Documentation/gpu/rfc/i915_vm_bind.rst - linux - Git at Google

 ==========================================
 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 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.

 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
	==========================================
	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 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.

	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