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-
-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
-