1 ========================================== 2 I915 VM_BIND feature design and use cases 3 ========================================== 4 5 VM_BIND feature 6 ================ 7 DRM_I915_GEM_VM_BIND/UNBIND ioctls allows UMD to bind/unbind GEM buffer 8 objects (BOs) or sections of a BOs at specified GPU virtual addresses on a 9 specified address space (VM). These mappings (also referred to as persistent 10 mappings) will be persistent across multiple GPU submissions (execbuf calls) 11 issued by the UMD, without user having to provide a list of all required 12 mappings during each submission (as required by older execbuf mode). 13 14 The VM_BIND/UNBIND calls allow UMDs to request a timeline out fence for 15 signaling the completion of bind/unbind operation. 16 17 VM_BIND feature is advertised to user via I915_PARAM_VM_BIND_VERSION. 18 User has to opt-in for VM_BIND mode of binding for an address space (VM) 19 during VM creation time via I915_VM_CREATE_FLAGS_USE_VM_BIND extension. 20 21 VM_BIND/UNBIND ioctl calls executed on different CPU threads concurrently are 22 not ordered. Furthermore, parts of the VM_BIND/UNBIND operations can be done 23 asynchronously, when valid out fence is specified. 24 25 VM_BIND features include: 26 27 * Multiple Virtual Address (VA) mappings can map to the same physical pages 28 of an object (aliasing). 29 * VA mapping can map to a partial section of the BO (partial binding). 30 * Support capture of persistent mappings in the dump upon GPU error. 31 * Support for userptr gem objects (no special uapi is required for this). 32 33 TLB flush consideration 34 ------------------------ 35 The i915 driver flushes the TLB for each submission and when an object's 36 pages are released. The VM_BIND/UNBIND operation will not do any additional 37 TLB flush. Any VM_BIND mapping added will be in the working set for subsequent 38 submissions on that VM and will not be in the working set for currently running 39 batches (which would require additional TLB flushes, which is not supported). 40 41 Execbuf ioctl in VM_BIND mode 42 ------------------------------- 43 A VM in VM_BIND mode will not support older execbuf mode of binding. 44 The execbuf ioctl handling in VM_BIND mode differs significantly from the 45 older execbuf2 ioctl (See struct drm_i915_gem_execbuffer2). 46 Hence, a new execbuf3 ioctl has been added to support VM_BIND mode. (See 47 struct drm_i915_gem_execbuffer3). The execbuf3 ioctl will not accept any 48 execlist. Hence, no support for implicit sync. It is expected that the below 49 work will be able to support requirements of object dependency setting in all 50 use cases: 51 52 "dma-buf: Add an API for exporting sync files" 53 (https://lwn.net/Articles/859290/) 54 55 The new execbuf3 ioctl only works in VM_BIND mode and the VM_BIND mode only 56 works with execbuf3 ioctl for submission. All BOs mapped on that VM (through 57 VM_BIND call) at the time of execbuf3 call are deemed required for that 58 submission. 59 60 The execbuf3 ioctl directly specifies the batch addresses instead of as 61 object handles as in execbuf2 ioctl. The execbuf3 ioctl will also not 62 support many of the older features like in/out/submit fences, fence array, 63 default gem context and many more (See struct drm_i915_gem_execbuffer3). 64 65 In VM_BIND mode, VA allocation is completely managed by the user instead of 66 the i915 driver. Hence all VA assignment, eviction are not applicable in 67 VM_BIND mode. Also, for determining object activeness, VM_BIND mode will not 68 be using the i915_vma active reference tracking. It will instead use dma-resv 69 object for that (See `VM_BIND dma_resv usage`_). 70 71 So, a lot of existing code supporting execbuf2 ioctl, like relocations, VA 72 evictions, vma lookup table, implicit sync, vma active reference tracking etc., 73 are not applicable for execbuf3 ioctl. Hence, all execbuf3 specific handling 74 should be in a separate file and only functionalities common to these ioctls 75 can be the shared code where possible. 76 77 VM_PRIVATE objects 78 ------------------- 79 By default, BOs can be mapped on multiple VMs and can also be dma-buf 80 exported. Hence these BOs are referred to as Shared BOs. 81 During each execbuf submission, the request fence must be added to the 82 dma-resv fence list of all shared BOs mapped on the VM. 83 84 VM_BIND feature introduces an optimization where user can create BO which 85 is private to a specified VM via I915_GEM_CREATE_EXT_VM_PRIVATE flag during 86 BO creation. Unlike Shared BOs, these VM private BOs can only be mapped on 87 the VM they are private to and can't be dma-buf exported. 88 All private BOs of a VM share the dma-resv object. Hence during each execbuf 89 submission, they need only one dma-resv fence list updated. Thus, the fast 90 path (where required mappings are already bound) submission latency is O(1) 91 w.r.t the number of VM private BOs. 92 93 VM_BIND locking hierarchy 94 ------------------------- 95 The locking design here supports the older (execlist based) execbuf mode, the 96 newer VM_BIND mode, the VM_BIND mode with GPU page faults and possible future 97 system allocator support (See `Shared Virtual Memory (SVM) support`_). 98 The older execbuf mode and the newer VM_BIND mode without page faults manages 99 residency of backing storage using dma_fence. The VM_BIND mode with page faults 100 and the system allocator support do not use any dma_fence at all. 101 102 VM_BIND locking order is as below. 103 104 1) Lock-A: A vm_bind mutex will protect vm_bind lists. This lock is taken in 105 vm_bind/vm_unbind ioctl calls, in the execbuf path and while releasing the 106 mapping. 107 108 In future, when GPU page faults are supported, we can potentially use a 109 rwsem instead, so that multiple page fault handlers can take the read side 110 lock to lookup the mapping and hence can run in parallel. 111 The older execbuf mode of binding do not need this lock. 112 113 2) Lock-B: The object's dma-resv lock will protect i915_vma state and needs to 114 be held while binding/unbinding a vma in the async worker and while updating 115 dma-resv fence list of an object. Note that private BOs of a VM will all 116 share a dma-resv object. 117 118 The future system allocator support will use the HMM prescribed locking 119 instead. 120 121 3) Lock-C: Spinlock/s to protect some of the VM's lists like the list of 122 invalidated vmas (due to eviction and userptr invalidation) etc. 123 124 When GPU page faults are supported, the execbuf path do not take any of these 125 locks. There we will simply smash the new batch buffer address into the ring and 126 then tell the scheduler run that. The lock taking only happens from the page 127 fault handler, where we take lock-A in read mode, whichever lock-B we need to 128 find the backing storage (dma_resv lock for gem objects, and hmm/core mm for 129 system allocator) and some additional locks (lock-D) for taking care of page 130 table races. Page fault mode should not need to ever manipulate the vm lists, 131 so won't ever need lock-C. 132 133 VM_BIND LRU handling 134 --------------------- 135 We need to ensure VM_BIND mapped objects are properly LRU tagged to avoid 136 performance degradation. We will also need support for bulk LRU movement of 137 VM_BIND objects to avoid additional latencies in execbuf path. 138 139 The page table pages are similar to VM_BIND mapped objects (See 140 `Evictable page table allocations`_) and are maintained per VM and needs to 141 be pinned in memory when VM is made active (ie., upon an execbuf call with 142 that VM). So, bulk LRU movement of page table pages is also needed. 143 144 VM_BIND dma_resv usage 145 ----------------------- 146 Fences needs to be added to all VM_BIND mapped objects. During each execbuf 147 submission, they are added with DMA_RESV_USAGE_BOOKKEEP usage to prevent 148 over sync (See enum dma_resv_usage). One can override it with either 149 DMA_RESV_USAGE_READ or DMA_RESV_USAGE_WRITE usage during explicit object 150 dependency setting. 151 152 Note that DRM_I915_GEM_WAIT and DRM_I915_GEM_BUSY ioctls do not check for 153 DMA_RESV_USAGE_BOOKKEEP usage and hence should not be used for end of batch 154 check. Instead, the execbuf3 out fence should be used for end of batch check 155 (See struct drm_i915_gem_execbuffer3). 