[PATCH v4 33/33] drm/doc: gpusvm: Add GPU SVM documentation

[Matthew Brost <matthew.brost@intel.com>]

Add documentation for agree upon GPU SVM design principles, current
status, and future plans.

v4:
 - Address Thomas's feedback

Signed-off-by: Matthew Brost <matthew.brost@intel.com>
---
 Documentation/gpu/rfc/gpusvm.rst | 84 ++++++++++++++++++++++++++++++++
 Documentation/gpu/rfc/index.rst  |  4 ++
 2 files changed, 88 insertions(+)
 create mode 100644 Documentation/gpu/rfc/gpusvm.rst

diff --git a/Documentation/gpu/rfc/gpusvm.rst b/Documentation/gpu/rfc/gpusvm.rst
new file mode 100644
index 000000000000..2d88f5981981
--- /dev/null
+++ b/Documentation/gpu/rfc/gpusvm.rst
@@ -0,0 +1,84 @@
+===============
+GPU SVM Section
+===============
+
+Agreed upon design principles
+=============================
+
+* migrate_to_ram path
+	* Rely only on core MM concepts (migration PTEs, page references, and
+	  page locking). The reasoning is that this is not required, can lead to
+	  livelock cases, and is generally not a good idea to seal races using
+	  driver-invented locks.
+	* No driver specific locks other than locks for hardware interaction in
+	  this path.
+	* Partial migration is supported (i.e., a subset of pages attempting to
+	  migrate can actually migrate, with only the faulting page guaranteed
+	  to migrate).
+	* Driver handles mixed migrations via retry loops rather than locking.
+* Eviction
+	* Only looking at physical memory data structures and locks as opposed to
+	  looking at virtual memory data structures and locks.
+	* No looking at mm/vma structs or relying on those being locked.
+* GPU fault side
+	* mmap_read only used around core MM functions which require this lock
+	  and should strive to take mmap_read lock only in GPU SVM layer.
+	* Big retry loop to handle all races with the mmu notifier under the gpu
+	  pagetable locks/mmu notifier range lock/whatever we end up calling
+          those.
+	* Races (especially against concurrent eviction or migrate_to_ram)
+	  should not be handled on the fault side by trying to hold locks;
+	  rather, they should be handled using retry loops. One possible
+	  exception is holding a BO's dma-resv lock during the initial migration
+	  to VRAM, as this is a well-defined lock that can be taken underneath
+	  the mmap_read lock.
+* Physical memory to virtual backpointer
+	* Does not work, no pointers from physical memory to virtual should
+	  exist.
+	* Physical memory backpointer (page->zone_device_data) should be stable
+	  from allocation to page free.
+* GPU pagetable locking
+	* Notifier lock only protects range tree, pages valid state for a range
+	  (rather than seqno due to wider notifiers), pagetable entries, and
+	  mmu notifier seqno tracking, it is not a global lock to protect
+          against races.
+	* All races handled with big retry as mentioned above.
+
+Overview of current design
+==========================
+
+Current design is simple as possible to get a working basline in which can be
+built upon.
+
+.. kernel-doc:: drivers/gpu/drm/xe/drm_gpusvm.c
+   :doc: Overview
+   :doc: Locking
+   :doc: Migrataion
+   :doc: Partial Unmapping of Ranges
+   :doc: Examples
+
+Possible future design features
+===============================
+
+* Concurrent GPU faults
+	* CPU faults are concurrent so makes sense to have concurrent GPU
+	  faults.
+	* Should be possible with fined grained locking in the driver GPU
+	  fault handler.
+	* No expected GPU SVM changes required.
+* Ranges with mixed system and device pages
+	* Can be added if required to drm_gpusvm_get_pages fairly easily.
+* Multi-GPU support
+	* Work in progress and patches expected after initially landing on GPU
+	  SVM.
+	* Ideally can be done with little to no changes to GPU SVM.
+* Drop ranges in favor of radix tree
+	* May be desirable for faster notifiers.
+* Compound device pages
+	* Nvidia, AMD, and Intel all have agreed expensive core MM functions in
+	  migrate device layer are a performance bottleneck, having compound
+	  device pages should help increase performance by reducing the number
+	  of these expensive calls.
+* Higher order dma mapping for migration
+	* 4k dma mapping adversely affects migration performance on Intel
+	  hardware, higher order (2M) dma mapping should help here.
diff --git a/Documentation/gpu/rfc/index.rst b/Documentation/gpu/rfc/index.rst
index 476719771eef..396e535377fb 100644
--- a/Documentation/gpu/rfc/index.rst
+++ b/Documentation/gpu/rfc/index.rst
@@ -16,6 +16,10 @@ host such documentation:
 * Once the code has landed move all the documentation to the right places in
   the main core, helper or driver sections.
 
+.. toctree::
+
+    gpusvm.rst
+
 .. toctree::
 
     i915_gem_lmem.rst
-- 
2.34.1