247 lines
9.2 KiB
ReStructuredText
247 lines
9.2 KiB
ReStructuredText
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==================================
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Memory Attribute Aliasing on IA-64
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==================================
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Bjorn Helgaas <bjorn.helgaas@hp.com>
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May 4, 2006
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Memory Attributes
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=================
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Itanium supports several attributes for virtual memory references.
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The attribute is part of the virtual translation, i.e., it is
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contained in the TLB entry. The ones of most interest to the Linux
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kernel are:
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== ======================
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WB Write-back (cacheable)
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UC Uncacheable
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WC Write-coalescing
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== ======================
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System memory typically uses the WB attribute. The UC attribute is
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used for memory-mapped I/O devices. The WC attribute is uncacheable
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like UC is, but writes may be delayed and combined to increase
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performance for things like frame buffers.
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The Itanium architecture requires that we avoid accessing the same
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page with both a cacheable mapping and an uncacheable mapping[1].
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The design of the chipset determines which attributes are supported
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on which regions of the address space. For example, some chipsets
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support either WB or UC access to main memory, while others support
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only WB access.
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Memory Map
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==========
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Platform firmware describes the physical memory map and the
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supported attributes for each region. At boot-time, the kernel uses
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the EFI GetMemoryMap() interface. ACPI can also describe memory
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devices and the attributes they support, but Linux/ia64 currently
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doesn't use this information.
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The kernel uses the efi_memmap table returned from GetMemoryMap() to
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learn the attributes supported by each region of physical address
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space. Unfortunately, this table does not completely describe the
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address space because some machines omit some or all of the MMIO
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regions from the map.
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The kernel maintains another table, kern_memmap, which describes the
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memory Linux is actually using and the attribute for each region.
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This contains only system memory; it does not contain MMIO space.
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The kern_memmap table typically contains only a subset of the system
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memory described by the efi_memmap. Linux/ia64 can't use all memory
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in the system because of constraints imposed by the identity mapping
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scheme.
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The efi_memmap table is preserved unmodified because the original
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boot-time information is required for kexec.
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Kernel Identity Mappings
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========================
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Linux/ia64 identity mappings are done with large pages, currently
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either 16MB or 64MB, referred to as "granules." Cacheable mappings
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are speculative[2], so the processor can read any location in the
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page at any time, independent of the programmer's intentions. This
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means that to avoid attribute aliasing, Linux can create a cacheable
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identity mapping only when the entire granule supports cacheable
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access.
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Therefore, kern_memmap contains only full granule-sized regions that
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can referenced safely by an identity mapping.
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Uncacheable mappings are not speculative, so the processor will
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generate UC accesses only to locations explicitly referenced by
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software. This allows UC identity mappings to cover granules that
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are only partially populated, or populated with a combination of UC
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and WB regions.
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User Mappings
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=============
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User mappings are typically done with 16K or 64K pages. The smaller
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page size allows more flexibility because only 16K or 64K has to be
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homogeneous with respect to memory attributes.
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Potential Attribute Aliasing Cases
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==================================
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There are several ways the kernel creates new mappings:
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mmap of /dev/mem
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----------------
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This uses remap_pfn_range(), which creates user mappings. These
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mappings may be either WB or UC. If the region being mapped
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happens to be in kern_memmap, meaning that it may also be mapped
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by a kernel identity mapping, the user mapping must use the same
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attribute as the kernel mapping.
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If the region is not in kern_memmap, the user mapping should use
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an attribute reported as being supported in the EFI memory map.
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Since the EFI memory map does not describe MMIO on some
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machines, this should use an uncacheable mapping as a fallback.
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mmap of /sys/class/pci_bus/.../legacy_mem
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-----------------------------------------
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This is very similar to mmap of /dev/mem, except that legacy_mem
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only allows mmap of the one megabyte "legacy MMIO" area for a
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specific PCI bus. Typically this is the first megabyte of
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physical address space, but it may be different on machines with
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several VGA devices.
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"X" uses this to access VGA frame buffers. Using legacy_mem
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rather than /dev/mem allows multiple instances of X to talk to
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different VGA cards.
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The /dev/mem mmap constraints apply.
