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

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Date:   Fri, 25 Nov 2016 09:45:27 -0700
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Christian König <christian.koenig@....com>,
        Jason Gunthorpe <jgunthorpe@...idianresearch.com>,
        Dan Williams <dan.j.williams@...el.com>
Cc:     Serguei Sagalovitch <serguei.sagalovitch@....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 25/11/16 06:06 AM, Christian König wrote:
> Well Serguei send me a couple of documents about QPI when we started to
> discuss this internally as well and that's exactly one of the cases I
> had in mind when writing this.
> 
> If I understood it correctly for such systems P2P is technical possible,
> but not necessary a good idea. Usually it is faster to just use a
> bouncing buffer when the peers are a bit "father" apart.
> 
> That this problem is solved on newer hardware is good, but doesn't helps
> us at all if we at want to support at least systems from the last five
> years or so.

Well we have been testing with Sandy Bridge, I think the problem was
supposed to be fixed in Ivy but we never tested it so I can't say what
the performance turned out to be. Ivy is nearly 5 years old. I expect
this is something that will be improved more and more with subsequent
generations.

A white list may end up being rather complicated if it has to cover
different CPU generations and system architectures. I feel this is a
decision user space could easily make.

Logan