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

[Matthew Brost]

On Sat, Jul 15, 2023 at 05:45:43PM +0200, Thomas Hellström wrote:
> 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@xxxxxxxxxxxxxxx>

Reviewed-by: Matthew Brost <matthew.brost@xxxxxxxxx>

> Acked-by: Nirmoy Das <nirmoy.das@xxxxxxxxx>
> ---
>  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
>