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

Wed Mar 9 15:58:09 UTC 2022

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/

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
>