1 =========================
2 Unaligned Memory Accesses
3 =========================
4
5 :Author: Daniel Drake <dsd@gentoo.org>,
6 :Author: Johannes Berg <johannes@sipsolutions.net>
7
8 :With help from: Alan Cox, Avuton Olrich, Heikki Orsila, Jan Engelhardt,
9 Kyle McMartin, Kyle Moffett, Randy Dunlap, Robert Hancock, Uli Kunitz,
10 Vadim Lobanov
11
12
13 Linux runs on a wide variety of architectures which have varying behaviour
14 when it comes to memory access. This document presents some details about
15 unaligned accesses, why you need to write code that doesn't cause them,
16 and how to write such code!
17
18
19 The definition of an unaligned access
20 =====================================
21
22 Unaligned memory accesses occur when you try to read N bytes of data starting
23 from an address that is not evenly divisible by N (i.e. addr % N != 0).
24 For example, reading 4 bytes of data from address 0x10004 is fine, but
25 reading 4 bytes of data from address 0x10005 would be an unaligned memory
26 access.
27
28 The above may seem a little vague, as memory access can happen in different
29 ways. The context here is at the machine code level: certain instructions read
30 or write a number of bytes to or from memory (e.g. movb, movw, movl in x86
31 assembly). As will become clear, it is relatively easy to spot C statements
32 which will compile to multiple-byte memory access instructions, namely when
33 dealing with types such as u16, u32 and u64.
34
35
36 Natural alignment
37 =================
38
39 The rule mentioned above forms what we refer to as natural alignment:
40 When accessing N bytes of memory, the base memory address must be evenly
41 divisible by N, i.e. addr % N == 0.
42
43 When writing code, assume the target architecture has natural alignment
44 requirements.
45
46 In reality, only a few architectures require natural alignment on all sizes
47 of memory access. However, we must consider ALL supported architectures;
48 writing code that satisfies natural alignment requirements is the easiest way
49 to achieve full portability.
50
51
52 Why unaligned access is bad
53 ===========================
54
55 The effects of performing an unaligned memory access vary from architecture
56 to architecture. It would be easy to write a whole document on the differences
57 here; a summary of the common scenarios is presented below:
58
59 - Some architectures are able to perform unaligned memory accesses
60 transparently, but there is usually a significant performance cost.
61 - Some architectures raise processor exceptions when unaligned accesses
62 happen. The exception handler is able to correct the unaligned access,
63 at significant cost to performance.
64 - Some architectures raise processor exceptions when unaligned accesses
65 happen, but the exceptions do not contain enough information for the
66 unaligned access to be corrected.
67 - Some architectures are not capable of unaligned memory access, but will
68 silently perform a different memory access to the one that was requested,
69 resulting in a subtle code bug that is hard to detect!
70
71 It should be obvious from the above that if your code causes unaligned
72 memory accesses to happen, your code will not work correctly on certain
73 platforms and will cause performance problems on others.
74
75
76 Code that does not cause unaligned access
77 =========================================
78
79 At first, the concepts above may seem a little hard to relate to actual
80 coding practice. After all, you don't have a great deal of control over
81 memory addresses of certain variables, etc.
82
83 Fortunately things are not too complex, as in most cases, the compiler
84 ensures that things will work for you. For example, take the following
85 structure::
86
87 struct foo {
88 u16 field1;
89 u32 field2;
90 u8 field3;
91 };
92
93 Let us assume that an instance of the above structure resides in memory
94 starting at address 0x10000. With a basic level of understanding, it would
95 not be unreasonable to expect that accessing field2 would cause an unaligned
96 access. You'd be expecting field2 to be located at offset 2 bytes into the
97 structure, i.e. address 0x10002, but that address is not evenly divisible
98 by 4 (remember, we're reading a 4 byte value here).
99
100 Fortunately, the compiler understands the alignment constraints, so in the
101 above case it would insert 2 bytes of padding in between field1 and field2.
102 Therefore, for standard structure types you can always rely on the compiler
103 to pad structures so that accesses to fields are suitably aligned (assuming
104 you do not cast the field to a type of different length).
105
106 Similarly, you can also rely on the compiler to align variables and function
107 parameters to a naturally aligned scheme, based on the size of the type of
108 the variable.
109
110 At this point, it should be clear that accessing a single byte (u8 or char)
111 will never cause an unaligned access, because all memory addresses are evenly
112 divisible by one.
113
114 On a related topic, with the above considerations in mind you may observe
115 that you could reorder the fields in the structure in order to place fields
116 where padding would otherwise be inserted, and hence reduce the overall
117 resident memory size of structure instances. The optimal layout of the
118 above example is::
119
120 struct foo {
121 u32 field2;
122 u16 field1;
123 u8 field3;
124 };
125
126 For a natural alignment scheme, the compiler would only have to add a single
127 byte of padding at the end of the structure. This padding is added in order
128 to satisfy alignment constraints for arrays of these structures.
129
130 Another point worth mentioning is the use of __attribute__((packed)) on a
131 structure type. This GCC-specific attribute tells the compiler never to
132 insert any padding within structures, useful when you want to use a C struct
133 to represent some data that comes in a fixed arrangement 'off the wire'.
