[PATCH v5] Documentation/gpu: Add a VM_BIND async draft document

Sat Jul 15 15:45:43 UTC 2023

Add a motivation for and description of asynchronous VM_BIND operation

v2:
- Fix typos (Nirmoy Das)
- Improve the description of a memory fence (Oak Zeng)
- Add a reference to the document in the Xe RFC.
- Add pointers to sample uAPI suggestions
v3:
- Address review comments (Danilo Krummrich)
- Formatting fixes
v4:
- Address typos (Francois Dugast)
- Explain why in-fences are not allowed for VM_BIND operations for long-
  running workloads (Matthew Brost)
v5:
- More typo- and style fixing
- Further clarify the implications of disallowing in-fences for VM_BIND
  operations for long-running workloads (Matthew Brost)

Signed-off-by: Thomas Hellström <thomas.hellstrom at linux.intel.com>
Acked-by: Nirmoy Das <nirmoy.das at intel.com>
---
 Documentation/gpu/drm-vm-bind-async.rst | 171 ++++++++++++++++++++++++
 Documentation/gpu/rfc/xe.rst            |   4 +-
 2 files changed, 173 insertions(+), 2 deletions(-)
 create mode 100644 Documentation/gpu/drm-vm-bind-async.rst

diff --git a/Documentation/gpu/drm-vm-bind-async.rst b/Documentation/gpu/drm-vm-bind-async.rst
new file mode 100644
index 000000000000..d2b02a38198a
--- /dev/null
+++ b/Documentation/gpu/drm-vm-bind-async.rst
@@ -0,0 +1,171 @@
+====================
+Asynchronous VM_BIND
+====================
+
+Nomenclature:
+=============
+
+* ``VRAM``: On-device memory. Sometimes referred to as device local memory.
+
+* ``gpu_vm``: A GPU address space. Typically per process, but can be shared by
+  multiple processes.
+
+* ``VM_BIND``: An operation or a list of operations to modify a gpu_vm using
+  an IOCTL. The operations include mapping and unmapping system- or
+  VRAM memory.
+
+* ``syncobj``: A container that abstracts synchronization objects. The
+  synchronization objects can be either generic, like dma-fences or
+  driver specific. A syncobj typically indicates the type of the
+  underlying synchronization object.
+
+* ``in-syncobj``: Argument to a VM_BIND IOCTL, the VM_BIND operation waits
+  for these before starting.
+
+* ``out-syncobj``: Argument to a VM_BIND_IOCTL, the VM_BIND operation
+  signals these when the bind operation is complete.
+
+* ``memory fence``: A synchronization object, different from a dma-fence.
+  A memory fence uses the value of a specified memory location to determine
+  signaled status. A memory fence can be awaited and signaled by both
+  the GPU and CPU. Memory fences are sometimes referred to as
+  user-fences, userspace-fences or gpu futexes and do not necessarily obey
+  the dma-fence rule of signaling within a "reasonable amount of time".
+  The kernel should thus avoid waiting for memory fences with locks held.
+
+* ``long-running workload``: A workload that may take more than the
+  current stipulated dma-fence maximum signal delay to complete and
+  which therefore needs to set the gpu_vm or the GPU execution context in
+  a certain mode that disallows completion dma-fences.
+
+* ``exec function``: An exec function is a function that revalidates all
+  affected gpu_vmas, submits a GPU command batch and registers the
+  dma_fence representing the GPU command's activity with all affected
+  dma_resvs. For completeness, although not covered by this document,
+  it's worth mentioning that an exec function may also be the
+  revalidation worker that is used by some drivers in compute /
+  long-running mode.
+
+* ``bind context``: A context identifier used for the VM_BIND
+  operation. VM_BIND operations that use the same bind context can be
+  assumed, where it matters, to complete in order of submission. No such
+  assumptions can be made for VM_BIND operations using separate bind contexts.
+
+* ``UMD``: User-mode driver.
+
+* ``KMD``: Kernel-mode driver.
+
+
+Synchronous / Asynchronous VM_BIND operation
+============================================
+
+Synchronous VM_BIND
+___________________
+With Synchronous VM_BIND, the VM_BIND operations all complete before the
+IOCTL returns. A synchronous VM_BIND takes neither in-fences nor
+out-fences. Synchronous VM_BIND may block and wait for GPU operations;
+for example swap-in or clearing, or even previous binds.
+
+Asynchronous VM_BIND
+____________________
+Asynchronous VM_BIND accepts both in-syncobjs and out-syncobjs. While the
+IOCTL may return immediately, the VM_BIND operations wait for the in-syncobjs
+before modifying the GPU page-tables, and signal the out-syncobjs when
+the modification is done in the sense that the next exec function that
+awaits for the out-syncobjs will see the change. Errors are reported
+synchronously assuming that the asynchronous part of the job never errors.
+In low-memory situations the implementation may block, performing the
+VM_BIND synchronously, because there might not be enough memory
+immediately available for preparing the asynchronous operation.
+
+If the VM_BIND IOCTL takes a list or an array of operations as an argument,
+the in-syncobjs needs to signal before the first operation starts to
+execute, and the out-syncobjs signal after the last operation
+completes. Operations in the operation list can be assumed, where it
+matters, to complete in order.
+
+Since asynchronous VM_BIND operations may use dma-fences embedded in
