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

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Date:   Thu, 24 Nov 2016 12:00:14 -0500
From:   Serguei Sagalovitch <serguei.sagalovitch@....com>
To:     Jason Gunthorpe <jgunthorpe@...idianresearch.com>,
        Christian König <christian.koenig@....com>
CC:     Dan Williams <dan.j.williams@...el.com>,
        Logan Gunthorpe <logang@...tatee.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>,
        "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 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").