| 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, | |
| vm_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 vm_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 | |