On Fri, Jun 24, 2022 at 10:49:34AM -0700, Niranjana Vishwanathapura wrote: > VM_BIND design document with description of intended use cases. > > v2: Reduce the scope to simple Mesa use case. > v3: Expand documentation on dma-resv usage, TLB flushing and > execbuf3. > v4: Remove vm_bind tlb flush request support. > v5: Update TLB flushing documentation. > > Signed-off-by: Niranjana Vishwanathapura <niranjana.vishwanathapura@xxxxxxxxx> > --- > Documentation/gpu/rfc/i915_vm_bind.rst | 246 +++++++++++++++++++++++++ > Documentation/gpu/rfc/index.rst | 4 + > 2 files changed, 250 insertions(+) > create mode 100644 Documentation/gpu/rfc/i915_vm_bind.rst > > diff --git a/Documentation/gpu/rfc/i915_vm_bind.rst b/Documentation/gpu/rfc/i915_vm_bind.rst > new file mode 100644 > index 000000000000..534adf0c6c7a > --- /dev/null > +++ b/Documentation/gpu/rfc/i915_vm_bind.rst > @@ -0,0 +1,246 @@ > +========================================== > +I915 VM_BIND feature design and use cases > +========================================== > + > +VM_BIND feature > +================ > +DRM_I915_GEM_VM_BIND/UNBIND ioctls allows UMD to bind/unbind GEM buffer > +objects (BOs) or sections of a BOs at specified GPU virtual addresses on a > +specified address space (VM). These mappings (also referred to as persistent > +mappings) will be persistent across multiple GPU submissions (execbuf calls) > +issued by the UMD, without user having to provide a list of all required > +mappings during each submission (as required by older execbuf mode). > + > +The VM_BIND/UNBIND calls allow UMDs to request a timeline fence for signaling > +the completion of bind/unbind operation. > + > +VM_BIND feature is advertised to user via I915_PARAM_HAS_VM_BIND. > +User has to opt-in for VM_BIND mode of binding for an address space (VM) > +during VM creation time via I915_VM_CREATE_FLAGS_USE_VM_BIND extension. > + > +The bind/unbind operation can get completed asynchronously and out of > +submission order. The out fence when specified will be signaled upon > +completion of bind/unbind operation. > + > +VM_BIND features include: > + > +* Multiple Virtual Address (VA) mappings can map to the same physical pages > + of an object (aliasing). > +* VA mapping can map to a partial section of the BO (partial binding). > +* Support capture of persistent mappings in the dump upon GPU error. > +* Support for userptr gem objects (no special uapi is required for this). > + > +TLB flush consideration > +------------------------ > +The i915 driver flushes the TLB for each submission and when an object's > +pages are released. The VM_BIND/UNBIND operation will not do any additional > +TLB flush. Any VM_BIND mapping added will be in the working set for subsequent > +submissions on that VM and will not be in the working set for currently running > +batches (which would require additional TLB flushes, which is not supported). > + > +Execbuf ioctl in VM_BIND mode > +------------------------------- > +A VM in VM_BIND mode will not support older execbuf mode of binding. > +The execbuf ioctl handling in VM_BIND mode differs significantly from the > +older execbuf2 ioctl (See struct drm_i915_gem_execbuffer2). > +Hence, a new execbuf3 ioctl has been added to support VM_BIND mode. (See > +struct drm_i915_gem_execbuffer3). The execbuf3 ioctl will not accept any > +execlist. Hence, no support for implicit sync. It is expected that the below > +work will be able to support requirements of object dependency setting in all > +use cases: > + > +"dma-buf: Add an API for exporting sync files" > +(https://lwn.net/Articles/859290/) > + > +The new execbuf3 ioctl only works in VM_BIND mode and the VM_BIND mode only > +works with execbuf3 ioctl for submission. All BOs mapped on that VM (through > +VM_BIND call) at the time of execbuf3 call are deemed required for that > +submission. > + > +The execbuf3 ioctl directly specifies the batch addresses instead of as > +object handles as in execbuf2 ioctl. The execbuf3 ioctl will also not > +support many of the older features like in/out/submit fences, fence array, > +default gem context and many more (See struct drm_i915_gem_execbuffer3). > + > +In VM_BIND mode, VA allocation is completely managed by the user instead of > +the i915 driver. Hence all VA assignment, eviction are not applicable in > +VM_BIND mode. Also, for determining object activeness, VM_BIND mode will not > +be using the i915_vma active reference tracking. It will instead use dma-resv > +object for that (See `VM_BIND dma_resv usage`_). > + > +So, a lot of existing code supporting execbuf2 ioctl, like relocations, VA > +evictions, vma lookup table, implicit sync, vma active reference tracking etc., > +are not applicable for execbuf3 ioctl. Hence, all execbuf3 specific handling > +should be in a separate file and only functionalities common to these ioctls > +can be the shared code