
PAT (Page Attribute Table)

x86 Page Attribute Table (PAT) allows for setting the memory attribute at the
page level granularity. PAT is complementary to the MTRR settings which allows
for setting of memory types over physical address ranges. However, PAT is
more flexible than MTRR due to its capability to set attributes at page level
and also due to the fact that there are no hardware limitations on number of
such attribute settings allowed. Added flexibility comes with guidelines for
not having memory type aliasing for the same physical memory with multiple
virtual addresses.

PAT allows for different types of memory attributes. The most commonly used
ones that will be supported at this time are Write-back, Uncached,
Write-combined and Uncached Minus.

There are many different APIs in the kernel that allows setting of memory
attributes at the page level. In order to avoid aliasing, these interfaces
should be used thoughtfully. Below is a table of interfaces available,
their intended usage and their memory attribute relationships. Internally,
these APIs use a reserve_memtype()/free_memtype() interface on the physical
address range to avoid any aliasing.


-------------------------------------------------------------------
API                    |    RAM   |  ACPI,...  |  Reserved/Holes  |
-----------------------|----------|------------|------------------|
                       |          |            |                  |
ioremap                |    --    |    UC      |       UC         |
                       |          |            |                  |
ioremap_cache          |    --    |    WB      |       WB         |
                       |          |            |                  |
ioremap_nocache        |    --    |    UC      |       UC         |
                       |          |            |                  |
ioremap_wc             |    --    |    --      |       WC         |
                       |          |            |                  |
set_memory_uc          |    UC    |    --      |       --         |
 set_memory_wb         |          |            |                  |
                       |          |            |                  |
set_memory_wc          |    WC    |    --      |       --         |
 set_memory_wb         |          |            |                  |
                       |          |            |                  |
pci sysfs resource     |    --    |    --      |       UC         |
                       |          |            |                  |
pci sysfs resource_wc  |    --    |    --      |       WC         |
 is IORESOURCE_PREFETCH|          |            |                  |
                       |          |            |                  |
pci proc               |    --    |    --      |       UC         |
 !PCIIOC_WRITE_COMBINE |          |            |                  |
                       |          |            |                  |
pci proc               |    --    |    --      |       WC         |
 PCIIOC_WRITE_COMBINE  |          |            |                  |
                       |          |            |                  |
/dev/mem               |    --    |    UC      |       UC         |
 read-write            |          |            |                  |
                       |          |            |                  |
/dev/mem               |    --    |    UC      |       UC         |
 mmap SYNC flag        |          |            |                  |
                       |          |            |                  |
/dev/mem               |    --    |  WB/WC/UC  |    WB/WC/UC      |
 mmap !SYNC flag       |          |(from exist-|  (from exist-    |
 and                   |          |  ing alias)|    ing alias)    |
 any alias to this area|          |            |                  |
                       |          |            |                  |
/dev/mem               |    --    |    WB      |       WB         |
 mmap !SYNC flag       |          |            |                  |
 no alias to this area |          |            |                  |
 and                   |          |            |                  |
 MTRR says WB          |          |            |                  |
                       |          |            |                  |
/dev/mem               |    --    |    --      |    UC_MINUS      |
 mmap !SYNC flag       |          |            |                  |
 no alias to this area |          |            |                  |
 and                   |          |            |                  |
 MTRR says !WB         |          |            |                  |
                       |          |            |                  |
-------------------------------------------------------------------

Notes:

-- in the above table mean "Not suggested usage for the API". Some of the --'s
are strictly enforced by the kernel. Some others are not really enforced
today, but may be enforced in future.

For ioremap and pci access through /sys or /proc - The actual type returned
can be more restrictive, in case of any existing aliasing for that address.
For example: If there is an existing uncached mapping, a new ioremap_wc can
return uncached mapping in place of write-combine requested.

set_memory_[uc|wc] and set_memory_wb should be used in pairs, where driver will
first make a region uc or wc and switch it back to wb after use.

Over time writes to /proc/mtrr will be deprecated in favor of using PAT based
interfaces. Users writing to /proc/mtrr are suggested to use above interfaces.

Drivers should use ioremap_[uc|wc] to access PCI BARs with [uc|wc] access
types.

Drivers should use set_memory_[uc|wc] to set access type for RAM ranges.