linux-kernel - Re: [RFC PATCH 3/5] mm/vma: add support for peer to peer to device vma

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Date:   Thu, 31 Jan 2019 19:58:05 +0000
From:   Jason Gunthorpe <jgg@...lanox.com>
To:     Jerome Glisse <jglisse@...hat.com>
CC:     Christoph Hellwig <hch@....de>,
        Logan Gunthorpe <logang@...tatee.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Christian Koenig <christian.koenig@....com>,
        Felix Kuehling <Felix.Kuehling@....com>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>,
        Joerg Roedel <jroedel@...e.de>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>
Subject: Re: [RFC PATCH 3/5] mm/vma: add support for peer to peer to device
 vma

On Thu, Jan 31, 2019 at 02:35:14PM -0500, Jerome Glisse wrote:

> > Basically invert the API flow - the DMA map would be done close to
> > GUP, not buried in the driver. This absolutely doesn't work for every
> > flow we have, but it does enable the ones that people seem to care
> > about when talking about P2P.
> 
> This does not work for GPU really i do not want to have to rewrite GPU
> driver for this. Struct page is a burden and it does not bring anything
> to the table. I rather provide an all in one stop for driver to use
> this without having to worry between regular vma and special vma.

I'm talking about almost exactly what you've done in here - make a
'sgl' that is dma addresses only. 

In these VMA patches you used a simple array of physical addreses -
I'm only talking about moving that array into a 'dma sgl'.

The flow is still basically the same - the driver directly gets DMA
physical addresses with no possibility to get a struct page or CPU
memory.

And then we can build more stuff around the 'dma sgl', including
the in-kernel users Logan is worrying about.

Jason