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

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Date:   Wed, 30 Nov 2016 12:28:24 -0500
From:   Serguei Sagalovitch <serguei.sagalovitch@....com>
To:     Jason Gunthorpe <jgunthorpe@...idianresearch.com>,
        Haggai Eran <haggaie@...lanox.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        "linux-nvdimm@...1.01.org" <linux-nvdimm@...1.01.org>,
        "christian.koenig@....com" <christian.koenig@....com>,
        "Suravee.Suthikulpanit@....com" <Suravee.Suthikulpanit@....com>,
        "John.Bridgman@....com" <John.Bridgman@....com>,
        "Alexander.Deucher@....com" <Alexander.Deucher@....com>,
        "Linux-media@...r.kernel.org" <Linux-media@...r.kernel.org>,
        "dan.j.williams@...el.com" <dan.j.williams@...el.com>,
        "logang@...tatee.com" <logang@...tatee.com>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        Max Gurtovoy <maxg@...lanox.com>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "Paul.Blinzer@....com" <Paul.Blinzer@....com>,
        "Felix.Kuehling@....com" <Felix.Kuehling@....com>,
        "ben.sander@....com" <ben.sander@....com>
Subject: Re: Enabling peer to peer device transactions for PCIe devices

On 2016-11-30 11:23 AM, Jason Gunthorpe wrote:
>> Yes, that sounds fine. Can we simply kill the process from the GPU driver?
>> Or do we need to extend the OOM killer to manage GPU pages?
> I don't know..
We could use send_sig_info to send signal from  kernel  to user space. 
So theoretically GPU driver
could issue KILL signal to some process.

> On Wed, Nov 30, 2016 at 12:45:58PM +0200, Haggai Eran wrote:
>> I think we can achieve the kernel's needs with ZONE_DEVICE and DMA-API support
>> for peer to peer. I'm not sure we need vmap. We need a way to have a scatterlist
>> of MMIO pfns, and ZONE_DEVICE allows that.
I do not think that using DMA-API as it is is the best solution (at 
least in the current form):

-  It deals with handles/fd for the whole allocation but client 
could/will use sub-allocation as
well as theoretically possible to "merge" several allocations in one 
from GPU perspective.
-  It require knowledge to export but because "sharing" is controlled 
from user space it
means that we must "export" all allocation by default
- It deals with 'fd'/handles but user application may work with 
addresses/pointers.

Also current  DMA-API force each time to do all DMA table programming 
unrelated if
location was changed or not. With  vma / mmu  we are  able to install 
notifier to intercept
changes in location and update  translation tables only as needed (we do 
not need to keep
get_user_pages()  lock).