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

["Thomas Hellström" <thomas.hellstrom@linux.intel.com>]

On Wed, 2025-01-29 at 11:52 -0800, Matthew Brost wrote:
> 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

baseline

With that fixed,
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>

> 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