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2 PAT (Page Attribute Table)
4 x86 Page Attribute Table (PAT) allows for setting the memory attribute at the
5 page level granularity. PAT is complementary to the MTRR settings which allows
6 for setting of memory types over physical address ranges. However, PAT is
7 more flexible than MTRR due to its capability to set attributes at page level
8 and also due to the fact that there are no hardware limitations on number of
9 such attribute settings allowed. Added flexibility comes with guidelines for
10 not having memory type aliasing for the same physical memory with multiple
11 virtual addresses.
13 PAT allows for different types of memory attributes. The most commonly used
14 ones that will be supported at this time are Write-back, Uncached,
15 Write-combined and Uncached Minus.
18 PAT APIs
19 --------
21 There are many different APIs in the kernel that allows setting of memory
22 attributes at the page level. In order to avoid aliasing, these interfaces
23 should be used thoughtfully. Below is a table of interfaces available,
24 their intended usage and their memory attribute relationships. Internally,
25 these APIs use a reserve_memtype()/free_memtype() interface on the physical
26 address range to avoid any aliasing.
29 -------------------------------------------------------------------
30 API | RAM | ACPI,... | Reserved/Holes |
31 -----------------------|----------|------------|------------------|
32 | | | |
33 ioremap | -- | UC- | UC- |
34 | | | |
35 ioremap_cache | -- | WB | WB |
36 | | | |
37 ioremap_nocache | -- | UC- | UC- |
38 | | | |
39 ioremap_wc | -- | -- | WC |
40 | | | |
41 set_memory_uc | UC- | -- | -- |
42 set_memory_wb | | | |
43 | | | |
44 set_memory_wc | WC | -- | -- |
45 set_memory_wb | | | |
46 | | | |
47 pci sysfs resource | -- | -- | UC- |
48 | | | |
49 pci sysfs resource_wc | -- | -- | WC |
50 is IORESOURCE_PREFETCH| | | |
51 | | | |
52 pci proc | -- | -- | UC- |
53 !PCIIOC_WRITE_COMBINE | | | |
54 | | | |
55 pci proc | -- | -- | WC |
56 PCIIOC_WRITE_COMBINE | | | |
57 | | | |
58 /dev/mem | -- | WB/WC/UC- | WB/WC/UC- |
59 read-write | | | |
60 | | | |
61 /dev/mem | -- | UC- | UC- |
62 mmap SYNC flag | | | |
63 | | | |
64 /dev/mem | -- | WB/WC/UC- | WB/WC/UC- |
65 mmap !SYNC flag | |(from exist-| (from exist- |
66 and | | ing alias)| ing alias) |
67 any alias to this area| | | |
68 | | | |
69 /dev/mem | -- | WB | WB |
70 mmap !SYNC flag | | | |
71 no alias to this area | | | |
72 and | | | |
73 MTRR says WB | | | |
74 | | | |
75 /dev/mem | -- | -- | UC- |
76 mmap !SYNC flag | | | |
77 no alias to this area | | | |
78 and | | | |
79 MTRR says !WB | | | |
80 | | | |
81 -------------------------------------------------------------------
83 Advanced APIs for drivers
84 -------------------------
85 A. Exporting pages to users with remap_pfn_range, io_remap_pfn_range,
86 vm_insert_pfn
88 Drivers wanting to export some pages to userspace do it by using mmap
89 interface and a combination of
90 1) pgprot_noncached()
91 2) io_remap_pfn_range() or remap_pfn_range() or vm_insert_pfn()
93 With PAT support, a new API pgprot_writecombine is being added. So, drivers can
94 continue to use the above sequence, with either pgprot_noncached() or
95 pgprot_writecombine() in step 1, followed by step 2.
97 In addition, step 2 internally tracks the region as UC or WC in memtype
98 list in order to ensure no conflicting mapping.
100 Note that this set of APIs only works with IO (non RAM) regions. If driver
101 wants to export a RAM region, it has to do set_memory_uc() or set_memory_wc()
102 as step 0 above and also track the usage of those pages and use set_memory_wb()
103 before the page is freed to free pool.
107 Notes:
109 -- in the above table mean "Not suggested usage for the API". Some of the --'s
110 are strictly enforced by the kernel. Some others are not really enforced
111 today, but may be enforced in future.
113 For ioremap and pci access through /sys or /proc - The actual type returned
114 can be more restrictive, in case of any existing aliasing for that address.
115 For example: If there is an existing uncached mapping, a new ioremap_wc can
116 return uncached mapping in place of write-combine requested.
118 set_memory_[uc|wc] and set_memory_wb should be used in pairs, where driver will
119 first make a region uc or wc and switch it back to wb after use.
121 Over time writes to /proc/mtrr will be deprecated in favor of using PAT based
122 interfaces. Users writing to /proc/mtrr are suggested to use above interfaces.
124 Drivers should use ioremap_[uc|wc] to access PCI BARs with [uc|wc] access
125 types.
127 Drivers should use set_memory_[uc|wc] to set access type for RAM ranges.
130 PAT debugging
131 -------------
133 With CONFIG_DEBUG_FS enabled, PAT memtype list can be examined by
135 # mount -t debugfs debugfs /sys/kernel/debug
136 # cat /sys/kernel/debug/x86/pat_memtype_list
137 PAT memtype list:
138 uncached-minus @ 0x7fadf000-0x7fae0000
139 uncached-minus @ 0x7fb19000-0x7fb1a000
140 uncached-minus @ 0x7fb1a000-0x7fb1b000
141 uncached-minus @ 0x7fb1b000-0x7fb1c000
142 uncached-minus @ 0x7fb1c000-0x7fb1d000
143 uncached-minus @ 0x7fb1d000-0x7fb1e000
144 uncached-minus @ 0x7fb1e000-0x7fb25000
145 uncached-minus @ 0x7fb25000-0x7fb26000
146 uncached-minus @ 0x7fb26000-0x7fb27000
147 uncached-minus @ 0x7fb27000-0x7fb28000
148 uncached-minus @ 0x7fb28000-0x7fb2e000
149 uncached-minus @ 0x7fb2e000-0x7fb2f000
150 uncached-minus @ 0x7fb2f000-0x7fb30000
151 uncached-minus @ 0x7fb31000-0x7fb32000
152 uncached-minus @ 0x80000000-0x90000000
154 This list shows physical address ranges and various PAT settings used to
155 access those physical address ranges.
157 Another, more verbose way of getting PAT related debug messages is with
158 "debugpat" boot parameter. With this parameter, various debug messages are
159 printed to dmesg log.