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Date:   Wed, 13 Oct 2021 15:34:45 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Alex Sierra <alex.sierra@....com>, Felix.Kuehling@....com,
        linux-mm@...ck.org, rcampbell@...dia.com,
        linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org,
        amd-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        hch@....de, jglisse@...hat.com, apopple@...dia.com
Subject: Re: [PATCH v1 00/12] MEMORY_DEVICE_COHERENT for CPU-accessible
 coherent device memory

On Tue, Oct 12, 2021 at 03:56:29PM -0300, Jason Gunthorpe wrote:
> On Tue, Oct 12, 2021 at 11:39:57AM -0700, Andrew Morton wrote:
> > On Tue, 12 Oct 2021 12:12:35 -0500 Alex Sierra <alex.sierra@....com> wrote:
> > 
> > > This patch series introduces MEMORY_DEVICE_COHERENT, a type of memory
> > > owned by a device that can be mapped into CPU page tables like
> > > MEMORY_DEVICE_GENERIC and can also be migrated like MEMORY_DEVICE_PRIVATE.
> > > With MEMORY_DEVICE_COHERENT, we isolate the new memory type from other
> > > subsystems as far as possible, though there are some small changes to
> > > other subsystems such as filesystem DAX, to handle the new memory type
> > > appropriately.
> > > 
> > > We use ZONE_DEVICE for this instead of NUMA so that the amdgpu
> > > allocator can manage it without conflicting with core mm for non-unified
> > > memory use cases.
> > > 
> > > How it works: The system BIOS advertises the GPU device memory (aka VRAM)
> > > as SPM (special purpose memory) in the UEFI system address map.
> > > The amdgpu driver registers the memory with devmap as
> > > MEMORY_DEVICE_COHERENT using devm_memremap_pages.
> > > 
> > > The initial user for this hardware page migration capability will be
> > > the Frontier supercomputer project.
> > 
> > To what other uses will this infrastructure be put?
> > 
> > Because I must ask: if this feature is for one single computer which
> > presumably has a custom kernel, why add it to mainline Linux?
> 
> Well, it certainly isn't just "one single computer". Overall I know of
> about, hmm, ~10 *datacenters* worth of installations that are using
> similar technology underpinnings.
> 
> "Frontier" is the code name for a specific installation but as the
> technology is proven out there will be many copies made of that same
> approach.
> 
> The previous program "Summit" was done with NVIDIA GPUs and PowerPC
> CPUs and also included a very similar capability. I think this is a
> good sign that this coherently attached accelerator will continue to
> be a theme in computing going foward. IIRC this was done using out of
> tree kernel patches and NUMA localities.
> 
> Specifically with CXL now being standardized and on a path to ubiquity
> I think we will see an explosion in deployments of coherently attached
> accelerator memory. This is the high end trickling down to wider
> usage.
> 
> I strongly think many CXL accelerators are going to want to manage
> their on-accelerator memory in this way as it makes universal sense to
> want to carefully manage memory access locality to optimize for
> performance.

Yeah with CXL this will be used by a lot more drivers/devices, not
even including nvidia's blob.

I guess if you want make sure get an ack on this from CXL folks, so that
we don't end up with a mess.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch

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