linux-kernel - Re: [HMM 03/15] mm/unaddressable-memory: new type of ZONE

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Message-ID: <b94c74b6-14a1-a537-ada2-ee98bdafb255@jonmasters.org>
Date:   Tue, 25 Apr 2017 03:08:43 -0400
From:   Jon Masters <jcm@...masters.org>
To:     John Hubbard <jhubbard@...dia.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Jerome Glisse <jglisse@...hat.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        David Nellans <dnellans@...dia.com>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>
Subject: Re: [HMM 03/15] mm/unaddressable-memory: new type of ZONE_DEVICE for
 unaddressable memory

On 04/23/2017 08:39 PM, John Hubbard wrote:

> Actually, MEMORY_DEVICE_PRIVATE / _PUBLIC seems like a good choice to 
> me, because the memory may not remain CPU-unaddressable in the future. 
> By that, I mean that I know of at least one company (ours) that is 
> working on products that will support hardware-based memory coherence 
> (and access counters to go along with that). If someone were to enable 
> HMM on such a system, then the device memory would be, in fact, directly 
> addressable by a CPU--thus exactly contradicting the "unaddressable" name.

I'm expecting similar with CCIX-like coherently attached accelerators
running within FPGAs and as discrete devices as well. Everyone and their
dog is working on hardware based coherence as a programming convenience
and so the notion of ZONE_DEVICE as it stood is going to rapidly evolve
over the next 18 months, maybe less. Short term anyway.

Jon.