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diff --git a/Documentation/gpu/rfc/i915_vm_bind.rst b/Documentation/gpu/rfc/i915_vm_bind.rst new file mode 100644 index 0000000000..0b3b525ac6 --- /dev/null +++ b/Documentation/gpu/rfc/i915_vm_bind.rst @@ -0,0 +1,245 @@ +========================================== +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 out fence for +signaling the completion of bind/unbind operation. + +VM_BIND feature is advertised to user via I915_PARAM_VM_BIND_VERSION. +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. + +VM_BIND/UNBIND ioctl calls executed on different CPU threads concurrently are +not ordered. Furthermore, parts of the VM_BIND/UNBIND operations can be done +asynchronously, when valid out fence is specified. + +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 hierarchy +------------------------- +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. + +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). + +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. + +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 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 |