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Date:   Wed, 12 Jul 2023 10:01:44 -0300
From:   Jason Gunthorpe <jgg@ziepe.ca>
To:     Mina Almasry <almasrymina@google.com>
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Subject: Re: Memory providers multiplexing (Was: [PATCH net-next v4 4/5]
 page_pool: remove PP_FLAG_PAGE_FRAG flag)
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On Tue, Jul 11, 2023 at 08:42:24PM -0700, Mina Almasry wrote:

> 1. The device memory driver would be the p2pdma provider. It would
> expose a user API which allocates a device memory region, calls
> pci_p2pdma_add_resource() and pci_p2pmem_publish() on it, and returns
> a reference to it to the userspace.

This is not quite right, if you convert any of the GPU drivers to use
P2PDMA you are going to need to restructure the p2pmem stuff to
seperate the genalloc. The GPU driver must continue to be the owner
and allocator of the MMIO memory it already controls, we can't have
two allocators working in parallel. 

The genalloc stuff supports the special NVMe use case, I don't know of
anything else that would like to work that way.

> 2. The NIC driver would be the p2pdma client and orchestrator. It
> would expose a user API which binds an rxq to a pci device. Prior to
> the bind the user API would check that the pci device has published
> p2p memory (pci_has_p2pmem()), and check the the p2p mem is accessible
> to the driver (pci_p2pdma_distance() I think), etc.

This doesn't fit the programming model for GPUs at all. You don't want
to get packets landing in random GPU memory that a kernel side
allocator selects, you want packets landing in GPU memory owned by a
specific process that owns the TCP connection.

This is why DMABUF is used here as it gives a handle to the GPU
memory. What you want is to get the P2P pages either directly from the
DMABUF or via pin_user_pages() on the DMABUF's mmap.

> AFAICT, all the concerns brought up in this thread are sidestepped by
> using p2pdma. I need not allocate struct pages in the core dma-buf
> code anymore (or anywhere), and I need not allocate pgmaps. I would
> just re-use the p2pdma support.

Well, as I said it is going to be a big ask to P2P enable any of the
DRM drivers.

And you still have the netmem vs zone_device struct page conflict to
figure out

But it is alot closer to reasonable than this RFC.

Jason