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

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Message-ID: <20190129224016.GD4713@mellanox.com>
Date:   Tue, 29 Jan 2019 23:02:25 +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 Tue, Jan 29, 2019 at 03:44:00PM -0500, Jerome Glisse wrote:

> > But this API doesn't seem to offer any control - I thought that
> > control was all coming from the mm/hmm notifiers triggering p2p_unmaps?
> 
> The control is within the driver implementation of those callbacks. 

Seems like what you mean by control is 'the exporter gets to choose
the physical address at the instant of map' - which seems reasonable
for GPU.


> will only allow p2p map to succeed for objects that have been tagged by the
> userspace in some way ie the userspace application is in control of what
> can be map to peer device.

I would have thought this means the VMA for the object is created
without the map/unmap ops? Or are GPU objects and VMAs unrelated?

> For moving things around after a successful p2p_map yes the exporting
> device have to call for instance zap_vma_ptes() or something
> similar.

Okay, great, RDMA needs this flow for hotplug - we zap the VMA's when
unplugging the PCI device and we can delay the PCI unplug completion
until all the p2p_unmaps are called...

But in this case a future p2p_map will have to fail as the BAR no
longer exists. How to handle this?

> > I would think that the importing driver can assume the BAR page is
> > kept alive until it calls unmap (presumably triggered by notifiers)?
> > 
> > ie the exporting driver sees the BAR page as pinned until unmap.
> 
> The intention with this patchset is that it is not pin ie the importer
> device _must_ abide by all mmu notifier invalidations and they can
> happen at anytime. The importing device can however re-p2p_map the
> same range after an invalidation.
>
> I would like to restrict this to importer that can invalidate for
> now because i believe all the first device to use can support the
> invalidation.

This seems reasonable (and sort of says importers not getting this
from HMM need careful checking), was this in the comment above the
ops?

Jason