1 2PAT (Page Attribute Table) 3 4x86 Page Attribute Table (PAT) allows for setting the memory attribute at the 5page level granularity. PAT is complementary to the MTRR settings which allows 6for setting of memory types over physical address ranges. However, PAT is 7more flexible than MTRR due to its capability to set attributes at page level 8and also due to the fact that there are no hardware limitations on number of 9such attribute settings allowed. Added flexibility comes with guidelines for 10not having memory type aliasing for the same physical memory with multiple 11virtual addresses. 12 13PAT allows for different types of memory attributes. The most commonly used 14ones that will be supported at this time are Write-back, Uncached, 15Write-combined, Write-through and Uncached Minus. 16 17 18PAT APIs 19-------- 20 21There are many different APIs in the kernel that allows setting of memory 22attributes at the page level. In order to avoid aliasing, these interfaces 23should be used thoughtfully. Below is a table of interfaces available, 24their intended usage and their memory attribute relationships. Internally, 25these APIs use a reserve_memtype()/free_memtype() interface on the physical 26address range to avoid any aliasing. 27 28 29------------------------------------------------------------------- 30API | RAM | ACPI,... | Reserved/Holes | 31-----------------------|----------|------------|------------------| 32 | | | | 33ioremap | -- | UC- | UC- | 34 | | | | 35ioremap_cache | -- | WB | WB | 36 | | | | 37ioremap_uc | -- | UC | UC | 38 | | | | 39ioremap_nocache | -- | UC- | UC- | 40 | | | | 41ioremap_wc | -- | -- | WC | 42 | | | | 43ioremap_wt | -- | -- | WT | 44 | | | | 45set_memory_uc | UC- | -- | -- | 46 set_memory_wb | | | | 47 | | | | 48set_memory_wc | WC | -- | -- | 49 set_memory_wb | | | | 50 | | | | 51set_memory_wt | WT | -- | -- | 52 set_memory_wb | | | | 53 | | | | 54pci sysfs resource | -- | -- | UC- | 55 | | | | 56pci sysfs resource_wc | -- | -- | WC | 57 is IORESOURCE_PREFETCH| | | | 58 | | | | 59pci proc | -- | -- | UC- | 60 !PCIIOC_WRITE_COMBINE | | | | 61 | | | | 62pci proc | -- | -- | WC | 63 PCIIOC_WRITE_COMBINE | | | | 64 | | | | 65/dev/mem | -- | WB/WC/UC- | WB/WC/UC- | 66 read-write | | | | 67 | | | | 68/dev/mem | -- | UC- | UC- | 69 mmap SYNC flag | | | | 70 | | | | 71/dev/mem | -- | WB/WC/UC- | WB/WC/UC- | 72 mmap !SYNC flag | |(from exist-| (from exist- | 73 and | | ing alias)| ing alias) | 74 any alias to this area| | | | 75 | | | | 76/dev/mem | -- | WB | WB | 77 mmap !SYNC flag | | | | 78 no alias to this area | | | | 79 and | | | | 80 MTRR says WB | | | | 81 | | | | 82/dev/mem | -- | -- | UC- | 83 mmap !SYNC flag | | | | 84 no alias to this area | | | | 85 and | | | | 86 MTRR says !WB | | | | 87 | | | | 88------------------------------------------------------------------- 89 90Advanced APIs for drivers 91------------------------- 92A. Exporting pages to users with remap_pfn_range, io_remap_pfn_range, 93vm_insert_pfn 94 95Drivers wanting to export some pages to userspace do it by using mmap 96interface and a combination of 971) pgprot_noncached() 982) io_remap_pfn_range() or remap_pfn_range() or vm_insert_pfn() 99 100With PAT support, a new API pgprot_writecombine is being added. So, drivers can 101continue to use the above sequence, with either pgprot_noncached() or 102pgprot_writecombine() in step 1, followed by step 2. 103 104In addition, step 2 internally tracks the region as UC or WC in memtype 105list in order to ensure no conflicting mapping. 106 107Note that this set of APIs only works with IO (non RAM) regions. If driver 108wants to export a RAM region, it has to do set_memory_uc() or set_memory_wc() 109as step 0 above and also track the usage of those pages and use set_memory_wb() 110before the page is freed to free pool. 111 112MTRR effects on PAT / non-PAT systems 113------------------------------------- 114 115The following table provides the effects of using write-combining MTRRs when 116using ioremap*() calls on x86 for both non-PAT and PAT systems. Ideally 117mtrr_add() usage will be phased out in favor of arch_phys_wc_add() which will 118be a no-op on PAT enabled systems. The region over which a arch_phys_wc_add() 119is made, should already have been ioremapped with WC attributes or PAT entries, 120this can be done by using ioremap_wc() / set_memory_wc(). Devices which 121combine areas of IO memory desired to remain uncacheable with areas where 122write-combining is desirable should consider use of ioremap_uc() followed by 123set_memory_wc() to white-list effective write-combined areas. Such use is 124nevertheless discouraged as the effective memory type is considered 125implementation defined, yet this strategy can be used as last resort on devices 126with size-constrained regions where otherwise MTRR write-combining would 127otherwise not be effective. 