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    SubjectRe: Enabling peer to peer device transactions for PCIe devices
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    On 2016-11-24 11:26 AM, Jason Gunthorpe wrote:
    > On Thu, Nov 24, 2016 at 10:45:18AM +0100, Christian König wrote:
    >> Am 24.11.2016 um 00:25 schrieb Jason Gunthorpe:
    >>> There is certainly nothing about the hardware that cares
    >>> about ZONE_DEVICE vs System memory.
    >> Well that is clearly not so simple. When your ZONE_DEVICE pages describe a
    >> PCI BAR and another PCI device initiates a DMA to this address the DMA
    >> subsystem must be able to check if the interconnection really works.
    > I said the hardware doesn't care.. You are right, we still have an
    > outstanding problem in Linux of how to generically DMA map a P2P
    > address - which is a different issue from getting the P2P address from
    > a __user pointer...
    >
    > Jason
    I agreed but the problem is that one issue immediately introduce another
    one
    to solve and so on (if we do not want to cut corners). I would think that
    a lot of them interconnected because the way how one problem could be
    solved may impact solution for another.

    btw: about "DMA map a p2p address": Right now to enable p2p between
    devices
    it is required/recommended to disable iommu support (e.g. intel iommu
    driver
    has special logic for graphics and comment "Reserve all PCI MMIO to avoid
    peer-to-peer access").

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