+out-syncobjs and internally in KMD to signal bind completion,  any
+memory fences given as VM_BIND in-fences need to be awaited
+synchronously before the VM_BIND ioctl returns, since dma-fences,
+required to signal in a reasonable amount of time, can never be made
+to depend on memory fences that don't have such a restriction.
+
+To aid in supporting user-space queues, the VM_BIND may take a bind context.
+
+The purpose of an Asynchronous VM_BIND operation is for user-mode
+drivers to be able to pipeline interleaved gpu_vm modifications and
+exec functions. For long-running workloads, such pipelining of a bind
+operation is not allowed and any in-fences need to be awaited
+synchronously. The reason for this is twofold. First, any memory
+fences gated by a long-running workload and used as in-syncobjs for the
+VM_BIND operation will need to be awaited synchronously anyway (see
+above). Second, any dma-fences used as in-syncobjs for VM_BIND
+operations for long-running workloads will not allow for pipelining
+anyway since long-running workloads don't allow for dma-fences as
+out-syncobjs, so while theoretically possible the use of them is
+questionable and should be rejected until there is a valuable use-case.
+Note that this is not a limitation imposed by dma-fence rules, but
+rather a limitation imposed to keep KMD implementation simple. It does
+not affect using dma-fences as dependencies for the long-running
+workload itself, which is allowed by dma-fence rules, but rather for
+the VM_BIND operation only.
+
+Also for VM_BINDS for long-running gpu_vms the user-mode driver should typically
+select memory fences as out-fences since that gives greater flexibility for
+the kernel mode driver to inject other operations into the bind /
+unbind operations. Like for example inserting breakpoints into batch
+buffers. The workload execution can then easily be pipelined behind
+the bind completion using the memory out-fence as the signal condition
+for a GPU semaphore embedded by UMD in the workload.
+
+Multi-operation VM_BIND IOCTL error handling and interrupts
+===========================================================
+
+The VM_BIND operations of the IOCTL may error due to lack of resources
+to complete and also due to interrupted waits. In both situations UMD
+should preferably restart the IOCTL after taking suitable action. If
+UMD has over-committed a memory resource, an -ENOSPC error will be
+returned, and UMD may then unbind resources that are not used at the
+moment and restart the IOCTL. On -EINTR, UMD should simply restart the
+IOCTL and on -ENOMEM user-space may either attempt to free known
+system memory resources or abort the operation. If aborting as a
+result of a failed operation in a list of operations, some operations
+may still have completed, and to get back to a known state, user-space
+should therefore attempt to unbind all virtual memory regions touched
+by the failing IOCTL.
+Unbind operations are guaranteed not to cause any errors due to
+resource constraints.
+In between a failed VM_BIND IOCTL and a successful restart there may
+be implementation defined restrictions on the use of the gpu_vm. For a
+description why, please see KMD implementation details under `error
+state saving`_.
+
+Sample uAPI implementations
+===========================
+Suggested uAPI implementations at the moment of writing can be found for
+the Nouveau driver `here
+<https://patchwork.freedesktop.org/patch/543260/?series=112994&rev=6>`_.
+and for the Xe driver `here
+<https://cgit.freedesktop.org/drm/drm-xe/diff/include/uapi/drm/xe_drm.h?h=drm-xe-next&id=9cb016ebbb6a275f57b1cb512b95d5a842391ad7>`_.
+
+KMD implementation details
+==========================
+
+Error state saving
+__________________
+Open: When the VM_BIND IOCTL returns an error, some or even parts of
+an operation may have been completed. If the IOCTL is restarted, in
+order to know where to restart, the KMD can either put the gpu_vm in
+an error state and save one instance of the needed restart state
+internally. In this case, KMD needs to block further modifications of
+the gpu_vm state that may cause additional failures requiring a
+restart state save, until the error has been fully resolved. If the
+uAPI instead defines a pointer to a UMD allocated cookie in the IOCTL
+struct, it could also choose to store the restart state in that cookie.
+
+The restart state may, for example, be the number of successfully
+completed operations.
+
+Easiest for UMD would of course be if KMD did a full unwind on error
+so that no error state needs to be saved.
diff --git a/Documentation/gpu/rfc/xe.rst b/Documentation/gpu/rfc/xe.rst
index 2516fe141db6..0f062e1346d2 100644
--- a/Documentation/gpu/rfc/xe.rst
+++ b/Documentation/gpu/rfc/xe.rst
@@ -138,8 +138,8 @@ memory fences. Ideally with helper support so people don't get it wrong in all
 possible ways.
 
 As a key measurable result, the benefits of ASYNC VM_BIND and a discussion of
-various flavors, error handling and a sample API should be documented here or in
-a separate document pointed to by this document.
+various flavors, error handling and sample API suggestions are documented in
+Documentation/gpu/drm-vm-bind-async.rst
 
 Userptr integration and vm_bind
 -------------------------------
-- 
2.40.1

