VM_BIND design document with description of intended use cases. v2: Add more documentation and format as per review comments from Daniel.
Signed-off-by: Niranjana Vishwanathapura <niranjana.vishwanathap...@intel.com> --- Documentation/driver-api/dma-buf.rst | 2 + Documentation/gpu/rfc/i915_vm_bind.rst | 304 +++++++++++++++++++++++++ Documentation/gpu/rfc/index.rst | 4 + 3 files changed, 310 insertions(+) create mode 100644 Documentation/gpu/rfc/i915_vm_bind.rst diff --git a/Documentation/driver-api/dma-buf.rst b/Documentation/driver-api/dma-buf.rst index 36a76cbe9095..64cb924ec5bb 100644 --- a/Documentation/driver-api/dma-buf.rst +++ b/Documentation/driver-api/dma-buf.rst @@ -200,6 +200,8 @@ DMA Fence uABI/Sync File .. kernel-doc:: include/linux/sync_file.h :internal: +.. _indefinite_dma_fences: + Indefinite DMA Fences ~~~~~~~~~~~~~~~~~~~~~ diff --git a/Documentation/gpu/rfc/i915_vm_bind.rst b/Documentation/gpu/rfc/i915_vm_bind.rst new file mode 100644 index 000000000000..f1be560d313c --- /dev/null +++ b/Documentation/gpu/rfc/i915_vm_bind.rst @@ -0,0 +1,304 @@ +========================================== +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 (execbuff calls) +issued by the UMD, without user having to provide a list of all required +mappings during each submission (as required by older execbuff mode). + +VM_BIND/UNBIND ioctls will support 'in' and 'out' fences to allow userpace +to specify how the binding/unbinding should sync with other operations +like the GPU job submission. These fences will be timeline 'drm_syncobj's +for non-Compute contexts (See struct drm_i915_vm_bind_ext_timeline_fences). +For Compute contexts, they will be user/memory fences (See struct +drm_i915_vm_bind_ext_user_fence). + +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. + +VM_BIND/UNBIND ioctl will immediately start binding/unbinding the mapping in an +async worker. The binding and unbinding will work like a special GPU engine. +The binding and unbinding operations are serialized and will wait on specified +input fences before the operation and will signal the output fences upon the +completion of the operation. Due to serialization, completion of an operation +will also indicate that all previous operations are also complete. + +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. +* TLB is flushed upon unbind completion. Batching of TLB flushes in some + use cases will be helpful. +* Asynchronous vm_bind and vm_unbind support with 'in' and 'out' fences. +* Support for userptr gem objects (no special uapi is required for this). + +Execbuff ioctl in VM_BIND mode +------------------------------- +The execbuff ioctl handling in VM_BIND mode differs significantly from the +older method. A VM in VM_BIND mode will not support older execbuff mode of +binding. In VM_BIND mode, execbuff 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/) + +This also means, we need an execbuff extension to pass in the batch +buffer addresses (See struct drm_i915_gem_execbuffer_ext_batch_addresses). + +If at all execlist support in execbuff ioctl is deemed necessary for +implicit sync in certain use cases, then support can be added later. + +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 in the execbuff path like relocations, VA evictions, +vma lookup table, implicit sync, vma active reference tracking etc., are not +applicable in VM_BIND mode. Hence, the execbuff path needs to be cleaned up +by clearly separating out the functionalities where the VM_BIND mode differs +from older method and they should be moved to separate files. + +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 execbuff 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 execbuff +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) execbuff 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 execbuff 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 execbuff 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 execbuff 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 execbuff 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 execbuff 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 execbuff call with +that VM). So, bulk LRU movement of page table pages is also needed. + +The i915 shrinker LRU has stopped being an LRU. So, it should also be moved +over to the ttm LRU in some fashion to make sure we once again have a reasonable +and consistent memory aging and reclaim architecture. + +VM_BIND dma_resv usage +----------------------- +Fences needs to be added to all VM_BIND mapped objects. During each execbuff +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 object dependency +setting (either through explicit or implicit mechanism). + +When vm_bind is called for a non-private object while the VM is already +active, the fences need to be copied from VM's shared dma-resv object +(common to all private objects of the VM) to this non-private object. +If this results in performance degradation, then some optimization will +be needed here. This is not a problem for VM's private objects