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Linux/Documentation/unaligned-memory-access.txt

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

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