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

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Date:   Wed, 30 Jan 2019 19:38:05 +0000
From:   Jason Gunthorpe <jgg@...lanox.com>
To:     Jerome Glisse <jglisse@...hat.com>
CC:     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>,
        Christoph Hellwig <hch@....de>,
        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 Wed, Jan 30, 2019 at 02:22:34PM -0500, Jerome Glisse wrote:

> For GPU it would not work, GPU might want to use main memory (because
> it is running out of BAR space) it is a lot easier if the p2p_map
> callback calls the right dma map function (for page or io) rather than
> having to define some format that would pass down the information.

This is already sort of built into the sgl, you are supposed to use
is_pci_p2pdma_page() and pci_p2pdma_map_sg() and somehow it is supposed
to work out - but I think this is also fairly incomplete.

ie the current APIs seem to assume the SGL is homogeneous :(

> > Worry about optimizing away the struct page overhead later?
> 
> Struct page do not fit well for GPU as the BAR address can be reprogram
> to point to any page inside the device memory (think 256M BAR versus
> 16GB device memory).

The struct page only points to the BAR - it is not related to the
actual GPU memory in any way. The struct page is just an alternative
way to specify the physical address of the BAR page.

I think this boils down to one call to setup the entire BAR, like nvme
does, and then using the struct page in the p2p_map SGL??

Jason