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

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Date:   Wed, 23 Nov 2016 14:42:12 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Jason Gunthorpe <jgunthorpe@...idianresearch.com>
Cc:     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>,
        Haggai Eran <haggaie@...lanox.com>
Subject: Re: Enabling peer to peer device transactions for PCIe devices

On Wed, Nov 23, 2016 at 1:55 PM, Jason Gunthorpe
<jgunthorpe@...idianresearch.com> wrote:
> On Wed, Nov 23, 2016 at 02:11:29PM -0700, Logan Gunthorpe wrote:
>> > As I said, there is no possible special handling. Standard IB hardware
>> > does not support changing the DMA address once a MR is created. Forget
>> > about doing that.
>>
>> Yeah, that's essentially the point I was trying to make. Not to mention
>> all the other unrelated hardware that can't DMA to an address that might
>> disappear mid-transfer.
>
> Right, it is impossible to ask for generic page migration with ongoing
> DMA. That is simply not supported by any of the hardware at all.
>
>> > Only ODP hardware allows changing the DMA address on the fly, and it
>> > works at the page table level. We do not need special handling for
>> > RDMA.
>>
>> I am aware of ODP but, noted by others, it doesn't provide a general
>> solution to the points above.
>
> How do you mean?
>
> Perhaps I am not following what Serguei is asking for, but I
> understood the desire was for a complex GPU allocator that could
> migrate pages between GPU and CPU memory under control of the GPU
> driver, among other things. The desire is for DMA to continue to work
> even after these migrations happen.
>
> Page table mirroring *is* the general solution for this problem. The
> GPU driver controls the VMA and the DMA driver mirrors that VMA.
>
> Do you know of another option that doesn't just degenerate to page
> table mirroring??
>
> Remember, there are two facets to the RDMA ODP implementation, I feel
> there is some confusion here..
>
> The crucial part for this discussion is the ability to fence and block
> DMA for a specific range. This is the hardware capability that lets
> page migration happen: fence&block DMA, migrate page, update page
> table in HCA, unblock DMA.

Wait, ODP requires migratable pages, ZONE_DEVICE pages are not
migratable. You can't replace a PCIe mapping with just any other
System RAM physical address, right? At least not without a filesystem
recording where things went, but at point we're no longer talking
about the base P2P-DMA mapping mechanism and are instead talking about
something like pnfs-rdma to a DAX filesystem.