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diff --git a/Documentation/x86/pat.rst b/Documentation/x86/pat.rst deleted file mode 100644 index 9a298fd97d74..000000000000 --- a/Documentation/x86/pat.rst +++ /dev/null @@ -1,242 +0,0 @@ -.. 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 - ========= ======================================= - |