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Subject: Re: [HMM-CDM 0/5] Cache coherent device memory (CDM) with HMM
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: Dave Hansen <dave.hansen@intel.com>,
        Jerome Glisse <jglisse@redhat.com>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        John Hubbard <jhubbard@nvidia.com>,
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        Dan Williams <dan.j.williams@intel.com>,
        Ross Zwisler <ross.zwisler@linux.intel.com>,
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        Vladimir Davydov <vdavydov.dev@gmail.com>,
        Balbir Singh <balbirs@au1.ibm.com>,
        Aneesh Kumar <aneesh.kumar@linux.vnet.ibm.com>,
        "Paul E . McKenney" <paulmck@linux.vnet.ibm.com>
Date: Thu, 15 Jun 2017 08:07:56 +1000
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On Wed, 2017-06-14 at 14:58 -0700, Dave Hansen wrote:
> > > http://www.uefi.org/sites/default/files/resources/ACPI_6_2.pdf
> > 
> > It doesn't interact with that. HMM-CDM is a set of helper that don't
> > do anything unless instructed so. So for device memory to be presented
> > as HMM-CDM you need to hotplug it as ZONE_DEVICE(DEVICE_PUBLIC) which
> > can be done with the helper introduced in patch 2 of this patchset.
> 
> I guess I'm asking whether we *should* instruct HMM-CDM to manage all
> coherent device memory.  If not, where do we draw the line for what we
> use HMM-CDM, and for what we use the core MM?

Well, if you want the features of HMM ... It basically boils down to
whether you have some kind of coherent processing unit close to that
memory and want to manage transparent migration of pages between system
and device memory, that sort of thing.

Cheers,
Ben.