xref: /openbmc/linux/Documentation/arch/x86/pat.rst (revision 1ac731c529cd4d6adbce134754b51ff7d822b145)

.. 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   |          |              |                  |
+------------------------+----------+--------------+------------------+
| /dev/mem               |    --    |   WB/WC/UC-  |    WB/WC/UC-     |
| read-write             |          |              |                  |
+------------------------+----------+--------------+------------------+
| /dev/mem               |    --    |    UC-       |       UC-        |
| mmap SYNC flag         |          |              |                  |
+------------------------+----------+--------------+------------------+
| /dev/mem               |    --    |   WB/WC/UC-  |  WB/WC/UC-       |
| mmap !SYNC flag        |          |              |                  |
| and                    |          |(from existing|  (from existing  |
| any alias to this area |          |alias)        |  alias)          |
+------------------------+----------+--------------+------------------+
| /dev/mem               |    --    |    WB        |       WB         |
| mmap !SYNC flag        |          |              |                  |
| no alias to this area  |          |              |                  |
| and                    |          |              |                  |
| MTRR says WB           |          |              |                  |
+------------------------+----------+--------------+------------------+
| /dev/mem               |    --    |    --        |       UC-        |
| mmap !SYNC flag        |          |              |                  |
| no alias to this area  |          |              |                  |
| and                    |          |              |                  |
| MTRR says !WB          |          |              |                  |
+------------------------+----------+--------------+------------------+


Advanced APIs for drivers
=========================

A. Exporting pages to users with remap_pfn_range, io_remap_pfn_range,
vmf_insert_pfn.

Drivers wanting to export some pages to userspace do it by using mmap
interface and a combination of:

  1) pgprot_noncached()
  2) io_remap_pfn_range() or remap_pfn_range() or vmf_insert_pfn()

With PAT support, a new API pgprot_writecombine is being added. So, drivers can
continue to use the above sequence, with either pgprot_noncached() or
pgprot_writecombine() in step 1, followed by step 2.

In addition, step 2 internally tracks the region as UC or WC in memtype
list in order to ensure no conflicting mapping.

Note that this set of APIs only works with IO (non RAM) regions. If driver
wants to export a RAM region, it has to do set_memory_uc() or set_memory_wc()
as step 0 above and also track the usage of those pages and use set_memory_wb()
before the page is freed to free pool.

MTRR effects on PAT / non-PAT systems
=====================================

The following table provides the effects of using write-combining MTRRs when
using ioremap*() calls on x86 for both non-PAT and PAT systems. Ideally
mtrr_add() usage will be phased out in favor of arch_phys_wc_add() which will
be a no-op on PAT enabled systems. The region over which a arch_phys_wc_add()
is made, should already have been ioremapped with WC attributes or PAT entries,
this can be done by using ioremap_wc() / set_memory_wc().  Devices which
combine areas of IO memory desired to remain uncacheable with areas where
write-combining is desirable should consider use of ioremap_uc() followed by
set_memory_wc() to white-list effective write-combined areas.  Such use is
nevertheless discouraged as the effective memory type is considered
implementation defined, yet this strategy can be used as last resort on devices
with size-constrained regions where otherwise MTRR write-combining would
otherwise not be effective.
::

  ====  =======  ===  =========================  =====================
  MTRR  Non-PAT  PAT  Linux ioremap value        Effective memory type
  ====  =======  ===  =========================  =====================
        PAT                                        Non-PAT |  PAT
        |PCD                                               |
        ||PWT                                              |
        |||                                                |
  WC    000      WB   _PAGE_CACHE_MODE_WB             WC   |   WC
  WC    001      WC   _PAGE_CACHE_MODE_WC             WC*  |   WC
  WC    010      UC-  _PAGE_CACHE_MODE_UC_MINUS       WC*  |   UC
  WC    011      UC   _PAGE_CACHE_MODE_UC             UC   |   UC
  ====  =======  ===  =========================  =====================

  (*) denotes implementation defined and is discouraged

.. note:: -- 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|wt] and set_memory_wb should be used in pairs, where driver
will first make a region uc, wc or wt 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|wt] to set access type for RAM ranges.


PAT debugging
=============

With CONFIG_DEBUG_FS enabled, PAT memtype list can be examined by::

  # mount -t debugfs debugfs /sys/kernel/debug
  # cat /sys/kernel/debug/x86/pat_memtype_list
  PAT memtype list:
  uncached-minus @ 0x7fadf000-0x7fae0000
  uncached-minus @ 0x7fb19000-0x7fb1a000
  uncached-minus @ 0x7fb1a000-0x7fb1b000
  uncached-minus @ 0x7fb1b000-0x7fb1c000
  uncached-minus @ 0x7fb1c000-0x7fb1d000
  uncached-minus @ 0x7fb1d000-0x7fb1e000
  uncached-minus @ 0x7fb1e000-0x7fb25000
  uncached-minus @ 0x7fb25000-0x7fb26000
  uncached-minus @ 0x7fb26000-0x7fb27000
  uncached-minus @ 0x7fb27000-0x7fb28000
  uncached-minus @ 0x7fb28000-0x7fb2e000
  uncached-minus @ 0x7fb2e000-0x7fb2f000
  uncached-minus @ 0x7fb2f000-0x7fb30000
  uncached-minus @ 0x7fb31000-0x7fb32000
  uncached-minus @ 0x80000000-0x90000000

This list shows physical address ranges and various PAT settings used to
access those physical address ranges.

Another, more verbose way of getting PAT related debug messages is with
"debugpat" boot parameter. With this parameter, various debug messages are
printed to dmesg log.

PAT Initialization
==================

The following table describes how PAT is initialized under various
configurations. The PAT MSR must be updated by Linux in order to support WC
and WT attributes. Otherwise, the PAT MSR has the value programmed in it
by the firmware. Note, Xen enables WC attribute in the PAT MSR for guests.

 ==== ===== ==========================  =========  =======
 MTRR PAT   Call Sequence               PAT State  PAT MSR
 ==== ===== ==========================  =========  =======
 E    E     MTRR -> PAT init            Enabled    OS
 E    D     MTRR -> PAT init            Disabled    -
 D    E     MTRR -> PAT disable         Disabled   BIOS
 D    D     MTRR -> PAT disable         Disabled    -
 -    np/E  PAT  -> PAT disable         Disabled   BIOS
 -    np/D  PAT  -> PAT disable         Disabled    -
 E    !P/E  MTRR -> PAT init            Disabled   BIOS
 D    !P/E  MTRR -> PAT disable         Disabled   BIOS
 !M   !P/E  MTRR stub -> PAT disable    Disabled   BIOS
 ==== ===== ==========================  =========  =======

  Legend

 ========= =======================================
 E         Feature enabled in CPU
 D	   Feature disabled/unsupported in CPU
 np	   "nopat" boot option specified
 !P	   CONFIG_X86_PAT option unset
 !M	   CONFIG_MTRR option unset
 Enabled   PAT state set to enabled
 Disabled  PAT state set to disabled
 OS        PAT initializes PAT MSR with OS setting
 BIOS      PAT keeps PAT MSR with BIOS setting
 ========= =======================================