motivated by how dma_map_sg is just a small
>> wrapper around the function pointer call...
>
> Yes, I noticed this problem too and that makes sense. It just means
> every dma_ops will probably need to be modified to either support p2p
> pages or fail on them. Though, the only real difficulty there is that it
> will be a lot of work.

I don't think you need to go touch all dma_ops, I think you can just
arrange for devices that are going to do dma to get redirected to a
p2p aware provider of operations that overrides the system default
dma_ops. I.e. just touch get_dma_ops().