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

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Date:   Tue, 18 Apr 2017 14:11:33 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Jason Gunthorpe <jgunthorpe@...idianresearch.com>
Cc:     Logan Gunthorpe <logang@...tatee.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        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 2:03 PM, Jason Gunthorpe
<jgunthorpe@...idianresearch.com> wrote:
> On Tue, Apr 18, 2017 at 12:48:35PM -0700, Dan Williams wrote:
>
>> > 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().
>
> I don't follow, when does get_dma_ops() return a p2p aware provider?
> It has no way to know if the DMA is going to involve p2p, get_dma_ops
> is called with the device initiating the DMA.
>
> So you'd always return the P2P shim on a system that has registered
> P2P memory?
>
> Even so, how does this shim work? dma_ops are not really intended to
> be stacked. How would we make unmap work, for instance? What happens
> when the underlying iommu dma ops actually natively understands p2p
> and doesn't want the shim?
>
> 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.