linux-kernel - Re: [RFC 0/8] Copy Offload with Peer-to-Peer PCI Memory

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Message-ID: <CAPcyv4itqGu+YekHcJ+_kyDSDch43yRSqK8NnGUh2DYYMd=F1Q@mail.gmail.com>
Date:   Tue, 18 Apr 2017 15:52:08 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:     Jason Gunthorpe <jgunthorpe@...idianresearch.com>,
        Logan Gunthorpe <logang@...tatee.com>,
        Bjorn Helgaas <helgaas@...nel.org>,
        Christoph Hellwig <hch@....de>,
        Sagi Grimberg <sagi@...mberg.me>,
        "James E.J. Bottomley" <jejb@...ux.vnet.ibm.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        Jens Axboe <axboe@...nel.dk>,
        Steve Wise <swise@...ngridcomputing.com>,
        Stephen Bates <sbates@...thlin.com>,
        Max Gurtovoy <maxg@...lanox.com>,
        Keith Busch <keith.busch@...el.com>, linux-pci@...r.kernel.org,
        linux-scsi <linux-scsi@...r.kernel.org>,
        linux-nvme@...ts.infradead.org, linux-rdma@...r.kernel.org,
        linux-nvdimm <linux-nvdimm@...1.01.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Jerome Glisse <jglisse@...hat.com>
Subject: Re: [RFC 0/8] Copy Offload with Peer-to-Peer PCI Memory

On Tue, Apr 18, 2017 at 3:46 PM, Benjamin Herrenschmidt
<benh@...nel.crashing.org> wrote:
> On Tue, 2017-04-18 at 10:27 -0700, Dan Williams wrote:
>> > FWIW, RDMA probably wouldn't want to use a p2mem device either, we
>> > already have APIs that map BAR memory to user space, and would like to
>> > keep using them. A 'enable P2P for bar' helper function sounds better
>> > to me.
>>
>> ...and I think it's not a helper function as much as asking the bus
>> provider "can these two device dma to each other". The "helper" is the
>> dma api redirecting through a software-iommu that handles bus address
>> translation differently than it would handle host memory dma mapping.
>
> Do we even need tat function ? The dma_ops have a dma_supported()
> call...
>
> If we have those override ops built into the "dma_target" object,
> then these things can make that decision knowing both the source
> and target device.
>

Yes.