Add documentation for agree upon GPU SVM design principles, current
status, and future plans.
v4:
- Address Thomas's feedback
v5:
- s/Current/Basline (Thomas)
v7:
- Add license (CI)
- Add examples for design guideline reasoning (Alistair)
- Add snippet about possible livelock with concurrent GPU and and CPU
access (Alistair)
Signed-off-by: Matthew Brost <matthew.brost@xxxxxxxxx>
Reviewed-by: Thomas Hellström <thomas.hellstrom@xxxxxxxxxxxxxxx>
---
Documentation/gpu/rfc/gpusvm.rst | 106 +++++++++++++++++++++++++++++++
Documentation/gpu/rfc/index.rst | 4 ++
2 files changed, 110 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..87d9f9506155
--- /dev/null
+++ b/Documentation/gpu/rfc/gpusvm.rst
@@ -0,0 +1,106 @@
+.. SPDX-License-Identifier: (GPL-2.0+ OR MIT)
+
+===============
+GPU SVM Section
+===============
+
+Agreed upon design principles
+=============================
+
+* migrate_to_ram path
+ * Rely only on core MM concepts (migration PTEs, page references, and
+ page locking).
+ * No driver specific locks other than locks for hardware interaction in
+ this path. These are not required and generally a bad idea to
+ invent driver defined locks to seal core MM races.
+ * An example of a driver-specific lock causing issues occurred before
+ fixing do_swap_page to lock the faulting page. A driver-exclusive lock
+ in migrate_to_ram produced a stable livelock if enough threads read
+ the faulting page.
+ * 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
+ * Eviction is defined as migrating data from the GPU back to the
+ CPU without a virtual address to free up GPU memory.
+ * 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.
+ * The rationale for the above two points is that CPU virtual addresses
+ can change at any moment, while the physical pages remain stable.
+ * GPU page table invalidation, which requires a GPU virtual address, is
+ handled via the notifier that has access to the GPU virtual address.
+* 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.
+ * One possible issue with the above approach is if a driver has a strict
+ migration policy requiring GPU access to occur in GPU memory.
+ Concurrent CPU access could cause a livelock due to endless retries.
+ While no current user (Xe) of GPU SVM has such a policy, it is likely
+ to be added in the future. Ideally, this should be resolved on the
+ core-MM side rather than through a driver-side lock.
+* Physical memory to virtual backpointer
+ * This does not work, as no pointers from physical memory to virtual
+ memory should exist. mremap() is an example of the core MM updating
+ the virtual address without notifying the driver.
+ * The physical memory backpointer (page->zone_device_data) should remain
+ stable from allocation to page free. Safely updating this against a
+ concurrent user would be very difficult unless the page is 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 baseline design
+===========================
+
+Baseline 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.
+* Build common userptr implementation on top of GPU SVM
+* Driver side madvise implementation and migration policies
+* Pull in pending dma-mapping API changes from Leon / Nvidia when these land
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