128 129---------------------------------------------------------------------- 130MTRR Non-PAT PAT Linux ioremap value Effective memory type 131---------------------------------------------------------------------- 132 Non-PAT | PAT 133 PAT 134 |PCD 135 ||PWT 136 ||| 137WC 000 WB _PAGE_CACHE_MODE_WB WC | WC 138WC 001 WC _PAGE_CACHE_MODE_WC WC* | WC 139WC 010 UC- _PAGE_CACHE_MODE_UC_MINUS WC* | UC 140WC 011 UC _PAGE_CACHE_MODE_UC UC | UC 141---------------------------------------------------------------------- 142 143(*) denotes implementation defined and is discouraged 144 145Notes: 146 147-- in the above table mean "Not suggested usage for the API". Some of the --'s 148are strictly enforced by the kernel. Some others are not really enforced 149today, but may be enforced in future. 150 151For ioremap and pci access through /sys or /proc - The actual type returned 152can be more restrictive, in case of any existing aliasing for that address. 153For example: If there is an existing uncached mapping, a new ioremap_wc can 154return uncached mapping in place of write-combine requested. 155 156set_memory_[uc|wc|wt] and set_memory_wb should be used in pairs, where driver 157will first make a region uc, wc or wt and switch it back to wb after use. 158 159Over time writes to /proc/mtrr will be deprecated in favor of using PAT based 160interfaces. Users writing to /proc/mtrr are suggested to use above interfaces. 161 162Drivers should use ioremap_[uc|wc] to access PCI BARs with [uc|wc] access 163types. 164 165Drivers should use set_memory_[uc|wc|wt] to set access type for RAM ranges. 166 167 168PAT debugging 169------------- 170 171With CONFIG_DEBUG_FS enabled, PAT memtype list can be examined by 172 173# mount -t debugfs debugfs /sys/kernel/debug 174# cat /sys/kernel/debug/x86/pat_memtype_list 175PAT memtype list: 176uncached-minus @ 0x7fadf000-0x7fae0000 177uncached-minus @ 0x7fb19000-0x7fb1a000 178uncached-minus @ 0x7fb1a000-0x7fb1b000 179uncached-minus @ 0x7fb1b000-0x7fb1c000 180uncached-minus @ 0x7fb1c000-0x7fb1d000 181uncached-minus @ 0x7fb1d000-0x7fb1e000 182uncached-minus @ 0x7fb1e000-0x7fb25000 183uncached-minus @ 0x7fb25000-0x7fb26000 184uncached-minus @ 0x7fb26000-0x7fb27000 185uncached-minus @ 0x7fb27000-0x7fb28000 186uncached-minus @ 0x7fb28000-0x7fb2e000 187uncached-minus @ 0x7fb2e000-0x7fb2f000 188uncached-minus @ 0x7fb2f000-0x7fb30000 189uncached-minus @ 0x7fb31000-0x7fb32000 190uncached-minus @ 0x80000000-0x90000000 191 192This list shows physical address ranges and various PAT settings used to 193access those physical address ranges. 194 195Another, more verbose way of getting PAT related debug messages is with 196"debugpat" boot parameter. With this parameter, various debug messages are 197printed to dmesg log. 198 199PAT Initialization 200------------------ 201 202The following table describes how PAT is initialized under various 203configurations. The PAT MSR must be updated by Linux in order to support WC 204and WT attributes. Otherwise, the PAT MSR has the value programmed in it 205by the firmware. Note, Xen enables WC attribute in the PAT MSR for guests. 206 207 MTRR PAT Call Sequence PAT State PAT MSR 208 ========================================================= 209 E E MTRR -> PAT init Enabled OS 210 E D MTRR -> PAT init Disabled - 211 D E MTRR -> PAT disable Disabled BIOS 212 D D MTRR -> PAT disable Disabled - 213 - np/E PAT -> PAT disable Disabled BIOS 214 - np/D PAT -> PAT disable Disabled - 215 E !P/E MTRR -> PAT init Disabled BIOS 216 D !P/E MTRR -> PAT disable Disabled BIOS 217 !M !P/E MTRR stub -> PAT disable Disabled BIOS 218 219 Legend 220 ------------------------------------------------ 221 E Feature enabled in CPU 222 D Feature disabled/unsupported in CPU 223 np "nopat" boot option specified 224 !P CONFIG_X86_PAT option unset 225 !M CONFIG_MTRR option unset 226 Enabled PAT state set to enabled 227 Disabled PAT state set to disabled 228 OS PAT initializes PAT MSR with OS setting 229 BIOS PAT keeps PAT MSR with BIOS setting 230 231