where possible. > + > +VM_PRIVATE objects > +------------------- > +By default, BOs can be mapped on multiple VMs and can also be dma-buf > +exported. Hence these BOs are referred to as Shared BOs. > +During each execbuf submission, the request fence must be added to the > +dma-resv fence list of all shared BOs mapped on the VM. > + > +VM_BIND feature introduces an optimization where user can create BO which > +is private to a specified VM via I915_GEM_CREATE_EXT_VM_PRIVATE flag during > +BO creation. Unlike Shared BOs, these VM private BOs can only be mapped on > +the VM they are private to and can't be dma-buf exported. > +All private BOs of a VM share the dma-resv object. Hence during each execbuf > +submission, they need only one dma-resv fence list updated. Thus, the fast > +path (where required mappings are already bound) submission latency is O(1) > +w.r.t the number of VM private BOs. > + > +VM_BIND locking hirarchy > +------------------------- > +The locking design here supports the older (execlist based) execbuf mode, the > +newer VM_BIND mode, the VM_BIND mode with GPU page faults and possible future > +system allocator support (See `Shared Virtual Memory (SVM) support`_). > +The older execbuf mode and the newer VM_BIND mode without page faults manages > +residency of backing storage using dma_fence. The VM_BIND mode with page faults > +and the system allocator support do not use any dma_fence at all. > + > +VM_BIND locking order is as below. > + > +1) Lock-A: A vm_bind mutex will protect vm_bind lists. This lock is taken in > + vm_bind/vm_unbind ioctl calls, in the execbuf path and while releasing the > + mapping. > + > + In future, when GPU page faults are supported, we can potentially use a > + rwsem instead, so that multiple page fault handlers can take the read side > + lock to lookup the mapping and hence can run in parallel. > + The older execbuf mode of binding do not need this lock. > + > +2) Lock-B: The object's dma-resv lock will protect i915_vma state and needs to > + be held while binding/unbinding a vma in the async worker and while updating > + dma-resv fence list of an object. Note that private BOs of a VM will all > + share a dma-resv object. > + > + The future system allocator support will use the HMM prescribed locking > + instead. > + > +3) Lock-C: Spinlock/s to protect some of the VM's lists like the list of > + invalidated vmas (due to eviction and userptr invalidation) etc. > + > +When GPU page faults are supported, the execbuf path do not take any of these > +locks. There we will simply smash the new batch buffer address into the ring and > +then tell the scheduler run that. The lock taking only happens from the page > +fault handler, where we take lock-A in read mode, whichever lock-B we need to > +find the backing storage (dma_resv lock for gem objects, and hmm/core mm for > +system allocator) and some additional locks (lock-D) for taking care of page > +table races. Page fault mode should not need to ever manipulate the vm lists, > +so won't ever need lock-C. > + > +VM_BIND LRU handling > +--------------------- > +We need to ensure VM_BIND mapped objects are properly LRU tagged to avoid > +performance degradation. We will also need support for bulk LRU movement of > +VM_BIND objects to avoid additional latencies in execbuf path. > + > +The page table pages are similar to VM_BIND mapped objects (See > +`Evictable page table allocations`_) and are maintained per VM and needs to > +be pinned in memory when VM is made active (ie., upon an execbuf call with > +that VM). So, bulk LRU movement of page table pages is also needed. > + > +VM_BIND dma_resv usage > +----------------------- > +Fences needs to be added to all VM_BIND mapped objects. During each execbuf > +submission, they are added with DMA_RESV_USAGE_BOOKKEEP usage to prevent > +over sync (See enum dma_resv_usage). One can override it with either > +DMA_RESV_USAGE_READ or DMA_RESV_USAGE_WRITE usage during explicit object > +dependency setting. I thought we've ditched that in favour of the import/export ioctls, and execbuf will only ever set BOOKKEEPING? > +Note that DRM_I915_GEM_WAIT and DRM_I915_GEM_BUSY ioctls do not check for > +DMA_RESV_USAGE_BOOKKEEP usage and hence should not be used for end of batch > +check. Instead, the execbuf3 out fence should be used for end of batch check > +(See struct drm_i915_gem_execbuffer3). This is a bit confused. Changing gem_wait and gem_busy ioctls to also look at bookkeeping will not fix the issue, because in a vm_bind vm _all_ objects (and hence also all batchbuffers mapped into the vm) always have the same fences in their dma_resv from that vm. Which means that if any batchbuffer is still running, all batchbuffers are considered active. And so checking fences in the dma_resv is useless for the umd and it needs to track batchbuffer usage itself somewhere. > + > +Also, in