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diff --git a/Documentation/core-api/swiotlb.rst b/Documentation/core-api/swiotlb.rst new file mode 100644 index 000000000000..5ad2c9ca85bc --- /dev/null +++ b/Documentation/core-api/swiotlb.rst @@ -0,0 +1,321 @@ +.. SPDX-License-Identifier: GPL-2.0 + +=============== +DMA and swiotlb +=============== + +swiotlb is a memory buffer allocator used by the Linux kernel DMA layer. It is +typically used when a device doing DMA can't directly access the target memory +buffer because of hardware limitations or other requirements. In such a case, +the DMA layer calls swiotlb to allocate a temporary memory buffer that conforms +to the limitations. The DMA is done to/from this temporary memory buffer, and +the CPU copies the data between the temporary buffer and the original target +memory buffer. This approach is generically called "bounce buffering", and the +temporary memory buffer is called a "bounce buffer". + +Device drivers don't interact directly with swiotlb. Instead, drivers inform +the DMA layer of the DMA attributes of the devices they are managing, and use +the normal DMA map, unmap, and sync APIs when programming a device to do DMA. +These APIs use the device DMA attributes and kernel-wide settings to determine +if bounce buffering is necessary. If so, the DMA layer manages the allocation, +freeing, and sync'ing of bounce buffers. Since the DMA attributes are per +device, some devices in a system may use bounce buffering while others do not. + +Because the CPU copies data between the bounce buffer and the original target +memory buffer, doing bounce buffering is slower than doing DMA directly to the +original memory buffer, and it consumes more CPU resources. So it is used only +when necessary for providing DMA functionality. + +Usage Scenarios +--------------- +swiotlb was originally created to handle DMA for devices with addressing +limitations. As physical memory sizes grew beyond 4 GiB, some devices could +only provide 32-bit DMA addresses. By allocating bounce buffer memory below +the 4 GiB line, these devices with addressing limitations could still work and +do DMA. + +More recently, Confidential Computing (CoCo) VMs have the guest VM's memory +encrypted by default, and the memory is not accessible by the host hypervisor +and VMM. For the host to do I/O on behalf of the guest, the I/O must be +directed to guest memory that is unencrypted. CoCo VMs set a kernel-wide option +to force all DMA I/O to use bounce buffers, and the bounce buffer memory is set +up as unencrypted. The host does DMA I/O to/from the bounce buffer memory, and +the Linux kernel DMA layer does "sync" operations to cause the CPU to copy the +data to/from the original target memory buffer. The CPU copying bridges between +the unencrypted and the encrypted memory. This use of bounce buffers allows +device drivers to "just work" in a CoCo VM, with no modifications +needed to handle the memory encryption complexity. + +Other edge case scenarios arise for bounce buffers. For example, when IOMMU +mappings are set up for a DMA operation to/from a device that is considered +"untrusted", the device should be given access only to the memory containing +the data being transferred. But if that memory occupies only part of an IOMMU +granule, other parts of the granule may contain unrelated kernel data. Since +IOMMU access control is per-granule, the untrusted device can gain access to +the unrelated kernel data. This problem is solved by bounce buffering the DMA +operation and ensuring that unused portions of the bounce buffers do not +contain any unrelated kernel data. + +Core Functionality +------------------ +The primary swiotlb APIs are swiotlb_tbl_map_single() and +swiotlb_tbl_unmap_single(). The "map" API allocates a bounce buffer of a +specified size in bytes and returns the physical address of the buffer. The +buffer memory is physically contiguous. The expectation is that the DMA layer +maps the physical memory address to a DMA address, and returns the DMA address +to the driver for programming into the device. If a DMA operation specifies +multiple memory buffer segments, a separate bounce buffer must be allocated for +each segment. swiotlb_tbl_map_single() always does a "sync" operation (i.e., a +CPU copy) to initialize the bounce buffer to match the contents of the original +buffer. + +swiotlb_tbl_unmap_single() does the reverse. If the DMA operation might have +updated the bounce buffer memory and DMA_ATTR_SKIP_CPU_SYNC is not set, the +unmap does a "sync" operation to cause a CPU copy of the data from the bounce +buffer back to the original buffer. Then the bounce buffer memory is freed. + +swiotlb also provides "sync" APIs that correspond to the dma_sync_*() APIs that +a driver may use when control of a buffer transitions between the CPU and the +device. The swiotlb "sync" APIs cause a CPU copy of the data between the +original buffer and the bounce buffer. Like the dma_sync_*() APIs, the swiotlb +"sync" APIs support doing a partial sync, where