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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 and Uncached Minus.
- 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_nocache | -- | UC | UC |
- | | | |
- ioremap_wc | -- | -- | WC |
- | | | |
- set_memory_uc | UC | -- | -- |
- set_memory_wb | | | |
- | | | |
- set_memory_wc | WC | -- | -- |
- 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 | -- | UC | 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_MINUS |
- mmap !SYNC flag | | | |
- no alias to this area | | | |
- and | | | |
- MTRR says !WB | | | |
- | | | |
- -------------------------------------------------------------------
- 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] and set_memory_wb should be used in pairs, where driver will
- first make a region uc or wc 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] to set access type for RAM ranges.
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