1.. SPDX-License-Identifier: GPL-2.0 2 3=============== 4DMA and swiotlb 5=============== 6 7swiotlb is a memory buffer allocator used by the Linux kernel DMA layer. It is 8typically used when a device doing DMA can't directly access the target memory 9buffer because of hardware limitations or other requirements. In such a case, 10the DMA layer calls swiotlb to allocate a temporary memory buffer that conforms 11to the limitations. The DMA is done to/from this temporary memory buffer, and 12the CPU copies the data between the temporary buffer and the original target 13memory buffer. This approach is generically called "bounce buffering", and the 14temporary memory buffer is called a "bounce buffer". 15 16Device drivers don't interact directly with swiotlb. Instead, drivers inform 17the DMA layer of the DMA attributes of the devices they are managing, and use 18the normal DMA map, unmap, and sync APIs when programming a device to do DMA. 19These APIs use the device DMA attributes and kernel-wide settings to determine 20if bounce buffering is necessary. If so, the DMA layer manages the allocation, 21freeing, and sync'ing of bounce buffers. Since the DMA attributes are per 22device, some devices in a system may use bounce buffering while others do not. 23 24Because the CPU copies data between the bounce buffer and the original target 25memory buffer, doing bounce buffering is slower than doing DMA directly to the 26original memory buffer, and it consumes more CPU resources. So it is used only 27when necessary for providing DMA functionality. 28 29Usage Scenarios 30--------------- 31swiotlb was originally created to handle DMA for devices with addressing 32limitations. As physical memory sizes grew beyond 4 GiB, some devices could 33only provide 32-bit DMA addresses. By allocating bounce buffer memory below 34the 4 GiB line, these devices with addressing limitations could still work and 35do DMA. 36 37More recently, Confidential Computing (CoCo) VMs have the guest VM's memory 38encrypted by default, and the memory is not accessible by the host hypervisor 39and VMM. For the host to do I/O on behalf of the guest, the I/O must be 40directed to guest memory that is unencrypted. CoCo VMs set a kernel-wide option 41to force all DMA I/O to use bounce buffers, and the bounce buffer memory is set 42up as unencrypted. The host does DMA I/O to/from the bounce buffer memory, and 43the Linux kernel DMA layer does "sync" operations to cause the CPU to copy the 44data to/from the original target memory buffer. The CPU copying bridges between 45the unencrypted and the encrypted memory. This use of bounce buffers allows 46device drivers to "just work" in a CoCo VM, with no modifications 47needed to handle the memory encryption complexity. 48 49Other edge case scenarios arise for bounce buffers. For example, when IOMMU 50mappings are set up for a DMA operation to/from a device that is considered 51"untrusted", the device should be given access only to the memory containing 52the data being transferred. But if that memory occupies only part of an IOMMU 53granule, other parts of the granule may contain unrelated kernel data. Since 54IOMMU access control is per-granule, the untrusted device can gain access to 55the unrelated kernel data. This problem is solved by bounce buffering the DMA 56operation and ensuring that unused portions of the bounce buffers do not 57contain any unrelated kernel data. 58 59Core Functionality 60------------------ 61The primary swiotlb APIs are swiotlb_tbl_map_single() and 62swiotlb_tbl_unmap_single(). The "map" API allocates a bounce buffer of a 63specified size in bytes and returns the physical address of the buffer. The 64buffer memory is physically contiguous. The expectation is that the DMA layer 65maps the physical memory address to a DMA address, and returns the DMA address 66to the driver for programming into the device. If a DMA operation specifies 67multiple memory buffer segments, a separate bounce buffer must be allocated for 68each segment. swiotlb_tbl_map_single() always does a "sync" operation (i.e., a 69CPU copy) to initialize the bounce buffer to match the contents of the original 70buffer. 71 72swiotlb_tbl_unmap_single() does the reverse. If the DMA operation might have 73updated the bounce buffer memory and DMA_ATTR_SKIP_CPU_SYNC is not set, the 74unmap does a "sync" operation to cause a CPU copy of the data from the bounce 75buffer back to the original buffer. Then the bounce buffer memory is freed. 76 77swiotlb also provides "sync" APIs that correspond to the dma_sync_*() APIs that 78a driver may use when control of a buffer transitions between the CPU and the 79device. The swiotlb "sync" APIs cause a CPU copy of the data between the 80original buffer and the bounce buffer. Like the dma_sync_*() APIs, the swiotlb 81"sync" APIs support doing a partial sync, where only a subset of the bounce 82buffer is copied to/from the original buffer. 83 84Core Functionality Constraints 85------------------------------ 86The swiotlb map/unmap/sync APIs must operate without blocking, as they are 87called by the corresponding DMA APIs which may run in contexts that cannot 88block. Hence the default memory pool for swiotlb allocations must be 89pre-allocated at boot time (but see Dynamic swiotlb below). Because swiotlb 90allocations must be physically contiguous, the entire default memory pool is 91allocated as a single contiguous block. 92 93The need to pre-allocate the default swiotlb pool creates a boot-time tradeoff. 94The pool should be large enough to ensure that bounce buffer requests can 95always be satisfied, as the non-blocking requirement means requests can't wait 96for space to become available. But a large pool potentially wastes memory, as 97this pre-allocated memory is not available for other uses in the system. The 98tradeoff is particularly acute in CoCo VMs that use bounce buffers for all DMA 99I/O. These VMs use a heuristic to set the default pool size to ~6% of memory, 100with a max of 1 GiB, which has the potential to be very wasteful of memory. 101Conversely, the heuristic might produce a size that is insufficient, depending 102on the I/O patterns of the workload in the VM. The dynamic swiotlb feature 103described below can help, but has limitations. Better management of the swiotlb 104default memory pool size remains an open issue. 105 106A single allocation from swiotlb is limited to IO_TLB_SIZE * IO_TLB_SEGSIZE 107bytes, which is 256 KiB with current definitions. When a device's DMA settings 108are such that the device might use swiotlb, the maximum size of a DMA segment 109must be limited to that 256 KiB. This value is communicated to higher-level 110kernel code via dma_map_mapping_size() and swiotlb_max_mapping_size(). If the 111higher-level code fails to account for this limit, it may make requests that 112are too large for swiotlb, and get a "swiotlb full" error. 113 114A key device DMA setting is "min_align_mask", which is a power of 2 minus 1 115so that some number of low order bits are set, or it may be zero. swiotlb 116allocations ensure these min_align_mask bits of the physical address of the 117bounce buffer match the same bits in the address of the original buffer. When 118min_align_mask is non-zero, it may produce an "alignment offset" in the address 119of the bounce buffer that slightly reduces the maximum size of an allocation. 120This potential alignment offset is reflected in the value returned by 121swiotlb_max_mapping_size(), which can show up in places like 122/sys/block/<device>/queue/max_sectors_kb. For example, if a device does not use 123swiotlb, max_sectors_kb might be 512 KiB or larger. If a device might use 124swiotlb, max_sectors_kb will be 256 KiB. When min_align_mask is non-zero, 125max_sectors_kb might be even smaller, such as 252 KiB. 126 127swiotlb_tbl_map_single() also takes an "alloc_align_mask" parameter. This 128parameter specifies the allocation of bounce buffer space must start at a 129physical address with the alloc_align_mask bits set to zero. But the actual 130bounce buffer might start at a larger address if min_align_mask is non-zero. 131Hence there may be pre-padding space that is allocated prior to the start of 132the bounce buffer. Similarly, the end of the bounce buffer is rounded up to an 133alloc_align_mask boundary, potentially resulting in post-padding space. Any 134pre-padding or post-padding space is not initialized by swiotlb code. The 135"alloc_align_mask" parameter is used by IOMMU code when mapping for untrusted 136devices. It is set to the granule size - 1 so that the bounce buffer is 137allocated entirely from granules that are not used for any other purpose. 138 139Data structures concepts 140------------------------ 141Memory used for swiotlb bounce buffers is allocated from overall system memory 142as one or more "pools". The default pool is allocated during system boot with a 143default size of 64 MiB. The default pool size may be modified with the 144"swiotlb=" kernel boot line parameter. The default size may also be adjusted 145due to other conditions, such as running in a CoCo VM, as described above. If 146CONFIG_SWIOTLB_DYNAMIC is enabled, additional pools may be allocated later in 147the life of the system. Each pool must be a contiguous range of physical 148memory. The default pool is allocated below the 4 GiB physical address line so 149it works for devices that can only address 32-bits of physical memory (unless 150architecture-specific code provides the SWIOTLB_ANY flag). In a CoCo VM, the 151pool memory must be decrypted before swiotlb is used. 152 153Each pool is divided into "slots" of size IO_TLB_SIZE, which is 2 KiB with 154current definitions. IO_TLB_SEGSIZE contiguous slots (128 slots) constitute 155what might be called a "slot set". When a bounce buffer is allocated, it 156occupies one or more contiguous slots. A slot is never shared by multiple 157bounce buffers. Furthermore, a bounce buffer must be allocated from a single 158slot set, which leads to the maximum bounce buffer size being IO_TLB_SIZE * 159IO_TLB_SEGSIZE. Multiple smaller bounce buffers may co-exist in a single slot 160set if the alignment and size constraints can be met. 161 162Slots are also grouped into "areas", with the constraint that a slot set exists 163entirely in a single area. Each area has its own spin lock that must be held to 164manipulate the slots in that area. The division into areas avoids contending 165for a single global spin lock when swiotlb is heavily used, such as in a CoCo 166VM. The number of areas defaults to the number of CPUs in the system for 167maximum parallelism, but since an area can't be smaller than IO_TLB_SEGSIZE 168slots, it might be necessary to assign multiple CPUs to the same area. The 169number of areas can also be set via the "swiotlb=" kernel boot parameter. 