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Date:   Sat, 27 Apr 2019 14:51:09 -0300
From:   Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
To:     Changbin Du <changbin.du@gmail.com>
Cc:     Jonathan Corbet <corbet@lwn.net>, tglx@linutronix.de,
        mingo@redhat.com, bp@alien8.de, x86@kernel.org,
        linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 11/27] Documentation: x86: convert pat.txt to reST
Message-ID: <20190427145109.1b2f93d9@coco.lan>
In-Reply-To: <20190426153150.21228-12-changbin.du@gmail.com>
References: <20190426153150.21228-1-changbin.du@gmail.com>
        <20190426153150.21228-12-changbin.du@gmail.com>
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Em Fri, 26 Apr 2019 23:31:34 +0800
Changbin Du <changbin.du@gmail.com> escreveu:

> 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>
> ---
>  Documentation/x86/index.rst |   1 +
>  Documentation/x86/pat.rst   | 235 ++++++++++++++++++++++++++++++++++++
>  Documentation/x86/pat.txt   | 230 -----------------------------------
>  3 files changed, 236 insertions(+), 230 deletions(-)
>  create mode 100644 Documentation/x86/pat.rst
>  delete mode 100644 Documentation/x86/pat.txt
> 
> diff --git a/Documentation/x86/index.rst b/Documentation/x86/index.rst
> index d805962a7238..e06b5c0ea883 100644
> --- a/Documentation/x86/index.rst
> +++ b/Documentation/x86/index.rst
> @@ -17,3 +17,4 @@ Linux x86 Support
>     zero-page
>     tlb
>     mtrr
> +   pat
> diff --git a/Documentation/x86/pat.rst b/Documentation/x86/pat.rst
> new file mode 100644
> index 000000000000..bf09cab2e0bf
> --- /dev/null
> +++ b/Documentation/x86/pat.rst
> @@ -0,0 +1,235 @@
> +.. 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 Write-back, Uncached,
> +Write-combined, Write-through and Uncached Minus.

I would rewrite the above to:

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
===  ==============

As, at the table below, it uses WB, UC, WC, ... instead the full name. By
doing this, it makes easier for readers when looking at the next table.

> +
> +
> +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        |          |(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-        |
> +  mmap !SYNC flag        |          |            |                  |
> +  no alias to this area  |          |            |                  |
> +  and                    |          |            |                  |
> +  MTRR says !WB          |          |            |                  |
> +                         |          |            |                  |
> +  -------------------------------------------------------------------

This is already a table. I would keep it like that, just replacing it to
the proper ReST markups, e. g.:

+------------------------+----------+--------------+------------------+
| 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
> +  ----------------------------------------------------------------------
> +                                                    Non-PAT |  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

The (*) here is part of the artwork. So, it should be indented, in order to
make easier for someone reading this on html mode.

As you know, before noticing your conversion, I made my own changes to
x86. There, for this particular artwork, I did some cosmetic changes,
to make it look more similar to other tables, as, IMHO, having a similar
visual makes easier for one reading this in text format:

  ====  =======  ===  =========================  =====================
  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

(unfortunately, even looking like a table, Sphinx won't parse it fine,
so, even if you opt to use the above, we still need to mark it as
a literal block)

It is up to you (and x86 maintainers) to choose between the
versions. Of course, I prefer the one I did, as it is visually
closer to what we're doing on most tables, so my brain can parse
it faster.

> +
> +.. 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
> +

Those are actually two tables. Please mark them as such, e. g.:

 ==== ===== ==========================  =========  =======
 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
 ========= =======================================


> diff --git a/Documentation/x86/pat.txt b/Documentation/x86/pat.txt
> deleted file mode 100644
> index 481d8d8536ac..000000000000
> --- a/Documentation/x86/pat.txt
> +++ /dev/null
> @@ -1,230 +0,0 @@
> -
> -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 Write-back, Uncached,
> -Write-combined, Write-through and 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       |          |(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-        |
> - 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
> -----------------------------------------------------------------------
> -                                                  Non-PAT |  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
> -
> -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|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
> -


Thanks,
Mauro