134
135 You might be inclined to believe that usage of this attribute can easily
136 lead to unaligned accesses when accessing fields that do not satisfy
137 architectural alignment requirements. However, again, the compiler is aware
138 of the alignment constraints and will generate extra instructions to perform
139 the memory access in a way that does not cause unaligned access. Of course,
140 the extra instructions obviously cause a loss in performance compared to the
141 non-packed case, so the packed attribute should only be used when avoiding
142 structure padding is of importance.
143
144
145 Code that causes unaligned access
146 =================================
147
148 With the above in mind, let's move onto a real life example of a function
149 that can cause an unaligned memory access. The following function taken
150 from include/linux/etherdevice.h is an optimized routine to compare two
151 ethernet MAC addresses for equality::
152
153 bool ether_addr_equal(const u8 *addr1, const u8 *addr2)
154 {
155 #ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
156 u32 fold = ((*(const u32 *)addr1) ^ (*(const u32 *)addr2)) |
157 ((*(const u16 *)(addr1 + 4)) ^ (*(const u16 *)(addr2 + 4)));
158
159 return fold == 0;
160 #else
161 const u16 *a = (const u16 *)addr1;
162 const u16 *b = (const u16 *)addr2;
163 return ((a[0] ^ b[0]) | (a[1] ^ b[1]) | (a[2] ^ b[2])) == 0;
164 #endif
165 }
166
167 In the above function, when the hardware has efficient unaligned access
168 capability, there is no issue with this code. But when the hardware isn't
169 able to access memory on arbitrary boundaries, the reference to a[0] causes
170 2 bytes (16 bits) to be read from memory starting at address addr1.
171
172 Think about what would happen if addr1 was an odd address such as 0x10003.
173 (Hint: it'd be an unaligned access.)
174
175 Despite the potential unaligned access problems with the above function, it
176 is included in the kernel anyway but is understood to only work normally on
177 16-bit-aligned addresses. It is up to the caller to ensure this alignment or
178 not use this function at all. This alignment-unsafe function is still useful
179 as it is a decent optimization for the cases when you can ensure alignment,
180 which is true almost all of the time in ethernet networking context.
181
182
183 Here is another example of some code that could cause unaligned accesses::
184
185 void myfunc(u8 *data, u32 value)
186 {
187 [...]
188 *((u32 *) data) = cpu_to_le32(value);
189 [...]
190 }
191
192 This code will cause unaligned accesses every time the data parameter points
193 to an address that is not evenly divisible by 4.
194
195 In summary, the 2 main scenarios where you may run into unaligned access
196 problems involve:
197
198 1. Casting variables to types of different lengths
199 2. Pointer arithmetic followed by access to at least 2 bytes of data
200
201
202 Avoiding unaligned accesses
203 ===========================
204
205 The easiest way to avoid unaligned access is to use the get_unaligned() and
206 put_unaligned() macros provided by the <linux/unaligned.h> header file.
207
208 Going back to an earlier example of code that potentially causes unaligned
209 access::
210
211 void myfunc(u8 *data, u32 value)
212 {
213 [...]
214 *((u32 *) data) = cpu_to_le32(value);
215 [...]
216 }
217
218 To avoid the unaligned memory access, you would rewrite it as follows::
219
220 void myfunc(u8 *data, u32 value)
221 {
222 [...]
223 value = cpu_to_le32(value);
224 put_unaligned(value, (u32 *) data);
225 [...]
226 }
227
228 The get_unaligned() macro works similarly. Assuming 'data' is a pointer to
229 memory and you wish to avoid unaligned access, its usage is as follows::
230
231 u32 value = get_unaligned((u32 *) data);
232
233 These macros work for memory accesses of any length (not just 32 bits as
234 in the examples above). Be aware that when compared to standard access of
235 aligned memory, using these macros to access unaligned memory can be costly in
236 terms of performance.
237
238 If use of such macros is not convenient, another option is to use memcpy(),
239 where the source or destination (or both) are of type u8* or unsigned char*.
240 Due to the byte-wise nature of this operation, unaligned accesses are avoided.
241
242
243 Alignment vs. Networking
244 ========================
245
246 On architectures that require aligned loads, networking requires that the IP
247 header is aligned on a four-byte boundary to optimise the IP stack. For
248 regular ethernet hardware, the constant NET_IP_ALIGN is used. On most
249 architectures this constant has the value 2 because the normal ethernet
250 header is 14 bytes long, so in order to get proper alignment one needs to
251 DMA to an address which can be expressed as 4*n + 2. One notable exception
252 here is powerpc which defines NET_IP_ALIGN to 0 because DMA to unaligned
253 addresses can be very expensive and dwarf the cost of unaligned loads.
254
255 For some ethernet hardware that cannot DMA to unaligned addresses like
256 4*n+2 or non-ethernet hardware, this can be a problem, and it is then
257 required to copy the incoming frame into an aligned buffer. Because this is
258 unnecessary on architectures that can do unaligned accesses, the code can be
259 made dependent on CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS like so::
260
261 #ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
262 skb = original skb
263 #else
264 skb = copy skb
265 #endif
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