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

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Date:   Fri, 6 Jan 2017 11:26:25 -0700
From:   Jason Gunthorpe <jgunthorpe@...idianresearch.com>
To:     Jerome Glisse <jglisse@...hat.com>
Cc:     Serguei Sagalovitch <serguei.sagalovitch@....com>,
        Jerome Glisse <j.glisse@...il.com>,
        "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>,
        "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 Fri, Jan 06, 2017 at 12:37:22PM -0500, Jerome Glisse wrote:
> On Fri, Jan 06, 2017 at 11:56:30AM -0500, Serguei Sagalovitch wrote:
> > On 2017-01-05 08:58 PM, Jerome Glisse wrote:
> > > On Thu, Jan 05, 2017 at 05:30:34PM -0700, Jason Gunthorpe wrote:
> > > > On Thu, Jan 05, 2017 at 06:23:52PM -0500, Jerome Glisse wrote:
> > > > 
> > > > > > I still don't understand what you driving at - you've said in both
> > > > > > cases a user VMA exists.
> > > > > In the former case no, there is no VMA directly but if you want one than
> > > > > a device can provide one. But such VMA is useless as CPU access is not
> > > > > expected.
> > > > I disagree it is useless, the VMA is going to be necessary to support
> > > > upcoming things like CAPI, you need it to support O_DIRECT from the
> > > > filesystem, DPDK, etc. This is why I am opposed to any model that is
> > > > not VMA based for setting up RDMA - that is shorted sighted and does
> > > > not seem to reflect where the industry is going.
> > > > 
> > > > So focus on having VMA backed by actual physical memory that covers
> > > > your GPU objects and ask how do we wire up the '__user *' to the DMA
> > > > API in the best way so the DMA API still has enough information to
> > > > setup IOMMUs and whatnot.
> > > I am talking about 2 different thing. Existing hardware and API where you
> > > _do not_ have a vma and you do not need one. This is just
> > > > existing stuff.

> > I do not understand why you assume that existing API doesn't  need one.
> > I would say that a lot of __existing__ user level API and their support in
> > kernel (especially outside of graphics domain) assumes that we have vma and
> > deal with __user * pointers.

+1

> Well i am thinking to GPUDirect here. Some of GPUDirect use case do not have
> vma (struct vm_area_struct) associated with them they directly apply to GPU
> object that aren't expose to CPU. Yes some use case have vma for share buffer.

Lets stop talkind about GPU direct. Today we can't even make VMA
pointing at a PCI bar work properly in the kernel - lets start there
please. People can argue over other options once that is done.

> For HMM plan is to restrict to ODP and either to replace ODP with HMM or change
> ODP to not use get_user_pages_remote() but directly fetch informations from
> CPU page table. Everything else stay as it is. I posted patchset to replace
> ODP with HMM in the past.

Make a generic API for all of this and you'd have my vote..

IMHO, you must support basic pinning semantics - that is necessary to
support generic short lived DMA (eg filesystem, etc). That hardware
can clearly do that if it can support ODP.

Jason