On Thu, Mar 06, 2025 at 04:45:31PM +1100, Alistair Popple wrote: > 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 <apopple@xxxxxxxxxx> > Thanks! > > 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. > > 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. > Yep, will fix up when merging. > > + * 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-balbirs@xxxxxxxxxx/ > Nice. Will take a look as we have time. I also want to pull in Leon's series [1] too. Matt [1] https://lore.kernel.org/all/cover.1738765879.git.leonro@xxxxxxxxxx/ > > +* 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 > >
