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Based on kernel version 3.13. Page generated on 2014-01-20 22:05 EST.

1	
2	PAT (Page Attribute Table)
3	
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.
12	
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.
16	
17	
18	PAT APIs
19	--------
20	
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.
27	
28	
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	-------------------------------------------------------------------
82	
83	Advanced APIs for drivers
84	-------------------------
85	A. Exporting pages to users with remap_pfn_range, io_remap_pfn_range,
86	vm_insert_pfn
87	
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()
92	
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.
96	
97	In addition, step 2 internally tracks the region as UC or WC in memtype
98	list in order to ensure no conflicting mapping.
99	
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.
104	
105	
106	
107	Notes:
108	
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.
112	
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.
117	
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.
120	
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.
123	
124	Drivers should use ioremap_[uc|wc] to access PCI BARs with [uc|wc] access
125	types.
126	
127	Drivers should use set_memory_[uc|wc] to set access type for RAM ranges.
128	
129	
130	PAT debugging
131	-------------
132	
133	With CONFIG_DEBUG_FS enabled, PAT memtype list can be examined by
134	
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
153	
154	This list shows physical address ranges and various PAT settings used to
155	access those physical address ranges.
156	
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.
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