[PATCH v2] Documentation/gpu: Add a VM_BIND async draft document
Thomas Hellström
thomas.hellstrom at linux.intel.com
Wed Jun 21 14:14:14 UTC 2023
Thanks for reviewing.
On Wed, 2023-06-21 at 13:23 +0200, Danilo Krummrich wrote:
> On Wed, Jun 21, 2023 at 12:04:35PM +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
> >
> > 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 | 145
> > ++++++++++++++++++++++++
> > Documentation/gpu/rfc/xe.rst | 4 +-
> > 2 files changed, 147 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..69aff250b62f
> > --- /dev/null
> > +++ b/Documentation/gpu/drm-vm-bind-async.rst
> > @@ -0,0 +1,145 @@
> > +====================
> > +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.
>
> Rather obvious, but maybe specify as "GPU virtual address space" or
> just
> "GPU VA space".
>
> Personally, I don't like "vm" as an abbreviation for "GPU VA space"
> or "virtual
> address space" in general, but it is commonly used and I fail to find
> a better
> one to be honest.
I have another upcoming document related to VM_BIND locking variants /
userptr integration and there I call gpu_vm. It would make sense to
unify the naming in the documents anyway. I don't have a strong opinion
either as long as it's not too long.
>
> > +
> > +* 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.
> > + 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, and do not necessarily bey the dma-fence rule of
> > + signalling 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 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
>
> Just some nit-picking: IOCTL is mostly written in caps, maybe you
> want to stick
> to either of the two variants.
Sure,
>
> > +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
>
> Maybe add "execbuf" to the nomenclature.
>
> > +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
>
> I think "bind context" should also be explained in the nomenclature.
>
> > +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 preferably restart the IOCTL after taking suitable action.
> > If
> > +UMD has overcommitted 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]_.
> > +
> > +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
> > +==========================
>
> Maybe we can mention the GPUVA manager as a helper for implementing
> such an
> interface.
Sure will add this as well.
Thanks,
Thomas
>
> Anyway, I will surely add a link pointing to this document to the
> documentation
> of the GPUVA manager.
>
> - Danilo
>
> > +
> > +.. [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.
> > 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
> >
>
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