Documentation: x86: convert pat.txt to reST

This converts the plain text documentation to reStructuredText format and
add it to Sphinx TOC tree. No essential content change.

Signed-off-by: Changbin Du <changbin.du@gmail.com>
Cc: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

1 files changed, 242 insertions, 0 deletions

diff --git a/Documentation/x86/pat.rst b/Documentation/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_nocache        |    --    |    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
+ ========= =======================================
+