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Date:   Sun, 20 Jun 2021 10:14:54 -0400
From:   "Theodore Ts'o" <tytso@....edu>
To:     Alex Sierra <alex.sierra@....com>
Cc:     akpm@...ux-foundation.org, 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, jgg@...dia.com, jglisse@...hat.com
Subject: Re: [PATCH v3 0/8] Support DEVICE_GENERIC memory in migrate_vma_*

On Thu, Jun 17, 2021 at 10:16:57AM -0500, Alex Sierra wrote:
> v1:
> AMD is building a system architecture for the Frontier supercomputer with a
> coherent interconnect between CPUs and GPUs. This hardware architecture allows
> the CPUs to coherently access GPU device memory. We have hardware in our labs
> and we are working with our partner HPE on the BIOS, firmware and software
> for delivery to the DOE.
> 
> The system BIOS advertises the GPU device memory (aka VRAM) as SPM
> (special purpose memory) in the UEFI system address map. The amdgpu driver looks
> it up with lookup_resource and registers it with devmap as MEMORY_DEVICE_GENERIC
> using devm_memremap_pages.
> 
> Now we're trying to migrate data to and from that memory using the migrate_vma_*
> helpers so we can support page-based migration in our unified memory allocations,
> while also supporting CPU access to those pages.
> 
> This patch series makes a few changes to make MEMORY_DEVICE_GENERIC pages behave
> correctly in the migrate_vma_* helpers. We are looking for feedback about this
> approach. If we're close, what's needed to make our patches acceptable upstream?
> If we're not close, any suggestions how else to achieve what we are trying to do
> (i.e. page migration and coherent CPU access to VRAM)?

Is there a way we can test the codepaths touched by this patchset?  It
doesn't have to be via a complete qemu simulation of the GPU device
memory, but some way of creating MEMORY_DEVICE_GENERIC subject to
migrate_vma_* helpers so we can test for regressions moving forward.

Thanks,

					- Ted

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