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. 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_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 | -- | 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 | | |