246 lines
11 KiB
ReStructuredText
246 lines
11 KiB
ReStructuredText
==========================================
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I915 VM_BIND feature design and use cases
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==========================================
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VM_BIND feature
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================
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DRM_I915_GEM_VM_BIND/UNBIND ioctls allows UMD to bind/unbind GEM buffer
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objects (BOs) or sections of a BOs at specified GPU virtual addresses on a
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specified address space (VM). These mappings (also referred to as persistent
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mappings) will be persistent across multiple GPU submissions (execbuf calls)
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issued by the UMD, without user having to provide a list of all required
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mappings during each submission (as required by older execbuf mode).
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The VM_BIND/UNBIND calls allow UMDs to request a timeline out fence for
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signaling the completion of bind/unbind operation.
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VM_BIND feature is advertised to user via I915_PARAM_VM_BIND_VERSION.
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User has to opt-in for VM_BIND mode of binding for an address space (VM)
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during VM creation time via I915_VM_CREATE_FLAGS_USE_VM_BIND extension.
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VM_BIND/UNBIND ioctl calls executed on different CPU threads concurrently are
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not ordered. Furthermore, parts of the VM_BIND/UNBIND operations can be done
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asynchronously, when valid out fence is specified.
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VM_BIND features include:
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* Multiple Virtual Address (VA) mappings can map to the same physical pages
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of an object (aliasing).
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* VA mapping can map to a partial section of the BO (partial binding).
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* Support capture of persistent mappings in the dump upon GPU error.
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* Support for userptr gem objects (no special uapi is required for this).
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TLB flush consideration
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------------------------
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The i915 driver flushes the TLB for each submission and when an object's
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pages are released. The VM_BIND/UNBIND operation will not do any additional
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TLB flush. Any VM_BIND mapping added will be in the working set for subsequent
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submissions on that VM and will not be in the working set for currently running
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batches (which would require additional TLB flushes, which is not supported).
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Execbuf ioctl in VM_BIND mode
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-------------------------------
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A VM in VM_BIND mode will not support older execbuf mode of binding.
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The execbuf ioctl handling in VM_BIND mode differs significantly from the
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older execbuf2 ioctl (See struct drm_i915_gem_execbuffer2).
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Hence, a new execbuf3 ioctl has been added to support VM_BIND mode. (See
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struct drm_i915_gem_execbuffer3). The execbuf3 ioctl will not accept any
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execlist. Hence, no support for implicit sync. It is expected that the below
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work will be able to support requirements of object dependency setting in all
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use cases:
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"dma-buf: Add an API for exporting sync files"
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(https://lwn.net/Articles/859290/)
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The new execbuf3 ioctl only works in VM_BIND mode and the VM_BIND mode only
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works with execbuf3 ioctl for submission. All BOs mapped on that VM (through
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VM_BIND call) at the time of execbuf3 call are deemed required for that
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submission.
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The execbuf3 ioctl directly specifies the batch addresses instead of as
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object handles as in execbuf2 ioctl. The execbuf3 ioctl will also not
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support many of the older features like in/out/submit fences, fence array,
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default gem context and many more (See struct drm_i915_gem_execbuffer3).
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In VM_BIND mode, VA allocation is completely managed by the user instead of
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the i915 driver. Hence all VA assignment, eviction are not applicable in
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VM_BIND mode. Also, for determining object activeness, VM_BIND mode will not
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be using the i915_vma active reference tracking. It will instead use dma-resv
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object for that (See `VM_BIND dma_resv usage`_).
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So, a lot of existing code supporting execbuf2 ioctl, like relocations, VA
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evictions, vma lookup table, implicit sync, vma active reference tracking etc.,
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are not applicable for execbuf3 ioctl. Hence, all execbuf3 specific handling
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should be in a separate file and only functionalities common to these ioctls
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can be the shared code where possible.
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VM_PRIVATE objects
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-------------------
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By default, BOs can be mapped on multiple VMs and can also be dma-buf
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exported. Hence these BOs are referred to as Shared BOs.
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During each execbuf submission, the request fence must be added to the
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dma-resv fence list of all shared BOs mapped on the VM.
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VM_BIND feature introduces an optimization where user can create BO which
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is private to a specified VM via I915_GEM_CREATE_EXT_VM_PRIVATE flag during
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BO creation. Unlike Shared BOs, these VM private BOs can only be mapped on
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the VM they are private to and can't be dma-buf exported.
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All private BOs of a VM share the dma-resv object. Hence during each execbuf
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submission, they need only one dma-resv fence list updated. Thus, the fast
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path (where required mappings are already bound) submission latency is O(1)
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w.r.t the number of VM private BOs.
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VM_BIND locking hierarchy
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-------------------------
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The locking design here supports the older (execlist based) execbuf mode, the
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newer VM_BIND mode, the VM_BIND mode with GPU page faults and possible future
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system allocator support (See `Shared Virtual Memory (SVM) support`_).
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The older execbuf mode and the newer VM_BIND mode without page faults manages
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residency of backing storage using dma_fence. The VM_BIND mode with page faults
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and the system allocator support do not use any dma_fence at all.
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VM_BIND locking order is as below.
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1) Lock-A: A vm_bind mutex will protect vm_bind lists. This lock is taken in
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vm_bind/vm_unbind ioctl calls, in the execbuf path and while releasing the
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mapping.
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In future, when GPU page faults are supported, we can potentially use a
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rwsem instead, so that multiple page fault handlers can take the read side
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lock to lookup the mapping and hence can run in parallel.
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The older execbuf mode of binding do not need this lock.