156 157 Also, in VM_BIND mode, use dma-resv apis for determining object activeness 158 (See dma_resv_test_signaled() and dma_resv_wait_timeout()) and do not use the 159 older i915_vma active reference tracking which is deprecated. This should be 160 easier to get it working with the current TTM backend. 161 162 Mesa use case 163 -------------- 164 VM_BIND can potentially reduce the CPU overhead in Mesa (both Vulkan and Iris), 165 hence improving performance of CPU-bound applications. It also allows us to 166 implement Vulkan's Sparse Resources. With increasing GPU hardware performance, 167 reducing CPU overhead becomes more impactful. 168 169 170 Other VM_BIND use cases 171 ======================== 172 173 Long running Compute contexts 174 ------------------------------ 175 Usage of dma-fence expects that they complete in reasonable amount of time. 176 Compute on the other hand can be long running. Hence it is appropriate for 177 compute to use user/memory fence (See `User/Memory Fence`_) and dma-fence usage 178 must be limited to in-kernel consumption only. 179 180 Where GPU page faults are not available, kernel driver upon buffer invalidation 181 will initiate a suspend (preemption) of long running context, finish the 182 invalidation, revalidate the BO and then resume the compute context. This is 183 done by having a per-context preempt fence which is enabled when someone tries 184 to wait on it and triggers the context preemption. 185 186 User/Memory Fence 187 ~~~~~~~~~~~~~~~~~~ 188 User/Memory fence is a <address, value> pair. To signal the user fence, the 189 specified value will be written at the specified virtual address and wakeup the 190 waiting process. User fence can be signaled either by the GPU or kernel async 191 worker (like upon bind completion). User can wait on a user fence with a new 192 user fence wait ioctl. 193 194 Here is some prior work on this: 195 https://patchwork.freedesktop.org/patch/349417/ 196 197 Low Latency Submission 198 ~~~~~~~~~~~~~~~~~~~~~~~ 199 Allows compute UMD to directly submit GPU jobs instead of through execbuf 200 ioctl. This is made possible by VM_BIND is not being synchronized against 201 execbuf. VM_BIND allows bind/unbind of mappings required for the directly 202 submitted jobs. 203 204 Debugger 205 --------- 206 With debug event interface user space process (debugger) is able to keep track 207 of and act upon resources created by another process (debugged) and attached 208 to GPU via vm_bind interface. 209 210 GPU page faults 211 ---------------- 212 GPU page faults when supported (in future), will only be supported in the 213 VM_BIND mode. While both the older execbuf mode and the newer VM_BIND mode of 214 binding will require using dma-fence to ensure residency, the GPU page faults 215 mode when supported, will not use any dma-fence as residency is purely managed 216 by installing and removing/invalidating page table entries. 217 218 Page level hints settings 219 -------------------------- 220 VM_BIND allows any hints setting per mapping instead of per BO. Possible hints 221 include placement and atomicity. Sub-BO level placement hint will be even more 222 relevant with upcoming GPU on-demand page fault support. 223 224 Page level Cache/CLOS settings 225 ------------------------------- 226 VM_BIND allows cache/CLOS settings per mapping instead of per BO. 227 228 Evictable page table allocations 229 --------------------------------- 230 Make pagetable allocations evictable and manage them similar to VM_BIND 231 mapped objects. Page table pages are similar to persistent mappings of a 232 VM (difference here are that the page table pages will not have an i915_vma 233 structure and after swapping pages back in, parent page link needs to be 234 updated). 235 236 Shared Virtual Memory (SVM) support 237 ------------------------------------ 238 VM_BIND interface can be used to map system memory directly (without gem BO 239 abstraction) using the HMM interface. SVM is only supported with GPU page 240 faults enabled. 241 242 VM_BIND UAPI 243 ============= 244 245 .. kernel-doc:: Documentation/gpu/rfc/i915_vm_bind.h
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