only a subset of the bounce +buffer is copied to/from the original buffer. + +Core Functionality Constraints +------------------------------ +The swiotlb map/unmap/sync APIs must operate without blocking, as they are +called by the corresponding DMA APIs which may run in contexts that cannot +block. Hence the default memory pool for swiotlb allocations must be +pre-allocated at boot time (but see Dynamic swiotlb below). Because swiotlb +allocations must be physically contiguous, the entire default memory pool is +allocated as a single contiguous block. + +The need to pre-allocate the default swiotlb pool creates a boot-time tradeoff. +The pool should be large enough to ensure that bounce buffer requests can +always be satisfied, as the non-blocking requirement means requests can't wait +for space to become available. But a large pool potentially wastes memory, as +this pre-allocated memory is not available for other uses in the system. The +tradeoff is particularly acute in CoCo VMs that use bounce buffers for all DMA +I/O. These VMs use a heuristic to set the default pool size to ~6% of memory, +with a max of 1 GiB, which has the potential to be very wasteful of memory. +Conversely, the heuristic might produce a size that is insufficient, depending +on the I/O patterns of the workload in the VM. The dynamic swiotlb feature +described below can help, but has limitations. Better management of the swiotlb +default memory pool size remains an open issue. + +A single allocation from swiotlb is limited to IO_TLB_SIZE * IO_TLB_SEGSIZE +bytes, which is 256 KiB with current definitions. When a device's DMA settings +are such that the device might use swiotlb, the maximum size of a DMA segment +must be limited to that 256 KiB. This value is communicated to higher-level +kernel code via dma_map_mapping_size() and swiotlb_max_mapping_size(). If the +higher-level code fails to account for this limit, it may make requests that +are too large for swiotlb, and get a "swiotlb full" error. + +A key device DMA setting is "min_align_mask", which is a power of 2 minus 1 +so that some number of low order bits are set, or it may be zero. swiotlb +allocations ensure these min_align_mask bits of the physical address of the +bounce buffer match the same bits in the address of the original buffer. When +min_align_mask is non-zero, it may produce an "alignment offset" in the address +of the bounce buffer that slightly reduces the maximum size of an allocation. +This potential alignment offset is reflected in the value returned by +swiotlb_max_mapping_size(), which can show up in places like +/sys/block/<device>/queue/max_sectors_kb. For example, if a device does not use +swiotlb, max_sectors_kb might be 512 KiB or larger. If a device might use +swiotlb, max_sectors_kb will be 256 KiB. When min_align_mask is non-zero, +max_sectors_kb might be even smaller, such as 252 KiB. + +swiotlb_tbl_map_single() also takes an "alloc_align_mask" parameter. This +parameter specifies the allocation of bounce buffer space must start at a +physical address with the alloc_align_mask bits set to zero. But the actual +bounce buffer might start at a larger address if min_align_mask is non-zero. +Hence there may be pre-padding space that is allocated prior to the start of +the bounce buffer. Similarly, the end of the bounce buffer is rounded up to an +alloc_align_mask boundary, potentially resulting in post-padding space. Any +pre-padding or post-padding space is not initialized by swiotlb code. The +"alloc_align_mask" parameter is used by IOMMU code when mapping for untrusted +devices. It is set to the granule size - 1 so that the bounce buffer is +allocated entirely from granules that are not used for any other purpose. + +Data structures concepts +------------------------ +Memory used for swiotlb bounce buffers is allocated from overall system memory +as one or more "pools". The default pool is allocated during system boot with a +default size of 64 MiB. The default pool size may be modified with the +"swiotlb=" kernel boot line parameter. The default size may also be adjusted +due to other conditions, such as running in a CoCo VM, as described above. If +CONFIG_SWIOTLB_DYNAMIC is enabled, additional pools may be allocated later in +the life of the system. Each pool must be a contiguous range of physical +memory. The default pool is allocated below the 4 GiB physical address line so +it works for devices that can only address 32-bits of physical memory (unless +architecture-specific code provides the SWIOTLB_ANY flag). In a CoCo VM, the +pool memory must be decrypted before swiotlb is used. + +Each pool is divided into "slots" of size IO_TLB_SIZE, which is 2 KiB with +current definitions. IO_TLB_SEGSIZE contiguous slots (128 slots) constitute +what might be called a "slot set". When a bounce buffer is allocated, it +occupies one or more contiguous slots. A slot is never shared by multiple +bounce buffers. Furthermore, a bounce buffer must be allocated from a single +slot set, which leads to the maximum bounce buffer