Documentation: add initial UALink Documentation

Document the details of UALink on the GPU.

v2: add updates from Felix

Cc: Felix.Kuehling@amd.com
Cc: Joseph.Greathouse@amd.com
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

Documentation/gpu/amdgpu/index.rst

@@ -23,4 +23,5 @@ Next (GCN), Radeon DNA (RDNA), and Compute DNA (CDNA) architectures.
    debugfs
    process-isolation
    amdgpu-glossary
+   ualink
    ptl

Documentation/gpu/amdgpu/ualink.rst

@@ -0,0 +1,75 @@
+==============
+UALink Support
+==============
+
+Overview
+========
+
+Connected GPUs in a pod can directly access the remove memory on another GPU
+over UALink.  Unlike RMDA, there is no copy involved; it is direct loads/stores
+over the fabric.  Shared memory can only be accessed by a remote GPU if the
+memory was exported and the importer has been authorized. For the memory to be
+shared, it must be part of a unified physical address space shared between
+nodes.  This address space is called NPA (Nework Physical Address) space.  This
+address space is partitioned between the GPUs so that each GPU has its own
+segment of the address space in which to export its memory.  Each GPU maintains
+a dedicated set of page tables for their NPA space similar to GPUVM.  Note that
+this mechanism only allows for GPU access to remote memory.  The remote memory
+is not CPU accessible.
+
+Exported memory is pinned.  This is similar to how dma-bufs in VRAM are pinned
+for P2P access.  Remote TLB shootdowns from the exporter are used when the
+exported memory is freed in order to remove access by remote GPUs.
+
+To access remote memory, the driver can map NPA addresses into its per process
+GPUVM page tables just like local memory.  Applications use opaque handles to
+represent remote memory.  GPUs in a pod communicate with eachother directly to
+exchange NPA addresses between importers and exporters.  If a node goes offline
+or is reset, their peers will clean up any remaining refrences that are lost
+when that happens.  NPA addresses are exchanged directly between the kernel
+drivers using the scale up fabric.  NPA addresses are never exposed to user
+space.
+
+On the importer, the NPA space is like another physical address space. NPA
+addresses can be used as physical addresses for GPUVM to provide GPU virtual
+addresses to the memory for processes using the GPU.
+
+On the exporter, the NPA space provides a way to expose discontiguous local
+memory as a contiguous address range for remote GPUs.  This allows the exporter
+to locally manage the pages mapped into the NPA space.
+
+Remote NPAs are managed like another device specific TTM pool similar to
+doorbells or VRAM, however they cannot be CPU mapped.
+
+
+User Interface
+==============
+Two IOCTLs are provided to export and import remote memory.
+
+Export Memory
+-------------
+To export memory, a UALINK handle must be created for an allocation that can be
+shared with another node in the pod.  To do this the exporter calls the GEM
+UALink IOCTL with the GEM handle to the buffer it wants to export.  The IOCTL
+returns a unique 128 bit handle which can be shared with the remote host.
+Calling export on the same GEM handle always returns the same UALink handle.
+The UALink handle is destroyed when the GEM object reference count reaches 0.
+
+Import Memory
+-------------
+To import remote memory, the UALink handle from the remote node must be
+converted from a UALink handle to a local GEM object which represents the local
+reference to the NPA space on the importer.  If the memory has already been
+imported, it just returns a new reference to the existing GEM object.  If not,
+the importer queries the exporter to get the NPA address.  Once it has that, the
+importer can create the GEM to represent the NPA space used by the allocation.
+The GEM object is then exported to the caller as a dma-buf. The dma-buf is
+leveraged for dynamic attachment which provides the ability to revoke access
+when necessary.
+
+
+Device to Device Communications
+===============================
+
+Devices communicate via a protocol implemented in firmware.  Mesages sent to a
+remote node generate an interrupt on that node for servicing.