Documentation/arch/x86/pat.rst

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.. SPDX-License-Identifier: GPL-2.0

==========================
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:

===  ==============
WB   Write-back
UC   Uncached
WC   Write-combined
WT   Write-through
UC-  Uncached Minus
===  ==============


PAT APIs
========

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_uc             |    --    |    UC        |       UC         |
+------------------------+----------+--------------+------------------+
| ioremap_wc             |    --    |    --        |       WC         |
+------------------------+----------+--------------+------------------+
| ioremap_wt             |    --    |    --        |       WT         |
+------------------------+----------+--------------+------------------+
| set_memory_uc,         |    UC-   |    --        |       --         |
| set_memory_wb          |          |              |                  |
+------------------------+----------+--------------+------------------+
| set_memory_wc,         |    WC    |    --        |       --         |
| set_memory_wb          |          |              |                  |
+------------------------+----------+--------------+------------------+
| set_memory_wt,         |    WT    |    --        |       --         |
| 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   |          |              |                  |
+------------------------+----------+--------------+------------------+

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