Message-Id: <20240424150253.560999-1-mhklinux@outlook.com>
Date: Wed, 24 Apr 2024 08:02:53 -0700
From: mhkelley58@...il.com
To: robin.murphy@....com,
	joro@...tes.org,
	will@...nel.org,
	hch@....de,
	m.szyprowski@...sung.com,
	corbet@....net,
	iommu@...ts.linux.dev,
	linux-kernel@...r.kernel.org,
	linux-doc@...r.kernel.org,
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	roberto.sassu@...weicloud.com
Subject: [PATCH v2 1/1] Documentation/core-api: Add swiotlb documentation

From: Michael Kelley <mhklinux@...look.com>

There's currently no documentation for the swiotlb. Add documentation
describing usage scenarios, the key APIs, and implementation details.
Group the new documentation with other DMA-related documentation.

Signed-off-by: Michael Kelley <mhklinux@...look.com>
---
Changes in v2:
* Use KiB/MiB/GiB units instead of Kbytes/Mbytes/Gbytes [Matthew Wilcox]

 Documentation/core-api/index.rst   |   1 +
 Documentation/core-api/swiotlb.rst | 381 +++++++++++++++++++++++++++++
 2 files changed, 382 insertions(+)
 create mode 100644 Documentation/core-api/swiotlb.rst

diff --git a/Documentation/core-api/index.rst b/Documentation/core-api/index.rst
index 7a3a08d81f11..89c517665763 100644
--- a/Documentation/core-api/index.rst
+++ b/Documentation/core-api/index.rst
@@ -102,6 +102,7 @@ more memory-management documentation in Documentation/mm/index.rst.
    dma-api-howto
    dma-attributes
    dma-isa-lpc
+   swiotlb
    mm-api
    genalloc
    pin_user_pages
diff --git a/Documentation/core-api/swiotlb.rst b/Documentation/core-api/swiotlb.rst
new file mode 100644
index 000000000000..fd73a1b1fbb5
--- /dev/null
+++ b/Documentation/core-api/swiotlb.rst
@@ -0,0 +1,381 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+===============
+DMA and swiotlb
+===============
+
+The 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 the 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 the 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
+---------------
+The 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 existing 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 bounce
+buffer memory 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 updated the bounce buffer memory, the DMA layer
+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.
+
+The 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 the 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 the 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 the swiotlb, and get a "swiotlb full" error.
+
+A key device DMA setting is "min_align_mask". When set,
+swiotlb allocations are done so that the min_align_mask
+bits of the physical address of the bounce buffer match the same
+bits in the address of the original buffer. This setting 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 the swiotlb, max_sectors_kb
+might be 512 KiB or larger. If a device might use the
+swiotlb, max_sectors_kb will be 256 KiB. If min_align_mask is
+also set, 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 set. 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
+the 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 the 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
+the area's spin lock must be obtained before trying an allocation,
+so contention may occur if the 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, the 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, and that
+search is linear and not particularly 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
+----------------------
+The 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
+the 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 the 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().
-- 
2.25.1

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