ece91c882d
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. v6: Update out of order completion documentation. Signed-off-by: Niranjana Vishwanathapura <niranjana.vishwanathapura@intel.com> Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch> Acked-by: Paulo Zanoni <paulo.r.zanoni@intel.com> Signed-off-by: Matt Roper <matthew.d.roper@intel.com> Link: https://patchwork.freedesktop.org/patch/msgid/20220701003110.24843-2-niranjana.vishwanathapura@intel.com
246 lines
11 KiB
ReStructuredText
246 lines
11 KiB
ReStructuredText
==========================================
|
|
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
|