From patchwork Sat Jul 15 15:45:43 2023 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Patchwork-Submitter: Thomas Hellstrom X-Patchwork-Id: 13314569 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from gabe.freedesktop.org (gabe.freedesktop.org [131.252.210.177]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 26ABCC0015E for ; Sat, 15 Jul 2023 15:46:08 +0000 (UTC) Received: from gabe.freedesktop.org (localhost [127.0.0.1]) by gabe.freedesktop.org (Postfix) with ESMTP id 14FD410E16B; Sat, 15 Jul 2023 15:46:07 +0000 (UTC) Received: from mga09.intel.com (mga09.intel.com [134.134.136.24]) by gabe.freedesktop.org (Postfix) with ESMTPS id CCA0B10E16B; Sat, 15 Jul 2023 15:46:05 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=intel.com; i=@intel.com; q=dns/txt; s=Intel; t=1689435965; x=1720971965; h=from:to:cc:subject:date:message-id:mime-version: content-transfer-encoding; bh=TdOFQx8Xw8wGYYZnQLWKR6xssjx8vw+ImsQnbjJdtIg=; b=TNvI8483sirnZ5iamQDxnvWoFzWNQSMXWFmi8LS641BMiIGpULuqMDyy /K3I0wNf3Gd7lzgHc5AXFrK2+dzih9ZKDTV58sZlUokMigZPBCahOKt0R ati2LVfS3hSLqYbK0aKDqpq70FCY4mD7lya74zDMhPZObehy19Q9AGlSn BMRyUjfaBU/JiLgChE8uR9iXZcECIbNGDBX5FexecSlmvlK3ReE7sweEZ PbBAO1T7gmOLpV14RW5O+a+FygdHAfvSSG9ckqmGyilPhIHDMCd6/7tib HAzqP43QBsBeoxXdOm2fVujFe66+kBF/9iDu+6dXIPZHTDhTWgkItulOx A==; X-IronPort-AV: E=McAfee;i="6600,9927,10772"; a="368315030" X-IronPort-AV: E=Sophos;i="6.01,208,1684825200"; d="scan'208";a="368315030" Received: from orsmga002.jf.intel.com ([10.7.209.21]) by orsmga102.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 15 Jul 2023 08:46:04 -0700 X-ExtLoop1: 1 X-IronPort-AV: E=McAfee;i="6600,9927,10772"; a="722725472" X-IronPort-AV: E=Sophos;i="6.01,208,1684825200"; d="scan'208";a="722725472" Received: from sbint17x-mobl.gar.corp.intel.com (HELO thellstr-mobl1.intel.com) ([10.249.254.142]) by orsmga002-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 15 Jul 2023 08:46:01 -0700 From: =?utf-8?q?Thomas_Hellstr=C3=B6m?= To: intel-xe@lists.freedesktop.org Subject: [PATCH v5] Documentation/gpu: Add a VM_BIND async draft document Date: Sat, 15 Jul 2023 17:45:43 +0200 Message-ID: <20230715154543.13183-1-thomas.hellstrom@linux.intel.com> X-Mailer: git-send-email 2.41.0 MIME-Version: 1.0 X-BeenThere: dri-devel@lists.freedesktop.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Direct Rendering Infrastructure - Development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Matthew Brost , =?utf-8?q?Thomas_Hellstr=C3=B6m?= , Francois Dugast , linux-kernel@vger.kernel.org, Oak Zeng , Danilo Krummrich , dri-devel@lists.freedesktop.org, Nirmoy Das Errors-To: dri-devel-bounces@lists.freedesktop.org Sender: "dri-devel" 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 Acked-by: Nirmoy Das Reviewed-by: Matthew Brost Reviewed-by: Rodrigo Vivi --- 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 +`_. +and for the Xe driver `here +`_. + +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 -------------------------------