VM_BIND mode, use dma-resv apis for determining object activeness > +(See dma_resv_test_signaled() and dma_resv_wait_timeout()) and do not use the > +older i915_vma active reference tracking which is deprecated. This should be > +easier to get it working with the current TTM backend. Hm I thought he plan is to require ttm, since ttm has the bulk lru stuff only and our i915-gem shrinker/lru code is very busted? Maybe needs to be clarified slightly. > + > +Mesa use case > +-------------- > +VM_BIND can potentially reduce the CPU overhead in Mesa (both Vulkan and Iris), > +hence improving performance of CPU-bound applications. It also allows us to > +implement Vulkan's Sparse Resources. With increasing GPU hardware performance, > +reducing CPU overhead becomes more impactful. > + > + > +Other VM_BIND use cases > +======================== > + > +Long running Compute contexts > +------------------------------ > +Usage of dma-fence expects that they complete in reasonable amount of time. > +Compute on the other hand can be long running. Hence it is appropriate for > +compute to use user/memory fence (See `User/Memory Fence`_) and dma-fence usage > +must be limited to in-kernel consumption only. > + > +Where GPU page faults are not available, kernel driver upon buffer invalidation > +will initiate a suspend (preemption) of long running context, finish the > +invalidation, revalidate the BO and then resume the compute context. This is > +done by having a per-context preempt fence which is enabled when someone tries > +to wait on it and triggers the context preemption. > + > +User/Memory Fence > +~~~~~~~~~~~~~~~~~~ > +User/Memory fence is a <address, value> pair. To signal the user fence, the > +specified value will be written at the specified virtual address and wakeup the > +waiting process. User fence can be signaled either by the GPU or kernel async > +worker (like upon bind completion). User can wait on a user fence with a new > +user fence wait ioctl. > + > +Here is some prior work on this: > +https://patchwork.freedesktop.org/patch/349417/ > + > +Low Latency Submission > +~~~~~~~~~~~~~~~~~~~~~~~ > +Allows compute UMD to directly submit GPU jobs instead of through execbuf > +ioctl. This is made possible by VM_BIND is not being synchronized against > +execbuf. VM_BIND allows bind/unbind of mappings required for the directly > +submitted jobs. > + > +Debugger > +--------- > +With debug event interface user space process (debugger) is able to keep track > +of and act upon resources created by another process (debugged) and attached > +to GPU via vm_bind interface. > + > +GPU page faults > +---------------- > +GPU page faults when supported (in future), will only be supported in the > +VM_BIND mode. While both the older execbuf mode and the newer VM_BIND mode of > +binding will require using dma-fence to ensure residency, the GPU page faults > +mode when supported, will not use any dma-fence as residency is purely managed > +by installing and removing/invalidating page table entries. > + > +Page level hints settings > +-------------------------- > +VM_BIND allows any hints setting per mapping instead of per BO. > +Possible hints include read-only mapping, placement and atomicity. > +Sub-BO level placement hint will be even more relevant with > +upcoming GPU on-demand page fault support. > + > +Page level Cache/CLOS settings > +------------------------------- > +VM_BIND allows cache/CLOS settings per mapping instead of per BO. > + > +Evictable page table allocations > +--------------------------------- > +Make pagetable allocations evictable and manage them similar to VM_BIND > +mapped objects. Page table pages are similar to persistent mappings of a > +VM (difference here are that the page table pages will not have an i915_vma > +structure and after swapping pages back in, parent page link needs to be > +updated). > + > +Shared Virtual Memory (SVM) support > +------------------------------------ > +VM_BIND interface can be used to map system memory directly (without gem BO > +abstraction) using the HMM interface. SVM is only supported with GPU page > +faults enabled. > + > +VM_BIND UAPI > +============= > + > +.. kernel-doc:: Documentation/gpu/rfc/i915_vm_bind.h > diff --git a/Documentation/gpu/rfc/index.rst b/Documentation/gpu/rfc/index.rst > index 91e93a705230..7d10c36b268d 100644 > --- a/Documentation/gpu/rfc/index.rst > +++ b/Documentation/gpu/rfc/index.rst > @@ -23,3 +23,7 @@ host such documentation: > .. toctree:: > > i915_scheduler.rst > + > +.. toctree:: > + > + i915_vm_bind.rst > -- > 2.21.0.rc0.32.g243a4c7e27 So I had a few comments/questions on the dma_resv side, but frankly we can also adjust them later on - either when we land the code or with a quick patch for the rfc docs here. Reviewed-by: Daniel Vetter <daniel.vetter@xxxxxxxx> > -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch
