Documentation/ia64/aliasing.rst

   1 ==================================
   2 Memory Attribute Aliasing on IA-64
   3 ==================================
   4
   5 Bjorn Helgaas <bjorn.helgaas@hp.com>
   6
   7 May 4, 2006
   8
   9
  10 Memory Attributes
  11 =================
  12
  13     Itanium supports several attributes for virtual memory references.
  14     The attribute is part of the virtual translation, i.e., it is
  15     contained in the TLB entry.  The ones of most interest to the Linux
  16     kernel are:
  17
  18         ==              ======================
  19         WB              Write-back (cacheable)
  20         UC              Uncacheable
  21         WC              Write-coalescing
  22         ==              ======================
  23
  24     System memory typically uses the WB attribute.  The UC attribute is
  25     used for memory-mapped I/O devices.  The WC attribute is uncacheable
  26     like UC is, but writes may be delayed and combined to increase
  27     performance for things like frame buffers.
  28
  29     The Itanium architecture requires that we avoid accessing the same
  30     page with both a cacheable mapping and an uncacheable mapping[1].
  31
  32     The design of the chipset determines which attributes are supported
  33     on which regions of the address space.  For example, some chipsets
  34     support either WB or UC access to main memory, while others support
  35     only WB access.
  36
  37 Memory Map
  38 ==========
  39
  40     Platform firmware describes the physical memory map and the
  41     supported attributes for each region.  At boot-time, the kernel uses
  42     the EFI GetMemoryMap() interface.  ACPI can also describe memory
  43     devices and the attributes they support, but Linux/ia64 currently
  44     doesn't use this information.
  45
  46     The kernel uses the efi_memmap table returned from GetMemoryMap() to
  47     learn the attributes supported by each region of physical address
  48     space.  Unfortunately, this table does not completely describe the
  49     address space because some machines omit some or all of the MMIO
  50     regions from the map.
  51
  52     The kernel maintains another table, kern_memmap, which describes the
  53     memory Linux is actually using and the attribute for each region.
  54     This contains only system memory; it does not contain MMIO space.
  55
  56     The kern_memmap table typically contains only a subset of the system
  57     memory described by the efi_memmap.  Linux/ia64 can't use all memory
  58     in the system because of constraints imposed by the identity mapping
  59     scheme.
  60
  61     The efi_memmap table is preserved unmodified because the original
  62     boot-time information is required for kexec.
  63
  64 Kernel Identify Mappings
  65 ========================
  66
  67     Linux/ia64 identity mappings are done with large pages, currently
  68     either 16MB or 64MB, referred to as "granules."  Cacheable mappings
  69     are speculative[2], so the processor can read any location in the
  70     page at any time, independent of the programmer's intentions.  This
  71     means that to avoid attribute aliasing, Linux can create a cacheable
  72     identity mapping only when the entire granule supports cacheable
  73     access.
  74
  75     Therefore, kern_memmap contains only full granule-sized regions that
  76     can referenced safely by an identity mapping.
  77
  78     Uncacheable mappings are not speculative, so the processor will
  79     generate UC accesses only to locations explicitly referenced by
  80     software.  This allows UC identity mappings to cover granules that
  81     are only partially populated, or populated with a combination of UC
  82     and WB regions.
  83
  84 User Mappings
  85 =============
  86
  87     User mappings are typically done with 16K or 64K pages.  The smaller
  88     page size allows more flexibility because only 16K or 64K has to be
  89     homogeneous with respect to memory attributes.
  90
  91 Potential Attribute Aliasing Cases
  92 ==================================
  93
  94     There are several ways the kernel creates new mappings:
  95
  96 mmap of /dev/mem
  97 ----------------
  98
  99         This uses remap_pfn_range(), which creates user mappings.  These
 100         mappings may be either WB or UC.  If the region being mapped
 101         happens to be in kern_memmap, meaning that it may also be mapped
 102         by a kernel identity mapping, the user mapping must use the same
 103         attribute as the kernel mapping.
 104
 105         If the region is not in kern_memmap, the user mapping should use
 106         an attribute reported as being supported in the EFI memory map.
 107
 108         Since the EFI memory map does not describe MMIO on some
 109         machines, this should use an uncacheable mapping as a fallback.
 110
 111 mmap of /sys/class/pci_bus/.../legacy_mem
 112 -----------------------------------------
 113
 114         This is very similar to mmap of /dev/mem, except that legacy_mem
 115         only allows mmap of the one megabyte "legacy MMIO" area for a
 116         specific PCI bus.  Typically this is the first megabyte of
 117         physical address space, but it may be different on machines with
 118         several VGA devices.
 119
 120         "X" uses this to access VGA frame buffers.  Using legacy_mem
 121         rather than /dev/mem allows multiple instances of X to talk to
 122         different VGA cards.
 123
 124         The /dev/mem mmap constraints apply.
 125
 126 mmap of /proc/bus/pci/.../??.?
