[Intel-gfx] [PATCH v2 1/3] drm/doc/rfc: VM_BIND feature design document

Tue Jun 21 08:35:16 UTC 2022

On 20/06/2022 17:29, Niranjana Vishwanathapura wrote:
> On Mon, Jun 20, 2022 at 11:43:10AM +0100, Tvrtko Ursulin wrote:
>>
>> Hi,
>>
>> On 17/06/2022 06:14, Niranjana Vishwanathapura wrote:
>>> VM_BIND design document with description of intended use cases.
>>>
>>> v2: Reduce the scope to simple Mesa use case.
>>
>> since I expressed interest please add me to cc when sending out.
>>
> 
> Hi Tvrtko,
> I did include you in the cc list with git send-email, but looks like 
> some patches
> in this series has the full cc list, but some don't (you are on cc list 
> of this
> patch though). I am not sure why.

Odd, I'm not on CC on the (only for me) copy I found in the mailing list.

>> How come the direction changed to simplify all of a sudden? I did not 
>> spot any discussion to that effect. Was it internal talks?
>>
> 
> Yah, some of us had offline discussion involving the Mesa team.
> I did update the thread (previous version of this patch series) about that.
> Plan was to align our roadmap to focus on the deliverables at this point
> without further complicating the uapi.
>>>
>>> Signed-off-by: Niranjana Vishwanathapura 
>>> <niranjana.vishwanathapura at intel.com>
>>> ---
>>>  Documentation/gpu/rfc/i915_vm_bind.rst | 238 +++++++++++++++++++++++++
>>>  Documentation/gpu/rfc/index.rst        |   4 +
>>>  2 files changed, 242 insertions(+)
>>>  create mode 100644 Documentation/gpu/rfc/i915_vm_bind.rst
>>>
>>> diff --git a/Documentation/gpu/rfc/i915_vm_bind.rst 
>>> b/Documentation/gpu/rfc/i915_vm_bind.rst
>>> new file mode 100644
>>> index 000000000000..4ab590ef11fd
>>> --- /dev/null
>>> +++ b/Documentation/gpu/rfc/i915_vm_bind.rst
>>> @@ -0,0 +1,238 @@
>>> +==========================================
>>> +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 fence for 
>>> signaling
>>> +the completion of bind/unbind operation.
>>> +
>>> +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.
>>> +
>>> +Normally, vm_bind/unbind operations will get completed synchronously,
>>
>> To me synchronously, at this point in the text, reads as ioctl will 
>> return only when the operation is done. Rest of the paragraph however 
>> disagrees (plus existence of out fence). It is not clear to me what is 
>> the actual behaviour. Will it be clear to userspace developers reading 
>> uapi kerneldoc? If it is async, what are the ordering rules in this 
>> version?
>>
> 
> Yah, here I am simply stating the i915_vma_pin_ww() behavior which mostly
> does the binding synchronously unless there is a moving fence associated
> with the object in which case, binding will complete later once that fence
> is signaled (hence the out fence).

So from userspace point of view it is fully asynchronous and out of 
order? I'd suggest spelling that out in the uapi kerneldoc.

>>> +but if the object is being moved, the binding will happen once that the
>>> +moving is complete and out fence will be signaled after binding is 
>>> complete.
>>> +The bind/unbind operation can get completed out of submission order.
>>> +
>>> +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.
>>> +* Support for userptr gem objects (no special uapi is required for 
>>> this).
>>> +
>>> +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/)
>>
>> What does this mean? If execbuf3 does not know about target objects 
>> how can we add a meaningful fence?
>>
> 
> Execbuf3 does know about the target objects. It is all the objects
> bound to that VM via vm_bind call.
> 
>>> +
>>> +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.
>>
>> Does this tie to my previous question? Design is to add each fence to 
>> literally _all_ BOs mapped to a VM, on every execbuf3? If so, is that 
>> definitely needed and for what use case? Mixing implicit and explicit, 
>> I mean bridging implicit and explicit sync clients?
>>
> 
> Yes. It is similar to how legacy execbuf2 does. ie., add request fence
> to all of the target BOs. Only difference is in execbuf2 case, target
> objects are the objects in execlist, whereas in execbuf2, it is all
> the BOs mapped to that VM via vm_bind call. It is needed as UMD says
> that it is needed by vm_bind'ing the BO before the execbuf3 call.

Sorry I did not understand why it is needed, the last sentence that is, 
what did that suppose to mean?

Regards,

Tvrtko

> Niranjana
> 
>> Regards,
>>
>> Tvrtko
>>
>>> +
>>> +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 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 execbuf 
>>> 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.
>>> +
>>> +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 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.
>>> +
>>> +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
>>> 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