Re: Interacting with coherent memory on external devices

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Tue, 2015-04-21 at 19:46 -0400, Jerome Glisse wrote:
> On Tue, Apr 21, 2015 at 02:44:45PM -0700, Paul E. McKenney wrote:
> > Hello!
> > 
> > We have some interest in hardware on devices that is cache-coherent
> > with main memory, and in migrating memory between host memory and
> > device memory.  We believe that we might not be the only ones looking
> > ahead to hardware like this, so please see below for a draft of some
> > approaches that we have been thinking of.
> > 
> > Thoughts?
> 
> I have posted several time a patchset just for doing that, i am sure
> Ben did see it. Search for HMM. I am about to repost it in next couple
> weeks.

Actually no :-) This is not at all HMM realm.

HMM deals with non-cachable (MMIO) device memory that isn't represented
by struct page and separate MMUs that allow pages to be selectively
unmapped from CPU vs. device.

This proposal is about a very different type of device where the device
memory is fully cachable from a CPU standpoint, and thus can be
represented by struct page, and the device has an MMU that is completely
shared with the CPU, ie, the device operates within a given context of
the system and if a page is marked read-only or inaccessible, this will
be true on both the CPU and the device.

Note: IBM is also interested in HMM for devices that don't qualify with
the above such as some GPUs or NICs, but this is something *else*.

Cheers,
Ben.