 127 ------------------------------
 128
 129         This is an MMIO mmap of PCI functions, which additionally may or
 130         may not be requested as using the WC attribute.
 131
 132         If WC is requested, and the region in kern_memmap is either WC
 133         or UC, and the EFI memory map designates the region as WC, then
 134         the WC mapping is allowed.
 135
 136         Otherwise, the user mapping must use the same attribute as the
 137         kernel mapping.
 138
 139 read/write of /dev/mem
 140 ----------------------
 141
 142         This uses copy_from_user(), which implicitly uses a kernel
 143         identity mapping.  This is obviously safe for things in
 144         kern_memmap.
 145
 146         There may be corner cases of things that are not in kern_memmap,
 147         but could be accessed this way.  For example, registers in MMIO
 148         space are not in kern_memmap, but could be accessed with a UC
 149         mapping.  This would not cause attribute aliasing.  But
 150         registers typically can be accessed only with four-byte or
 151         eight-byte accesses, and the copy_from_user() path doesn't allow
 152         any control over the access size, so this would be dangerous.
 153
 154 ioremap()
 155 ---------
 156
 157         This returns a mapping for use inside the kernel.
 158
 159         If the region is in kern_memmap, we should use the attribute
 160         specified there.
 161
 162         If the EFI memory map reports that the entire granule supports
 163         WB, we should use that (granules that are partially reserved
 164         or occupied by firmware do not appear in kern_memmap).
 165
 166         If the granule contains non-WB memory, but we can cover the
 167         region safely with kernel page table mappings, we can use
 168         ioremap_page_range() as most other architectures do.
 169
 170         Failing all of the above, we have to fall back to a UC mapping.
 171
 172 Past Problem Cases
 173 ==================
 174
 175 mmap of various MMIO regions from /dev/mem by "X" on Intel platforms
 176 --------------------------------------------------------------------
 177
 178       The EFI memory map may not report these MMIO regions.
 179
 180       These must be allowed so that X will work.  This means that
 181       when the EFI memory map is incomplete, every /dev/mem mmap must
 182       succeed.  It may create either WB or UC user mappings, depending
 183       on whether the region is in kern_memmap or the EFI memory map.
 184
 185 mmap of 0x0-0x9FFFF /dev/mem by "hwinfo" on HP sx1000 with VGA enabled
 186 ----------------------------------------------------------------------
 187
 188       The EFI memory map reports the following attributes:
 189
 190         =============== ======= ==================
 191         0x00000-0x9FFFF WB only
 192         0xA0000-0xBFFFF UC only (VGA frame buffer)
 193         0xC0000-0xFFFFF WB only
 194         =============== ======= ==================
 195
 196       This mmap is done with user pages, not kernel identity mappings,
 197       so it is safe to use WB mappings.
 198
 199       The kernel VGA driver may ioremap the VGA frame buffer at 0xA0000,
 200       which uses a granule-sized UC mapping.  This granule will cover some
 201       WB-only memory, but since UC is non-speculative, the processor will
 202       never generate an uncacheable reference to the WB-only areas unless
 203       the driver explicitly touches them.
 204
 205 mmap of 0x0-0xFFFFF legacy_mem by "X"
 206 -------------------------------------
 207
 208       If the EFI memory map reports that the entire range supports the
 209       same attributes, we can allow the mmap (and we will prefer WB if
 210       supported, as is the case with HP sx[12]000 machines with VGA
 211       disabled).
 212
 213       If EFI reports the range as partly WB and partly UC (as on sx[12]000
 214       machines with VGA enabled), we must fail the mmap because there's no
 215       safe attribute to use.
 216
 217       If EFI reports some of the range but not all (as on Intel firmware
 218       that doesn't report the VGA frame buffer at all), we should fail the
 219       mmap and force the user to map just the specific region of interest.
 220
 221 mmap of 0xA0000-0xBFFFF legacy_mem by "X" on HP sx1000 with VGA disabled
 222 ------------------------------------------------------------------------
 223
 224       The EFI memory map reports the following attributes::
 225
 226         0x00000-0xFFFFF WB only (no VGA MMIO hole)
 227
 228       This is a special case of the previous case, and the mmap should
 229       fail for the same reason as above.
 230
 231 read of /sys/devices/.../rom
 232 ----------------------------
 233
 234       For VGA devices, this may cause an ioremap() of 0xC0000.  This
 235       used to be done with a UC mapping, because the VGA frame buffer
 236       at 0xA0000 prevents use of a WB granule.  The UC mapping causes
 237       an MCA on HP sx[12]000 chipsets.
 238
 239       We should use WB page table mappings to avoid covering the VGA
 240       frame buffer.
 241
 242 Notes
 243 =====
 244
 245     [1] SDM rev 2.2, vol 2, sec 4.4.1.
 246     [2] SDM rev 2.2, vol 2, sec 4.4.6.