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| -rw-r--r-- | Documentation/x86/index.rst | 1 | ||||
| -rw-r--r-- | Documentation/x86/pat.rst | 242 | ||||
| -rw-r--r-- | Documentation/x86/pat.txt | 230 |
3 files changed, 243 insertions, 230 deletions
diff --git a/Documentation/x86/index.rst b/Documentation/x86/index.rst index 769c4491e9cb..f7012e4afacd 100644 --- a/Documentation/x86/index.rst +++ b/Documentation/x86/index.rst @@ -17,3 +17,4 @@ x86-specific Documentation zero-page tlb mtrr + pat diff --git a/Documentation/x86/pat.rst b/Documentation/x86/pat.rst new file mode 100644 index 000000000000..9a298fd97d74 --- /dev/null +++ b/Documentation/x86/pat.rst @@ -0,0 +1,242 @@ +.. 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 + ========= ======================================= + 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 - |