size being IO_TLB_SIZE * +IO_TLB_SEGSIZE. Multiple smaller bounce buffers may co-exist in a single slot +set if the alignment and size constraints can be met. + +Slots are also grouped into "areas", with the constraint that a slot set exists +entirely in a single area. Each area has its own spin lock that must be held to +manipulate the slots in that area. The division into areas avoids contending +for a single global spin lock when swiotlb is heavily used, such as in a CoCo +VM. The number of areas defaults to the number of CPUs in the system for +maximum parallelism, but since an area can't be smaller than IO_TLB_SEGSIZE +slots, it might be necessary to assign multiple CPUs to the same area. The +number of areas can also be set via the "swiotlb=" kernel boot parameter. + +When allocating a bounce buffer, if the area associated with the calling CPU +does not have enough free space, areas associated with other CPUs are tried +sequentially. For each area tried, the area's spin lock must be obtained before +trying an allocation, so contention may occur if swiotlb is relatively busy +overall. But an allocation request does not fail unless all areas do not have +enough free space. + +IO_TLB_SIZE, IO_TLB_SEGSIZE, and the number of areas must all be powers of 2 as +the code uses shifting and bit masking to do many of the calculations. The +number of areas is rounded up to a power of 2 if necessary to meet this +requirement. + +The default pool is allocated with PAGE_SIZE alignment. If an alloc_align_mask +argument to swiotlb_tbl_map_single() specifies a larger alignment, one or more +initial slots in each slot set might not meet the alloc_align_mask criterium. +Because a bounce buffer allocation can't cross a slot set boundary, eliminating +those initial slots effectively reduces the max size of a bounce buffer. +Currently, there's no problem because alloc_align_mask is set based on IOMMU +granule size, and granules cannot be larger than PAGE_SIZE. But if that were to +change in the future, the initial pool allocation might need to be done with +alignment larger than PAGE_SIZE. + +Dynamic swiotlb +--------------- +When CONFIG_DYNAMIC_SWIOTLB is enabled, swiotlb can do on-demand expansion of +the amount of memory available for allocation as bounce buffers. If a bounce +buffer request fails due to lack of available space, an asynchronous background +task is kicked off to allocate memory from general system memory and turn it +into an swiotlb pool. Creating an additional pool must be done asynchronously +because the memory allocation may block, and as noted above, swiotlb requests +are not allowed to block. Once the background task is kicked off, the bounce +buffer request creates a "transient pool" to avoid returning an "swiotlb full" +error. A transient pool has the size of the bounce buffer request, and is +deleted when the bounce buffer is freed. Memory for this transient pool comes +from the general system memory atomic pool so that creation does not block. +Creating a transient pool has relatively high cost, particularly in a CoCo VM +where the memory must be decrypted, so it is done only as a stopgap until the +background task can add another non-transient pool. + +Adding a dynamic pool has limitations. Like with the default pool, the memory +must be physically contiguous, so the size is limited to MAX_PAGE_ORDER pages +(e.g., 4 MiB on a typical x86 system). Due to memory fragmentation, a max size +allocation may not be available. The dynamic pool allocator tries smaller sizes +until it succeeds, but with a minimum size of 1 MiB. Given sufficient system +memory fragmentation, dynamically adding a pool might not succeed at all. + +The number of areas in a dynamic pool may be different from the number of areas +in the default pool. Because the new pool size is typically a few MiB at most, +the number of areas will likely be smaller. For example, with a new pool size +of 4 MiB and the 256 KiB minimum area size, only 16 areas can be created. If +the system has more than 16 CPUs, multiple CPUs must share an area, creating +more lock contention. + +New pools added via dynamic swiotlb are linked together in a linear list. +swiotlb code frequently must search for the pool containing a particular +swiotlb physical address, so that search is linear and not performant with a +large number of dynamic pools. The data structures could be improved for +faster searches. + +Overall, dynamic swiotlb works best for small configurations with relatively +few CPUs. It allows the default swiotlb pool to be smaller so that memory is +not wasted, with dynamic pools making more space available if needed (as long +as fragmentation isn't an obstacle). It is less useful for large CoCo VMs. + +Data Structure Details +---------------------- +swiotlb is managed with four primary data structures: io_tlb_mem, io_tlb_pool, +io_tlb_area, and io_tlb_slot. io_tlb_mem describes a swiotlb memory allocator, +which includes the default memory pool and any dynamic or transient pools +linked to it. Limited statistics