Hi Thomas, On 5/30/2023 10:42 AM, Thomas Hellström wrote:
Add a motivation for and description of asynchronous VM_BIND operation Signed-off-by: Thomas Hellström <thomas.hellstrom@xxxxxxxxxxxxxxx> --- Documentation/gpu/drm-vm-bind-async.rst | 138 ++++++++++++++++++++++++ 1 file changed, 138 insertions(+) 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..7f7f8f7ddfea --- /dev/null +++ b/Documentation/gpu/drm-vm-bind-async.rst @@ -0,0 +1,138 @@ +==================== +Asynchronous VM_BIND +==================== + +Nomenclature: +============= + +* VRAM: On-device memory. Sometimes referred to as device local memory. + +* 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 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-syncbj: 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 + that 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. + +* 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 VM or the GPU execution context in + a certain mode that disallows completion dma-fences. + +* 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 swapin 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 execbuf 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. + +To aid in supporting user-space queues, the VM_BIND may take a bind context +AKA bind engine identifier argument. All VM_BIND operations using the same +bind engine can then be assumed, where it matters, to complete in +order. No such assumptions can be made between VM_BIND operations +using separate bind contexts. + +The purpose of an Asynchronous VM_BIND operation is for user-mode +drivers to be able to pipeline interleaved vm modifications and +execbufs. For long-running workloads, such pipelining of a bind +operation is not allowed and any in-fences need to be awaited +synchronously. + +Also for VM_BINDS for long-running 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 preferrably
s/preferrably/preferably
restart the IOCTL after taking suitable action. If +UMD has overcommited
s/overcommited/overcommitted Thanks for documenting this complex topic. Acked-by: Nirmoy Das <nirmoy.das@xxxxxxxxx> Regards, Nirmoy
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 VM. For a +description why, please see KMD implementation details under [error +state saving]_. + + +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 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 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.
