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 >
