[RFC v2 1/2] drm/doc/rfc: VM_BIND feature design document

Thu Apr 21 02:08:14 UTC 2022

On Wed, Mar 09, 2022 at 10:58:09AM -0500, Alex Deucher wrote:
>On Mon, Mar 7, 2022 at 3:30 PM Niranjana Vishwanathapura
><niranjana.vishwanathapura at intel.com> wrote:
>>
>> VM_BIND design document with description of intended use cases.
>>
>> Signed-off-by: Niranjana Vishwanathapura <niranjana.vishwanathapura at intel.com>
>> ---
>>  Documentation/gpu/rfc/i915_vm_bind.rst | 210 +++++++++++++++++++++++++
>>  Documentation/gpu/rfc/index.rst        |   4 +
>>  2 files changed, 214 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..cdc6bb25b942
>> --- /dev/null
>> +++ b/Documentation/gpu/rfc/i915_vm_bind.rst
>> @@ -0,0 +1,210 @@
>> +==========================================
>> +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) 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 ioctl deferes binding the mappings until next execbuff submission
>> +where it will be required, or immediately if I915_GEM_VM_BIND_IMMEDIATE
>> +flag is set (useful if mapping is required for an active context).
>> +
>> +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.
>> +A VM in VM_BIND mode will not support older execbuff mode of binding.
>> +
>> +UMDs can still send BOs of these persistent mappings in execlist of execbuff
>> +for specifying BO dependencies (implicit fencing) and to use BO as a batch,
>> +but those BOs should be mapped ahead via vm_bind ioctl.
>> +
>> +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
>> +  usecases will be helpful.
>> +- Asynchronous vm_bind and vm_unbind support.
>> +- VM_BIND uses user/memory fence mechanism for signaling bind completion
>> +  and for signaling batch completion in long running contexts (explained
>> +  below).
>> +
>> +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
>> +-------------------------
>> +VM_BIND locking order is as below.
>> +
>> +1) 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 pagefault handlers can take the read side
>> +   lock to lookup the mapping and hence can run in parallel.
>> +
>> +2) The BO's dma-resv lock will protect i915_vma state and needs to be held
>> +   while binding a vma and while updating dma-resv fence list of a BO.
>> +   The private BOs of a VM will all share a dma-resv object.
>> +
>> +   This lock is held in vm_bind call for immediate binding, during vm_unbind
>> +   call for unbinding and during execbuff path for binding the mapping and
>> +   updating the dma-resv fence list of the BO.
>> +
>> +3) Spinlock/s to protect some of the VM's lists.
>> +
>> +We will also need support for bluk LRU movement of persistent mapping to
>> +avoid additional latencies in execbuff path.
>> +
>> +GPU page faults
>> +----------------
>> +Both older execbuff mode and the newer VM_BIND mode of binding will require
>> +using dma-fence to ensure residency.
>> +In future when GPU page faults are supported, no dma-fence usage is required
>> +as residency is purely managed by installing and removing/invalidating ptes.
>> +
>> +
>> +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 an 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 an 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/
>> +
>> +
>> +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 and dma-fence usage will be limited to
>> +in-kernel consumption only. This requires an execbuff uapi extension to pass
>> +in 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, execbuff out fence
>> +and implicit dependency setting is not allowed on long running contexts.
>> +
>> +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 fence (called suspend fence) proxying as
>> +i915_request fence. This suspend fence is enabled when there is a wait on it,
>> +which triggers the context preemption.
>> +
>> +This is much easier to support with VM_BIND 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. VM_BIND allows map/unmap of BOs required for 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 (debuggee) and attached
>> +to GPU via vm_bind interface.
>> +
>> +Mesa/Valkun
>
>s/Valkun/Vulkan/

Thanks Alex,
Will fix.

Niranjana

>
>Alex
>
>> +------------
>> +VM_BIND can potentially reduce the CPU-overhead in Mesa thus improving
>> +performance. For Vulkan it should be straightforward to use VM_BIND.
>> +For Iris implicit buffer tracking must be implemented before we can harness
>> +VM_BIND benefits. With increasing GPU hardware performance reducing CPU
>> +overhead becomes more important.
>> +
>> +Page level hints settings
>> +--------------------------
>> +VM_BIND allows any hints setting per mapping instead of per BO.
>> +Possible hints include read-only, 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.
>> +
>> +
>> +Broder i915 cleanups
>> +=====================
>> +Supporting this whole new vm_bind mode of binding which comes with its own
>> +usecases 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.
>> +
>> +- 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).
>> +- 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. Instead use underlying BO's
>> +  dma-resv fence list to determine if a i915_vma is active or not.
>> +
>> +These can be worked upon after intitial vm_bind support is added.
>> +
>> +
>> +UAPI
>> +=====
>> +Uapi definiton can be found here:
>> +.. 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
>>