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

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Message-ID: <20170105190113.GA12587@obsidianresearch.com>
Date:   Thu, 5 Jan 2017 12:01:13 -0700
From:   Jason Gunthorpe <jgunthorpe@...idianresearch.com>
To:     Jerome Glisse <j.glisse@...il.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@...1.01.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>,
        "Kuehling, Felix" <Felix.Kuehling@....com>,
        "Sagalovitch, Serguei" <Serguei.Sagalovitch@....com>,
        "Blinzer, Paul" <Paul.Blinzer@....com>,
        "Koenig, Christian" <Christian.Koenig@....com>,
        "Suthikulpanit, Suravee" <Suravee.Suthikulpanit@....com>,
        "Sander, Ben" <ben.sander@....com>, hch@...radead.org,
        david1.zhou@....com, qiang.yu@....com
Subject: Re: Enabling peer to peer device transactions for PCIe devices

On Thu, Jan 05, 2017 at 01:39:29PM -0500, Jerome Glisse wrote:

>   1) peer-to-peer because of userspace specific API like NVidia GPU
>     direct (AMD is pushing its own similar API i just can't remember
>     marketing name). This does not happen through a vma, this happens
>     through specific device driver call going through device specific
>     ioctl on both side (GPU and RDMA). So both kernel driver are aware
>     of each others.

Today you can only do user-initiated RDMA operations in conjection
with a VMA.

We'd need a really big and strong reason to create an entirely new
non-VMA based memory handle scheme for RDMA.

So my inclination is to just completely push back on this idea. You
need a VMA to do RMA.

GPUs need to create VMAs for the memory they want to RDMA from, even
if the VMA handle just causes SIGBUS for any CPU access.

Jason