on swiotlb usage are kept per memory allocator +and are stored in this data structure. These statistics are available under +/sys/kernel/debug/swiotlb when CONFIG_DEBUG_FS is set. + +io_tlb_pool describes a memory pool, either the default pool, a dynamic pool, +or a transient pool. The description includes the start and end addresses of +the memory in the pool, a pointer to an array of io_tlb_area structures, and a +pointer to an array of io_tlb_slot structures that are associated with the pool. + +io_tlb_area describes an area. The primary field is the spin lock used to +serialize access to slots in the area. The io_tlb_area array for a pool has an +entry for each area, and is accessed using a 0-based area index derived from the +calling processor ID. Areas exist solely to allow parallel access to swiotlb +from multiple CPUs. + +io_tlb_slot describes an individual memory slot in the pool, with size +IO_TLB_SIZE (2 KiB currently). The io_tlb_slot array is indexed by the slot +index computed from the bounce buffer address relative to the starting memory +address of the pool. The size of struct io_tlb_slot is 24 bytes, so the +overhead is about 1% of the slot size. + +The io_tlb_slot array is designed to meet several requirements. First, the DMA +APIs and the corresponding swiotlb APIs use the bounce buffer address as the +identifier for a bounce buffer. This address is returned by +swiotlb_tbl_map_single(), and then passed as an argument to +swiotlb_tbl_unmap_single() and the swiotlb_sync_*() functions. The original +memory buffer address obviously must be passed as an argument to +swiotlb_tbl_map_single(), but it is not passed to the other APIs. Consequently, +swiotlb data structures must save the original memory buffer address so that it +can be used when doing sync operations. This original address is saved in the +io_tlb_slot array. + +Second, the io_tlb_slot array must handle partial sync requests. In such cases, +the argument to swiotlb_sync_*() is not the address of the start of the bounce +buffer but an address somewhere in the middle of the bounce buffer, and the +address of the start of the bounce buffer isn't known to swiotlb code. But +swiotlb code must be able to calculate the corresponding original memory buffer +address to do the CPU copy dictated by the "sync". So an adjusted original +memory buffer address is populated into the struct io_tlb_slot for each slot +occupied by the bounce buffer. An adjusted "alloc_size" of the bounce buffer is +also recorded in each struct io_tlb_slot so a sanity check can be performed on +the size of the "sync" operation. The "alloc_size" field is not used except for +the sanity check. + +Third, the io_tlb_slot array is used to track available slots. The "list" field +in struct io_tlb_slot records how many contiguous available slots exist starting +at that slot. A "0" indicates that the slot is occupied. A value of "1" +indicates only the current slot is available. A value of "2" indicates the +current slot and the next slot are available, etc. The maximum value is +IO_TLB_SEGSIZE, which can appear in the first slot in a slot set, and indicates +that the entire slot set is available. These values are used when searching for +available slots to use for a new bounce buffer. They are updated when allocating +a new bounce buffer and when freeing a bounce buffer. At pool creation time, the +"list" field is initialized to IO_TLB_SEGSIZE down to 1 for the slots in every +slot set. + +Fourth, the io_tlb_slot array keeps track of any "padding slots" allocated to +meet alloc_align_mask requirements described above. When +swiotlb_tlb_map_single() allocates bounce buffer space to meet alloc_align_mask +requirements, it may allocate pre-padding space across zero or more slots. But +when swiotbl_tlb_unmap_single() is called with the bounce buffer address, the +alloc_align_mask value that governed the allocation, and therefore the +allocation of any padding slots, is not known. The "pad_slots" field records +the number of padding slots so that swiotlb_tbl_unmap_single() can free them. +The "pad_slots" value is recorded only in the first non-padding slot allocated +to the bounce buffer. + +Restricted pools +---------------- +The swiotlb machinery is also used for "restricted pools", which are pools of +memory separate from the default swiotlb pool, and that are dedicated for DMA +use by a particular device. Restricted pools provide a level of DMA memory +protection on systems with limited hardware protection capabilities, such as +those lacking an IOMMU. Such usage is specified by DeviceTree entries and +requires that CONFIG_DMA_RESTRICTED_POOL is set. Each restricted pool is based +on its own io_tlb_mem data structure that is independent of the main swiotlb +io_tlb_mem. + +Restricted pools add swiotlb_alloc() and swiotlb_free() APIs, which are called +from the dma_alloc_*() and dma_free_*() APIs. The swiotlb_alloc/free() APIs +allocate/free slots from/to the restricted pool directly and do not go through +swiotlb_tbl_map/unmap_single(). |