From patchwork Mon Mar 7 20:31:45 2022 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Niranjana Vishwanathapura X-Patchwork-Id: 12772304 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 0FA86C433F5 for ; Mon, 7 Mar 2022 20:30:08 +0000 (UTC) Received: from gabe.freedesktop.org (localhost [127.0.0.1]) by gabe.freedesktop.org (Postfix) with ESMTP id 7AB1410E17A; Mon, 7 Mar 2022 20:30:06 +0000 (UTC) Received: from mga02.intel.com (mga02.intel.com [134.134.136.20]) by gabe.freedesktop.org (Postfix) with ESMTPS id B12D010E173; Mon, 7 Mar 2022 20:30:05 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=intel.com; i=@intel.com; q=dns/txt; s=Intel; t=1646685005; x=1678221005; h=from:to:cc:subject:date:message-id:in-reply-to: references:mime-version:content-transfer-encoding; bh=/4F4SAwj8com9NdxTxbhkS5t+ooXG3AchyPGGvLaJgU=; b=j/kmm67geZxFdqJl8JclcFPf5cyXBV1JLREAu0SGQXUOAPC5oPHEwkO+ EMi7aW3YyQSk2SL+ERqDhR7qnX5wjr3zJ4xWqJMV1Snd/d4yzYO8ajkPi /WIPv3VXZ0tKQJkRqEZGRVdpjHzJ69DW0Qq1WZmrLXRA9yOy8JlTHAIye yhRmckrVFXYhoFV82ER3nO7lYPsY7Qy8TKNv7mu1UXojkeuPhqm7z0r/S vsfHeRvSk3Tdi6ZHNglD+m6TFTo3ydobI7GBQCewJwkrm9eQ/8JmaJms+ Xplagv90mLt6AlYYSyWabK6AhDG2bgFWtWN4rWkaBNqOFmDWoxxmPVUeY w==; X-IronPort-AV: E=McAfee;i="6200,9189,10279"; a="241932981" X-IronPort-AV: E=Sophos;i="5.90,163,1643702400"; d="scan'208";a="241932981" Received: from orsmga003.jf.intel.com ([10.7.209.27]) by orsmga101.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 07 Mar 2022 12:29:43 -0800 X-IronPort-AV: E=Sophos;i="5.90,163,1643702400"; d="scan'208";a="495188049" Received: from nvishwa1-desk.sc.intel.com ([172.25.29.76]) by orsmga003-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-SHA; 07 Mar 2022 12:29:41 -0800 From: Niranjana Vishwanathapura To: intel-gfx@lists.freedesktop.org, dri-devel@lists.freedesktop.org Subject: [RFC v2 1/2] drm/doc/rfc: VM_BIND feature design document Date: Mon, 7 Mar 2022 12:31:45 -0800 Message-Id: <20220307203146.648-2-niranjana.vishwanathapura@intel.com> X-Mailer: git-send-email 2.21.0.rc0.32.g243a4c7e27 In-Reply-To: <20220307203146.648-1-niranjana.vishwanathapura@intel.com> References: <20220307203146.648-1-niranjana.vishwanathapura@intel.com> 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: daniel.vetter@intel.com, thomas.hellstrom@intel.com, chris.p.wilson@intel.com Errors-To: dri-devel-bounces@lists.freedesktop.org Sender: "dri-devel" VM_BIND design document with description of intended use cases. Signed-off-by: Niranjana Vishwanathapura --- Documentation/gpu/rfc/i915_vm_bind.rst | 210 +++++++++++++++++++++++++ Documentation/gpu/rfc/index.rst | 4 + 2 files changed, 214 insertions(+) create mode 100644 Documentation/gpu/rfc/i915_vm_bind.rst diff --git a/Documentation/gpu/rfc/i915_vm_bind.rst b/Documentation/gpu/rfc/i915_vm_bind.rst new file mode 100644 index 000000000000..cdc6bb25b942 --- /dev/null +++ b/Documentation/gpu/rfc/i915_vm_bind.rst @@ -0,0 +1,210 @@ +========================================== +I915 VM_BIND feature design and use cases +========================================== + +VM_BIND feature +================ +DRM_I915_GEM_VM_BIND/UNBIND ioctls allows UMD to bind/unbind GEM buffer +objects (BOs) or sections of a BOs at specified GPU virtual addresses on +a specified address space (VM). + +These mappings (also referred to as persistent mappings) will be persistent +across multiple GPU submissions (execbuff) issued by the UMD, without user +having to provide a list of all required mappings during each submission +(as required by older execbuff mode). + +VM_BIND ioctl deferes binding the mappings until next execbuff submission +where it will be required, or immediately if I915_GEM_VM_BIND_IMMEDIATE +flag is set (useful if mapping is required for an active context). + +VM_BIND feature is advertised to user via I915_PARAM_HAS_VM_BIND. +User has to opt-in for VM_BIND mode of binding for an address space (VM) +during VM creation time via I915_VM_CREATE_FLAGS_USE_VM_BIND extension. +A VM in VM_BIND mode will not support older execbuff mode of binding. + +UMDs can still send BOs of these persistent mappings in execlist of execbuff +for specifying BO dependencies (implicit fencing) and to use BO as a batch, +but those BOs should be mapped ahead via vm_bind ioctl. + +VM_BIND features include, +- Multiple