Hi Thomas, Thanks for the document. See one question inline. Thanks, Oak > -----Original Message----- > From: dri-devel <dri-devel-bounces@xxxxxxxxxxxxxxxxxxxxx> On Behalf Of > Thomas Hellström > Sent: May 30, 2023 4:43 AM > To: intel-xe@xxxxxxxxxxxxxxxxxxxxx > Cc: Brost, Matthew <matthew.brost@xxxxxxxxx>; Thomas Hellström > <thomas.hellstrom@xxxxxxxxxxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx; dri- > devel@xxxxxxxxxxxxxxxxxxxxx; Danilo Krummrich <dakr@xxxxxxxxxx> > Subject: [RFC PATCH] Documentation/gpu: Add a VM_BIND async draft > document. > > 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. Are you saying memory fence (user fence) uses specific memory location to determine signaled status, while dma-fence doesn't use specific memory location to determine status? My understanding is, both user fence and dma fence use a memory to determine status...in the dma fence case, it is the seqno field of struct dma_fence. The difference b/t those two is, for dma-fence, people agreed it has to be signaled in certain amount of time; while user fence doesn't has such contract. -Oak 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 restart the IOCTL after taking suitable action. If > +UMD has overcommited 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. > -- > 2.39.2