as they use +shared dma-resv object which is always updated on each execbuff submission. + +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. We can remove the +i915_vma active reference tracking fully while supporting TTM backend for igfx. + +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). + +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. + + +VM_BIND Compute support +======================== + +User/Memory Fence +------------------ +The idea is to take a user specified virtual address and install an interrupt +handler to wake up the current task when the memory location passes the user +supplied filter. User/Memory fence is a <address, value> pair. To signal the +user fence, specified value will be written at the specified virtual address +and wakeup the waiting process. User can wait on a user fence with the +gem_wait_user_fence ioctl. + +It also allows the user to emit their own MI_FLUSH/PIPE_CONTROL notify +interrupt within their batches after updating the value to have sub-batch +precision on the wakeup. Each batch can signal a user fence to indicate +the completion of next level batch. The completion of very first level batch +needs to be signaled by the command streamer. The user must provide the +user/memory fence for this via the DRM_I915_GEM_EXECBUFFER_EXT_USER_FENCE +extension of execbuff ioctl, so that KMD can setup the command streamer to +signal it. + +User/Memory fence can also be supplied to the kernel driver to signal/wake up +the user process after completion of an asynchronous operation. + +When VM_BIND ioctl was provided with a user/memory fence via the +I915_VM_BIND_EXT_USER_FENCE extension, it will be signaled upon the completion +of binding of that mapping. All async binds/unbinds are serialized, hence +signaling of user/memory fence also indicate the completion of all previous +binds/unbinds. + +This feature will be derived from the below original work: +https://patchwork.freedesktop.org/patch/349417/ + +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 and dma-fence usage will be limited to +in-kernel consumption only. This requires an execbuff uapi extension to pass +in user fence (See struct drm_i915_vm_bind_ext_user_fence). Compute must opt-in +for this mechanism with I915_CONTEXT_CREATE_FLAGS_LONG_RUNNING flag during +context creation. The dma-fence based user interfaces like gem_wait ioctl and +execbuff out fence are not allowed on long running contexts. Implicit sync is +not valid as well and is anyway not supported in VM_BIND mode. + +Where GPU page faults are not available, kernel driver upon buffer invalidation +will initiate a suspend (preemption) of long running context with a dma-fence +attached to it. And upon completion of that suspend fence, finish the +invalidation, revalidate the BO and then resume the compute context. This is +done by having a per-context preempt fence (also called suspend fence) proxying +as i915_request fence. This suspend fence is enabled when someone tries to wait +on it, which then triggers the context preemption. + +As this support for context suspension using a preempt fence and the resume work +for the compute mode contexts can get tricky to get it right, it is better to +add this support in drm scheduler so that multiple drivers can make use of it. +That means, it will have a dependency on i915 drm scheduler conversion with GuC +scheduler backend. This should be fine, as the plan is to support compute mode +contexts only with GuC scheduler backend (at least initially). This is much +easier to support with VM_BIND mode compared to the current heavier execbuff +path resource attachment. + +Low Latency Submission +----------------------- +Allows compute UMD to directly submit GPU jobs instead of through execbuff +ioctl. This is made possible by VM_BIND is not being synchronized against +execbuff. VM_BIND allows bind/unbind of mappings required for the directly +submitted jobs. + +Other VM_BIND use cases +======================== + +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 execbuff 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. + +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. + + +Broder i915 cleanups +===================== +Supporting this whole new vm_bind mode of binding which comes with its own +use cases to support and the locking requirements requires proper integration +with the existing i915 driver. This calls for some broader i915 driver +cleanups/simplifications for maintainability of the driver going forward. +Here are few things identified and are being looked into. + +- Remove vma lookup cache (eb->gem_context->handles_vma). VM_BIND feature + do not use it and complexity it brings in is probably more than the + performance advantage we get in legacy execbuff case. +- Remove vma->open_count counting +- Remove i915_vma active reference tracking. VM_BIND feature will not be using + it. Instead use underlying BO's dma-resv fence list to determine if a i915_vma + is active or not. + + +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