linux-kernel - Re: Interacting with coherent memory on external devices

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Date:	Wed, 22 Apr 2015 10:36:09 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Jerome Glisse <j.glisse@...il.com>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	jglisse@...hat.com, mgorman@...e.de, aarcange@...hat.com,
	riel@...hat.com, airlied@...hat.com,
	aneesh.kumar@...ux.vnet.ibm.com,
	Cameron Buschardt <cabuschardt@...dia.com>,
	Mark Hairgrove <mhairgrove@...dia.com>,
	Geoffrey Gerfin <ggerfin@...dia.com>,
	John McKenna <jmckenna@...dia.com>, akpm@...ux-foundation.org
Subject: Re: Interacting with coherent memory on external devices

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.

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