Re: [RFC summary] Enable Coherent Device Memory

[Christoph Lameter <cl@linux.com>]

On Wed, 17 May 2017, Benjamin Herrenschmidt wrote:

> On Tue, 2017-05-16 at 09:43 +0100, Mel Gorman wrote:
> > I'm not sure what you're asking here. migration is only partially
> > transparent but a move_pages call will be necessary to force pages onto
> > CDM if binding policies are not used so the cost of migration will be
> > invisible. Even if you made it "transparent", the migration cost would
> > be incurred at fault time. If anything, using move_pages would be more
> > predictable as you control when the cost is incurred.
>
> One of the main point of this whole exercise is for applications to not
> have to bother with any of this and now you are bringing all back into
> their lap.

You can provide a library that does it?

> The base idea behind the counters we have on the link is for the HW to
> know when memory is accessed "remotely", so that the device driver can
> make decision about migrating pages into or away from the device,
> especially so that applications don't have to concern themselves with
> memory placement.

Library can enquire about the current placement of the pages and move them
if necessary?


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