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mmap of /proc/bus/pci/.../??.?
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------------------------------
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This is an MMIO mmap of PCI functions, which additionally may or
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may not be requested as using the WC attribute.
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If WC is requested, and the region in kern_memmap is either WC
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or UC, and the EFI memory map designates the region as WC, then
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the WC mapping is allowed.
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Otherwise, the user mapping must use the same attribute as the
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kernel mapping.
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read/write of /dev/mem
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----------------------
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This uses copy_from_user(), which implicitly uses a kernel
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identity mapping. This is obviously safe for things in
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kern_memmap.
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There may be corner cases of things that are not in kern_memmap,
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but could be accessed this way. For example, registers in MMIO
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space are not in kern_memmap, but could be accessed with a UC
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mapping. This would not cause attribute aliasing. But
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registers typically can be accessed only with four-byte or
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eight-byte accesses, and the copy_from_user() path doesn't allow
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any control over the access size, so this would be dangerous.
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ioremap()
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---------
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This returns a mapping for use inside the kernel.
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If the region is in kern_memmap, we should use the attribute
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specified there.
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If the EFI memory map reports that the entire granule supports
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WB, we should use that (granules that are partially reserved
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or occupied by firmware do not appear in kern_memmap).
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If the granule contains non-WB memory, but we can cover the
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region safely with kernel page table mappings, we can use
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ioremap_page_range() as most other architectures do.
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Failing all of the above, we have to fall back to a UC mapping.
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Past Problem Cases
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==================
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mmap of various MMIO regions from /dev/mem by "X" on Intel platforms
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--------------------------------------------------------------------
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The EFI memory map may not report these MMIO regions.
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These must be allowed so that X will work. This means that
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when the EFI memory map is incomplete, every /dev/mem mmap must
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succeed. It may create either WB or UC user mappings, depending
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on whether the region is in kern_memmap or the EFI memory map.
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mmap of 0x0-0x9FFFF /dev/mem by "hwinfo" on HP sx1000 with VGA enabled
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----------------------------------------------------------------------
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The EFI memory map reports the following attributes:
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=============== ======= ==================
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0x00000-0x9FFFF WB only
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0xA0000-0xBFFFF UC only (VGA frame buffer)
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0xC0000-0xFFFFF WB only
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=============== ======= ==================
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This mmap is done with user pages, not kernel identity mappings,
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so it is safe to use WB mappings.
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The kernel VGA driver may ioremap the VGA frame buffer at 0xA0000,
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which uses a granule-sized UC mapping. This granule will cover some
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WB-only memory, but since UC is non-speculative, the processor will
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never generate an uncacheable reference to the WB-only areas unless
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the driver explicitly touches them.
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mmap of 0x0-0xFFFFF legacy_mem by "X"
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-------------------------------------
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If the EFI memory map reports that the entire range supports the
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same attributes, we can allow the mmap (and we will prefer WB if
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supported, as is the case with HP sx[12]000 machines with VGA
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disabled).
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If EFI reports the range as partly WB and partly UC (as on sx[12]000
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machines with VGA enabled), we must fail the mmap because there's no
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safe attribute to use.
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If EFI reports some of the range but not all (as on Intel firmware
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that doesn't report the VGA frame buffer at all), we should fail the
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mmap and force the user to map just the specific region of interest.
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mmap of 0xA0000-0xBFFFF legacy_mem by "X" on HP sx1000 with VGA disabled
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------------------------------------------------------------------------
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The EFI memory map reports the following attributes::
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0x00000-0xFFFFF WB only (no VGA MMIO hole)
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This is a special case of the previous case, and the mmap should
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fail for the same reason as above.
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read of /sys/devices/.../rom
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----------------------------
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For VGA devices, this may cause an ioremap() of 0xC0000. This
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used to be done with a UC mapping, because the VGA frame buffer
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at 0xA0000 prevents use of a WB granule. The UC mapping causes
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an MCA on HP sx[12]000 chipsets.
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We should use WB page table mappings to avoid covering the VGA
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frame buffer.
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Notes
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=====
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[1] SDM rev 2.2, vol 2, sec 4.4.1.
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[2] SDM rev 2.2, vol 2, sec 4.4.6.
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