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

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Date:   Tue, 22 Nov 2016 12:01:17 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Serguei Sagalovitch <serguei.sagalovitch@....com>
Cc:     "Deucher, Alexander" <Alexander.Deucher@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        "Linux-media@...r.kernel.org" <Linux-media@...r.kernel.org>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "Bridgman, John" <John.Bridgman@....com>,
        "Kuehling, Felix" <Felix.Kuehling@....com>,
        "Blinzer, Paul" <Paul.Blinzer@....com>,
        "Koenig, Christian" <Christian.Koenig@....com>,
        "Suthikulpanit, Suravee" <Suravee.Suthikulpanit@....com>,
        "Sander, Ben" <ben.sander@....com>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: Enabling peer to peer device transactions for PCIe devices

On Tue, Nov 22, 2016 at 10:59 AM, Serguei Sagalovitch
<serguei.sagalovitch@....com> wrote:
> Dan,
>
> I personally like "device-DAX" idea but my concerns are:
>
> -  How well it will co-exists with the  DRM infrastructure / implementations
>    in part dealing with CPU pointers?

Inside the kernel a device-DAX range is "just memory" in the sense
that you can perform pfn_to_page() on it and issue I/O, but the vma is
not migratable. To be honest I do not know how well that co-exists
with drm infrastructure.

> -  How well we will be able to handle case when we need to "move"/"evict"
>    memory/data to the new location so CPU pointer should point to the new
> physical location/address
>     (and may be not in PCI device memory at all)?

So, device-DAX deliberately avoids support for in-kernel migration or
overcommit. Those cases are left to the core mm or drm. The device-dax
interface is for cases where all that is needed is a direct-mapping to
a statically-allocated physical-address range be it persistent memory
or some other special reserved memory range.