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2) Lock-B: The object's dma-resv lock will protect i915_vma state and needs to
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be held while binding/unbinding a vma in the async worker and while updating
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dma-resv fence list of an object. Note that private BOs of a VM will all
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share a dma-resv object.
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The future system allocator support will use the HMM prescribed locking
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instead.
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3) Lock-C: Spinlock/s to protect some of the VM's lists like the list of
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invalidated vmas (due to eviction and userptr invalidation) etc.
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When GPU page faults are supported, the execbuf path do not take any of these
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locks. There we will simply smash the new batch buffer address into the ring and
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then tell the scheduler run that. The lock taking only happens from the page
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fault handler, where we take lock-A in read mode, whichever lock-B we need to
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find the backing storage (dma_resv lock for gem objects, and hmm/core mm for
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system allocator) and some additional locks (lock-D) for taking care of page
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table races. Page fault mode should not need to ever manipulate the vm lists,
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so won't ever need lock-C.
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VM_BIND LRU handling
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---------------------
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We need to ensure VM_BIND mapped objects are properly LRU tagged to avoid
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performance degradation. We will also need support for bulk LRU movement of
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VM_BIND objects to avoid additional latencies in execbuf path.
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The page table pages are similar to VM_BIND mapped objects (See
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`Evictable page table allocations`_) and are maintained per VM and needs to
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be pinned in memory when VM is made active (ie., upon an execbuf call with
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that VM). So, bulk LRU movement of page table pages is also needed.
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VM_BIND dma_resv usage
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-----------------------
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Fences needs to be added to all VM_BIND mapped objects. During each execbuf
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submission, they are added with DMA_RESV_USAGE_BOOKKEEP usage to prevent
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over sync (See enum dma_resv_usage). One can override it with either
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DMA_RESV_USAGE_READ or DMA_RESV_USAGE_WRITE usage during explicit object
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dependency setting.
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Note that DRM_I915_GEM_WAIT and DRM_I915_GEM_BUSY ioctls do not check for
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DMA_RESV_USAGE_BOOKKEEP usage and hence should not be used for end of batch
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check. Instead, the execbuf3 out fence should be used for end of batch check
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(See struct drm_i915_gem_execbuffer3).
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Also, in VM_BIND mode, use dma-resv apis for determining object activeness
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(See dma_resv_test_signaled() and dma_resv_wait_timeout()) and do not use the
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older i915_vma active reference tracking which is deprecated. This should be
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easier to get it working with the current TTM backend.
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Mesa use case
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--------------
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VM_BIND can potentially reduce the CPU overhead in Mesa (both Vulkan and Iris),
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hence improving performance of CPU-bound applications. It also allows us to
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implement Vulkan's Sparse Resources. With increasing GPU hardware performance,
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reducing CPU overhead becomes more impactful.
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Other VM_BIND use cases
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========================
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Long running Compute contexts
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------------------------------
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Usage of dma-fence expects that they complete in reasonable amount of time.
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Compute on the other hand can be long running. Hence it is appropriate for
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compute to use user/memory fence (See `User/Memory Fence`_) and dma-fence usage
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must be limited to in-kernel consumption only.
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Where GPU page faults are not available, kernel driver upon buffer invalidation
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will initiate a suspend (preemption) of long running context, finish the
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invalidation, revalidate the BO and then resume the compute context. This is
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done by having a per-context preempt fence which is enabled when someone tries
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to wait on it and triggers the context preemption.
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User/Memory Fence
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~~~~~~~~~~~~~~~~~~
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User/Memory fence is a <address, value> pair. To signal the user fence, the
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specified value will be written at the specified virtual address and wakeup the
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waiting process. User fence can be signaled either by the GPU or kernel async
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worker (like upon bind completion). User can wait on a user fence with a new
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user fence wait ioctl.
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Here is some prior work on this:
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https://patchwork.freedesktop.org/patch/349417/
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Low Latency Submission
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~~~~~~~~~~~~~~~~~~~~~~~
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Allows compute UMD to directly submit GPU jobs instead of through execbuf
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ioctl. This is made possible by VM_BIND is not being synchronized against
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execbuf. VM_BIND allows bind/unbind of mappings required for the directly
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submitted jobs.
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Debugger
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---------
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With debug event interface user space process (debugger) is able to keep track
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of and act upon resources created by another process (debugged) and attached
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to GPU via vm_bind interface.
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GPU page faults
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----------------
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GPU page faults when supported (in future), will only be supported in the
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VM_BIND mode. While both the older execbuf mode and the newer VM_BIND mode of
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binding will require using dma-fence to ensure residency, the GPU page faults
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mode when supported, will not use any dma-fence as residency is purely managed
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by installing and removing/invalidating page table entries.
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Page level hints settings
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--------------------------
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VM_BIND allows any hints setting per mapping instead of per BO. Possible hints
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include placement and atomicity. Sub-BO level placement hint will be even more
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relevant with upcoming GPU on-demand page fault support.
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Page level Cache/CLOS settings
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-------------------------------
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VM_BIND allows cache/CLOS settings per mapping instead of per BO.
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Evictable page table allocations
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---------------------------------
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Make pagetable allocations evictable and manage them similar to VM_BIND
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mapped objects. Page table pages are similar to persistent mappings of a
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VM (difference here are that the page table pages will not have an i915_vma
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structure and after swapping pages back in, parent page link needs to be
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updated).
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Shared Virtual Memory (SVM) support
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------------------------------------
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VM_BIND interface can be used to map system memory directly (without gem BO
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abstraction) using the HMM interface. SVM is only supported with GPU page
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faults enabled.
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VM_BIND UAPI
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=============
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.. kernel-doc:: Documentation/gpu/rfc/i915_vm_bind.h
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