170 171When allocating a bounce buffer, if the area associated with the calling CPU 172does not have enough free space, areas associated with other CPUs are tried 173sequentially. For each area tried, the area's spin lock must be obtained before 174trying an allocation, so contention may occur if swiotlb is relatively busy 175overall. But an allocation request does not fail unless all areas do not have 176enough free space. 177 178IO_TLB_SIZE, IO_TLB_SEGSIZE, and the number of areas must all be powers of 2 as 179the code uses shifting and bit masking to do many of the calculations. The 180number of areas is rounded up to a power of 2 if necessary to meet this 181requirement. 182 183The default pool is allocated with PAGE_SIZE alignment. If an alloc_align_mask 184argument to swiotlb_tbl_map_single() specifies a larger alignment, one or more 185initial slots in each slot set might not meet the alloc_align_mask criterium. 186Because a bounce buffer allocation can't cross a slot set boundary, eliminating 187those initial slots effectively reduces the max size of a bounce buffer. 188Currently, there's no problem because alloc_align_mask is set based on IOMMU 189granule size, and granules cannot be larger than PAGE_SIZE. But if that were to 190change in the future, the initial pool allocation might need to be done with 191alignment larger than PAGE_SIZE. 192 193Dynamic swiotlb 194--------------- 195When CONFIG_DYNAMIC_SWIOTLB is enabled, swiotlb can do on-demand expansion of 196the amount of memory available for allocation as bounce buffers. If a bounce 197buffer request fails due to lack of available space, an asynchronous background 198task is kicked off to allocate memory from general system memory and turn it 199into an swiotlb pool. Creating an additional pool must be done asynchronously 200because the memory allocation may block, and as noted above, swiotlb requests 201are not allowed to block. Once the background task is kicked off, the bounce 202buffer request creates a "transient pool" to avoid returning an "swiotlb full" 203error. A transient pool has the size of the bounce buffer request, and is 204deleted when the bounce buffer is freed. Memory for this transient pool comes 205from the general system memory atomic pool so that creation does not block. 206Creating a transient pool has relatively high cost, particularly in a CoCo VM 207where the memory must be decrypted, so it is done only as a stopgap until the 208background task can add another non-transient pool. 209 210Adding a dynamic pool has limitations. Like with the default pool, the memory 211must be physically contiguous, so the size is limited to MAX_PAGE_ORDER pages 212(e.g., 4 MiB on a typical x86 system). Due to memory fragmentation, a max size 213allocation may not be available. The dynamic pool allocator tries smaller sizes 214until it succeeds, but with a minimum size of 1 MiB. Given sufficient system 215memory fragmentation, dynamically adding a pool might not succeed at all. 216 217The number of areas in a dynamic pool may be different from the number of areas 218in the default pool. Because the new pool size is typically a few MiB at most, 219the number of areas will likely be smaller. For example, with a new pool size 220of 4 MiB and the 256 KiB minimum area size, only 16 areas can be created. If 221the system has more than 16 CPUs, multiple CPUs must share an area, creating 222more lock contention. 223 224New pools added via dynamic swiotlb are linked together in a linear list. 225swiotlb code frequently must search for the pool containing a particular 226swiotlb physical address, so that search is linear and not performant with a 227large number of dynamic pools. The data structures could be improved for 228faster searches. 