Virtual Address (VA) mappings can map to the same physical pages + of an object (aliasing). +- VA mapping can map to a partial section of the BO (partial binding). +- Support capture of persistent mappings in the dump upon GPU error. +- TLB is flushed upon unbind completion. Batching of TLB flushes in some + usecases will be helpful. +- Asynchronous vm_bind and vm_unbind support. +- VM_BIND uses user/memory fence mechanism for signaling bind completion + and for signaling batch completion in long running contexts (explained + below). + +VM_PRIVATE objects +------------------ +By default, BOs can be mapped on multiple VMs and can also be dma-buf +exported. Hence these BOs are referred to as Shared BOs. +During each execbuff submission, the request fence must be added to the +dma-resv fence list of all shared BOs mapped on the VM. + +VM_BIND feature introduces an optimization where user can create BO which +is private to a specified VM via I915_GEM_CREATE_EXT_VM_PRIVATE flag during +BO creation. Unlike Shared BOs, these VM private BOs can only be mapped on +the VM they are private to and can't be dma-buf exported. +All private BOs of a VM share the dma-resv object. Hence during each execbuff +submission, they need only one dma-resv fence list updated. Thus the fast +path (where required mappings are already bound) submission latency is O(1) +w.r.t the number of VM private BOs. + +VM_BIND locking hirarchy +------------------------- +VM_BIND locking order is as below. + +1) A vm_bind mutex will protect vm_bind lists. This lock is taken in vm_bind/ + vm_unbind ioctl calls, in the execbuff path and while releasing the mapping. + + In future, when GPU page faults are supported, we can potentially use a + rwsem instead, so that multiple pagefault handlers can take the read side + lock to lookup the mapping and hence can run in parallel. + +2) The BO's dma-resv lock will protect i915_vma state and needs to be held + while binding a vma and while updating dma-resv fence list of a BO. + The private BOs of a VM will all share a dma-resv object. + + This lock is held in vm_bind call for immediate binding, during vm_unbind + call for unbinding and during execbuff path for binding the mapping and + updating the dma-resv fence list of the BO. + +3) Spinlock/s to protect some of the VM's lists. + +We will also need support for bluk LRU movement of persistent mapping to +avoid additional latencies in execbuff path. + +GPU page faults +---------------- +Both older execbuff mode and the newer VM_BIND mode of binding will require +using dma-fence to ensure residency. +In future when GPU page faults are supported, no dma-fence usage is required +as residency is purely managed by installing and removing/invalidating ptes. + + +User/Memory Fence +================== +The idea is to take a user specified virtual address and install an interrupt +handler to wake up the current task when the memory location passes the user +supplied filter. + +User/Memory fence is a pair. To signal the user fence, +specified value will be written at the specified virtual address and +wakeup the waiting process. User can wait on an user fence with the +gem_wait_user_fence ioctl. + +It also allows the user to emit their own MI_FLUSH/PIPE_CONTROL notify +interrupt within their batches after updating the value to have sub-batch +precision on the wakeup. Each batch can signal an user fence to indicate +the completion of next level batch. The completion of very first level batch +needs to be signaled by the command streamer. The user must provide the +user/memory fence for this via the DRM_I915_GEM_EXECBUFFER_EXT_USER_FENCE +extension of execbuff ioctl, so that KMD can setup the command streamer to +signal it. + +User/Memory fence can also be supplied to the kernel driver to signal/wake up +the user process after completion of an asynchronous operation. + +When VM_BIND ioctl was provided with a user/memory fence via the +I915_VM_BIND_EXT_USER_FENCE extension, it will be signaled upon the completion +of binding of that mapping. All async binds/unbinds are serialized, hence +signaling of user/memory fence also indicate the completion of all previous +binds/unbinds. + +This feature will be derived from the below original work: +https://patchwork.freedesktop.org/patch/349417/ + + +VM_BIND use cases +================== + +Long running Compute contexts +------------------------------ +Usage of dma-fence expects that they complete in reasonable amount of time. +Compute on the other hand can be long running. Hence it is appropriate for +compute to use user/memory fence and dma-fence usage will be limited to +in-kernel consumption only. This requires an execbuff uapi extension to pass +in user fence. Compute must opt-in for this mechanism with +I915_CONTEXT_CREATE_FLAGS_LONG_RUNNING flag during context creation. + +The dma-fence based user interfaces like gem_wait ioctl, execbuff out fence +and implicit dependency setting is not allowed on long running contexts. + +Where GPU page faults are not available, kernel driver upon buffer invalidation +will initiate a suspend (preemption) of long running context with a dma-fence +attached to it. And upon completion of that suspend fence, finish the +invalidation, revalidate the BO and then resume the compute context. This is +done by having a per-context fence (called suspend fence) proxying as +i915_request fence. This suspend fence is enabled when there is a wait on it, +which triggers the context preemption. + +This is much easier to support with VM_BIND compared to the current heavier +execbuff path resource attachment. + +Low Latency Submission +----------------------- +Allows compute UMD to directly submit GPU jobs instead of through execbuff +ioctl. VM_BIND allows map/unmap of BOs required for directly submitted jobs. + +Debugger +--------- +With debug event interface user space process (debugger) is able to keep track +of and act upon resources created by another process (debuggee) and attached +to GPU via vm_bind interface. + +Mesa/Valkun +------------ +VM_BIND can potentially reduce the CPU-overhead in Mesa thus improving +performance. For Vulkan it should be straightforward to use VM_BIND. +For Iris implicit buffer tracking must be implemented before we can harness +VM_BIND benefits. With increasing GPU hardware performance reducing CPU +overhead becomes more important. + +Page level hints settings +-------------------------- +VM_BIND allows any hints setting per mapping instead of per BO. +Possible hints include read-only, placement and atomicity. +Sub-BO level placement hint will be even more relevant with +upcoming GPU on-demand page fault support. + +Page level Cache/CLOS settings +------------------------------- +VM_BIND allows cache/CLOS settings per mapping instead of per BO. + +Shared Virtual Memory (SVM) support +------------------------------------ +VM_BIND interface can be used to map system memory directly (without gem BO +abstraction) using the HMM interface. + + +Broder i915 cleanups +===================== +Supporting this whole new vm_bind mode of binding which comes with its own +usecases to support and the locking requirements requires proper integration +with the existing i915 driver. This calls for some broader i915 driver +cleanups/simplifications for maintainability of the driver going forward. +Here are few things identified and are being looked into. + +- Make pagetable allocations evictable and manage them similar to VM_BIND + mapped objects. Page table pages are similar to persistent mappings of a + VM (difference here are that the page table pages will not + have an i915_vma structure and after swapping pages back in, parent page + link needs to be updated). +- Remove vma lookup cache (eb->gem_context->handles_vma). VM_BIND feature + do not use it and complexity it brings in is probably more than the + performance advantage we get in legacy execbuff case. +- Remove vma->open_count counting +- Remove i915_vma active reference tracking. Instead use underlying BO's + dma-resv fence list to determine if a i915_vma is active or not. + +These can be worked upon after intitial vm_bind support is added. + + +UAPI +===== +Uapi definiton can be found here: +.. kernel-doc:: Documentation/gpu/rfc/i915_vm_bind.h diff --git a/Documentation/gpu/rfc/index.rst b/Documentation/gpu/rfc/index.rst index 91e93a705230..7d10c36b268d 100644 --- a/Documentation/gpu/rfc/index.rst +++ b/Documentation/gpu/rfc/index.rst @@ -23,3 +23,7 @@ host such documentation: .. toctree:: i915_scheduler.rst + +.. toctree:: + + i915_vm_bind.rst