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

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Date:   Tue, 29 Jan 2019 13:39:49 -0700
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Jason Gunthorpe <jgg@...lanox.com>,
        Jerome Glisse <jglisse@...hat.com>
Cc:     "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 2019-01-29 12:32 p.m., Jason Gunthorpe wrote:
> Jerome, I think it would be nice to have a helper scheme - I think the
> simple case would be simple remapping of PCI BAR memory, so if we
> could have, say something like:
> 
> static const struct vm_operations_struct my_ops {
>   .p2p_map = p2p_ioremap_map_op,
>   .p2p_unmap = p2p_ioremap_unmap_op,
> }
> 
> struct ioremap_data {
>   [..]
> }
> 
> fops_mmap() {
>    vma->private_data = &driver_priv->ioremap_data;
>    return p2p_ioremap_device_memory(vma, exporting_device, [..]);
> }

This is roughly what I was expecting, except I don't see exactly what
the p2p_map and p2p_unmap callbacks are for. The importing driver should
see p2pdma/hmm struct pages and use the appropriate function to map
them. It shouldn't be the responsibility of the exporting driver to
implement the mapping. And I don't think we should have 'special' vma's
for this (though we may need something to ensure we don't get mapping
requests mixed with different types of pages...).

I also figured there'd be a fault version of p2p_ioremap_device_memory()
for when you are mapping P2P memory and you want to assign the pages
lazily. Though, this can come later when someone wants to implement that.

Logan