229 230Overall, dynamic swiotlb works best for small configurations with relatively 231few CPUs. It allows the default swiotlb pool to be smaller so that memory is 232not wasted, with dynamic pools making more space available if needed (as long 233as fragmentation isn't an obstacle). It is less useful for large CoCo VMs. 234 235Data Structure Details 236---------------------- 237swiotlb is managed with four primary data structures: io_tlb_mem, io_tlb_pool, 238io_tlb_area, and io_tlb_slot. io_tlb_mem describes a swiotlb memory allocator, 239which includes the default memory pool and any dynamic or transient pools 240linked to it. Limited statistics on swiotlb usage are kept per memory allocator 241and are stored in this data structure. These statistics are available under 242/sys/kernel/debug/swiotlb when CONFIG_DEBUG_FS is set. 243 244io_tlb_pool describes a memory pool, either the default pool, a dynamic pool, 245or a transient pool. The description includes the start and end addresses of 246the memory in the pool, a pointer to an array of io_tlb_area structures, and a 247pointer to an array of io_tlb_slot structures that are associated with the pool. 248 249io_tlb_area describes an area. The primary field is the spin lock used to 250serialize access to slots in the area. The io_tlb_area array for a pool has an 251entry for each area, and is accessed using a 0-based area index derived from the 252calling processor ID. Areas exist solely to allow parallel access to swiotlb 253from multiple CPUs. 254 255io_tlb_slot describes an individual memory slot in the pool, with size 256IO_TLB_SIZE (2 KiB currently). The io_tlb_slot array is indexed by the slot 257index computed from the bounce buffer address relative to the starting memory 258address of the pool. The size of struct io_tlb_slot is 24 bytes, so the 259overhead is about 1% of the slot size. 260 261The io_tlb_slot array is designed to meet several requirements. First, the DMA 262APIs and the corresponding swiotlb APIs use the bounce buffer address as the 263identifier for a bounce buffer. This address is returned by 264swiotlb_tbl_map_single(), and then passed as an argument to 265swiotlb_tbl_unmap_single() and the swiotlb_sync_*() functions. The original 266memory buffer address obviously must be passed as an argument to 267swiotlb_tbl_map_single(), but it is not passed to the other APIs. Consequently, 268swiotlb data structures must save the original memory buffer address so that it 269can be used when doing sync operations. This original address is saved in the 270io_tlb_slot array. 271 272Second, the io_tlb_slot array must handle partial sync requests. In such cases, 273the argument to swiotlb_sync_*() is not the address of the start of the bounce 274buffer but an address somewhere in the middle of the bounce buffer, and the 275address of the start of the bounce buffer isn't known to swiotlb code. But 276swiotlb code must be able to calculate the corresponding original memory buffer 277address to do the CPU copy dictated by the "sync". So an adjusted original 278memory buffer address is populated into the struct io_tlb_slot for each slot 279occupied by the bounce buffer. An adjusted "alloc_size" of the bounce buffer is 280also recorded in each struct io_tlb_slot so a sanity check can be performed on 281the size of the "sync" operation. The "alloc_size" field is not used except for 282the sanity check. 283 284Third, the io_tlb_slot array is used to track available slots. The "list" field 285in struct io_tlb_slot records how many contiguous available slots exist starting 286at that slot. A "0" indicates that the slot is occupied. A value of "1" 287indicates only the current slot is available. A value of "2" indicates the 288current slot and the next slot are available, etc. The maximum value is 289IO_TLB_SEGSIZE, which can appear in the first slot in a slot set, and indicates 290that the entire slot set is available. These values are used when searching for 291available slots to use for a new bounce buffer. They are updated when allocating 292a new bounce buffer and when freeing a bounce buffer. At pool creation time, the 293"list" field is initialized to IO_TLB_SEGSIZE down to 1 for the slots in every 294slot set. 295 296Fourth, the io_tlb_slot array keeps track of any "padding slots" allocated to 297meet alloc_align_mask requirements described above. When 298swiotlb_tlb_map_single() allocates bounce buffer space to meet alloc_align_mask 299requirements, it may allocate pre-padding space across zero or more slots. But 300when swiotbl_tlb_unmap_single() is called with the bounce buffer address, the 301alloc_align_mask value that governed the allocation, and therefore the 302allocation of any padding slots, is not known. The "pad_slots" field records 303the number of padding slots so that swiotlb_tbl_unmap_single() can free them. 304The "pad_slots" value is recorded only in the first non-padding slot allocated 305to the bounce buffer. 306 307Restricted pools 308---------------- 309The swiotlb machinery is also used for "restricted pools", which are pools of 310memory separate from the default swiotlb pool, and that are dedicated for DMA 311use by a particular device. Restricted pools provide a level of DMA memory 312protection on systems with limited hardware protection capabilities, such as 313those lacking an IOMMU. Such usage is specified by DeviceTree entries and 314requires that CONFIG_DMA_RESTRICTED_POOL is set. Each restricted pool is based 315on its own io_tlb_mem data structure that is independent of the main swiotlb 316io_tlb_mem. 317 318Restricted pools add swiotlb_alloc() and swiotlb_free() APIs, which are called 319from the dma_alloc_*() and dma_free_*() APIs. The swiotlb_alloc/free() APIs 320allocate/free slots from/to the restricted pool directly and do not go through 321swiotlb_tbl_map/unmap_single(). 322