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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 | There are many different APIs in the kernel that allows setting of memory |
18 | attributes at the page level. In order to avoid aliasing, these interfaces |
19 | should be used thoughtfully. Below is a table of interfaces available, |
20 | their intended usage and their memory attribute relationships. Internally, |
21 | these APIs use a reserve_memtype()/free_memtype() interface on the physical |
22 | address range to avoid any aliasing. |
23 | |
24 | |
25 | ------------------------------------------------------------------- |
26 | API | RAM | ACPI,... | Reserved/Holes | |
27 | -----------------------|----------|------------|------------------| |
28 | | | | | |
29 | ioremap | -- | UC | UC | |
30 | | | | | |
31 | ioremap_cache | -- | WB | WB | |
32 | | | | | |
33 | ioremap_nocache | -- | UC | UC | |
34 | | | | | |
35 | ioremap_wc | -- | -- | WC | |
36 | | | | | |
37 | set_memory_uc | UC | -- | -- | |
38 | set_memory_wb | | | | |
39 | | | | | |
40 | set_memory_wc | WC | -- | -- | |
41 | set_memory_wb | | | | |
42 | | | | | |
43 | pci sysfs resource | -- | -- | UC | |
44 | | | | | |
45 | pci sysfs resource_wc | -- | -- | WC | |
46 | is IORESOURCE_PREFETCH| | | | |
47 | | | | | |
48 | pci proc | -- | -- | UC | |
49 | !PCIIOC_WRITE_COMBINE | | | | |
50 | | | | | |
51 | pci proc | -- | -- | WC | |
52 | PCIIOC_WRITE_COMBINE | | | | |
53 | | | | | |
54 | /dev/mem | -- | UC | UC | |
55 | read-write | | | | |
56 | | | | | |
57 | /dev/mem | -- | UC | UC | |
58 | mmap SYNC flag | | | | |
59 | | | | | |
60 | /dev/mem | -- | WB/WC/UC | WB/WC/UC | |
61 | mmap !SYNC flag | |(from exist-| (from exist- | |
62 | and | | ing alias)| ing alias) | |
63 | any alias to this area| | | | |
64 | | | | | |
65 | /dev/mem | -- | WB | WB | |
66 | mmap !SYNC flag | | | | |
67 | no alias to this area | | | | |
68 | and | | | | |
69 | MTRR says WB | | | | |
70 | | | | | |
71 | /dev/mem | -- | -- | UC_MINUS | |
72 | mmap !SYNC flag | | | | |
73 | no alias to this area | | | | |
74 | and | | | | |
75 | MTRR says !WB | | | | |
76 | | | | | |
77 | ------------------------------------------------------------------- |
78 | |
79 | Notes: |
80 | |
81 | -- in the above table mean "Not suggested usage for the API". Some of the --'s |
82 | are strictly enforced by the kernel. Some others are not really enforced |
83 | today, but may be enforced in future. |
84 | |
85 | For ioremap and pci access through /sys or /proc - The actual type returned |
86 | can be more restrictive, in case of any existing aliasing for that address. |
87 | For example: If there is an existing uncached mapping, a new ioremap_wc can |
88 | return uncached mapping in place of write-combine requested. |
89 | |
90 | set_memory_[uc|wc] and set_memory_wb should be used in pairs, where driver will |
91 | first make a region uc or wc and switch it back to wb after use. |
92 | |
93 | Over time writes to /proc/mtrr will be deprecated in favor of using PAT based |
94 | interfaces. Users writing to /proc/mtrr are suggested to use above interfaces. |
95 | |
96 | Drivers should use ioremap_[uc|wc] to access PCI BARs with [uc|wc] access |
97 | types. |
98 | |
99 | Drivers should use set_memory_[uc|wc] to set access type for RAM ranges. |
100 | |