On Wed, Mar 05, 2025 at 05:26:57PM -0800, Matthew Brost wrote: > Add documentation for agree upon GPU SVM design principles, current > status, and future plans.
One minor nit and a comment below, but feel free to add: Acked-by: Alistair Popple <apop...@nvidia.com> > 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.br...@intel.com> > Reviewed-by: Thomas Hellström <thomas.hellst...@linux.intel.com> > --- > 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. Pretty minor nit, but this could be read as core MM won't send a mmu notifier when calling mremap(). That's not the case, as it will get a notifier to invalidate the address but won't explicitly get notified of the new address. Not worth sending an update just for that though. > + * 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. Balbir has also just posted an initial RFC implementation of this here: https://lore.kernel.org/linux-mm/20250306044239.3874247-1-balb...@nvidia.com/ > +* 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 >
