Re: Enabling peer to peer device transactions for PCIe devices

From: Serguei Sagalovitch
Date: Tue Nov 22 2016 - 15:35:30 EST

On 2016-11-22 03:10 PM, Daniel Vetter wrote:
On Tue, Nov 22, 2016 at 9:01 PM, Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
On Tue, Nov 22, 2016 at 10:59 AM, Serguei Sagalovitch
<serguei.sagalovitch@xxxxxxx> wrote:
I personally like "device-DAX" idea but my concerns are:

- How well it will co-exists with the DRM infrastructure / implementations
in part dealing with CPU pointers?
Inside the kernel a device-DAX range is "just memory" in the sense
that you can perform pfn_to_page() on it and issue I/O, but the vma is
not migratable. To be honest I do not know how well that co-exists
with drm infrastructure.

- How well we will be able to handle case when we need to "move"/"evict"
memory/data to the new location so CPU pointer should point to the new
physical location/address
(and may be not in PCI device memory at all)?
So, device-DAX deliberately avoids support for in-kernel migration or
overcommit. Those cases are left to the core mm or drm. The device-dax
interface is for cases where all that is needed is a direct-mapping to
a statically-allocated physical-address range be it persistent memory
or some other special reserved memory range.
For some of the fancy use-cases (e.g. to be comparable to what HMM can
pull off) I think we want all the magic in core mm, i.e. migration and
overcommit. At least that seems to be the very strong drive in all
general-purpose gpu abstractions and implementations, where memory is
allocated with malloc, and then mapped/moved into vram/gpu address
space through some magic,
It is possible that there is other way around: memory is requested to be
allocated and should be kept in vram for performance reason but due
to possible overcommit case we need at least temporally to "move" such
allocation to system memory.
but still visible on both the cpu and gpu
side in some form. Special device to allocate memory, and not being
able to migrate stuff around sound like misfeatures from that pov.