1=========================
2Unaligned 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
13Linux runs on a wide variety of architectures which have varying behaviour
14when it comes to memory access. This document presents some details about
15unaligned accesses, why you need to write code that doesn't cause them,
16and how to write such code!
17
18
19The definition of an unaligned access
20=====================================
21
22Unaligned memory accesses occur when you try to read N bytes of data starting
23from an address that is not evenly divisible by N (i.e. addr % N != 0).
24For example, reading 4 bytes of data from address 0x10004 is fine, but
25reading 4 bytes of data from address 0x10005 would be an unaligned memory
26access.
27
28The above may seem a little vague, as memory access can happen in different
29ways. The context here is at the machine code level: certain instructions read
30or write a number of bytes to or from memory (e.g. movb, movw, movl in x86
31assembly). As will become clear, it is relatively easy to spot C statements
32which will compile to multiple-byte memory access instructions, namely when
33dealing with types such as u16, u32 and u64.
34
35
36Natural alignment
37=================
38
39The rule mentioned above forms what we refer to as natural alignment:
40When accessing N bytes of memory, the base memory address must be evenly
41divisible by N, i.e. addr % N == 0.
42
43When writing code, assume the target architecture has natural alignment
44requirements.
45
46In reality, only a few architectures require natural alignment on all sizes
47of memory access. However, we must consider ALL supported architectures;
48writing code that satisfies natural alignment requirements is the easiest way
49to achieve full portability.
50
51
52Why unaligned access is bad
53===========================
54
55The effects of performing an unaligned memory access vary from architecture
56to architecture. It would be easy to write a whole document on the differences
57here; 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
71It should be obvious from the above that if your code causes unaligned
72memory accesses to happen, your code will not work correctly on certain
73platforms and will cause performance problems on others.
74
75
76Code that does not cause unaligned access
77=========================================
78
79At first, the concepts above may seem a little hard to relate to actual
80coding practice. After all, you don't have a great deal of control over
81memory addresses of certain variables, etc.
82
83Fortunately things are not too complex, as in most cases, the compiler
84ensures that things will work for you. For example, take the following
85structure::
86
87	struct foo {
88		u16 field1;
89		u32 field2;
90		u8 field3;
91	};
92
93Let us assume that an instance of the above structure resides in memory
94starting at address 0x10000. With a basic level of understanding, it would
95not be unreasonable to expect that accessing field2 would cause an unaligned
96access. You'd be expecting field2 to be located at offset 2 bytes into the
97structure, i.e. address 0x10002, but that address is not evenly divisible
98by 4 (remember, we're reading a 4 byte value here).
99
100Fortunately, the compiler understands the alignment constraints, so in the
101above case it would insert 2 bytes of padding in between field1 and field2.
102Therefore, for standard structure types you can always rely on the compiler
103to pad structures so that accesses to fields are suitably aligned (assuming
104you do not cast the field to a type of different length).
105
106Similarly, you can also rely on the compiler to align variables and function
107parameters to a naturally aligned scheme, based on the size of the type of
108the variable.
109
110At this point, it should be clear that accessing a single byte (u8 or char)
111will never cause an unaligned access, because all memory addresses are evenly
112divisible by one.
113
114On a related topic, with the above considerations in mind you may observe
115that you could reorder the fields in the structure in order to place fields
116where padding would otherwise be inserted, and hence reduce the overall
117resident memory size of structure instances. The optimal layout of the
118above example is::
119
120	struct foo {
121		u32 field2;
122		u16 field1;
123		u8 field3;
124	};
125
126For a natural alignment scheme, the compiler would only have to add a single
127byte of padding at the end of the structure. This padding is added in order
128to satisfy alignment constraints for arrays of these structures.
129
130Another point worth mentioning is the use of __attribute__((packed)) on a
131structure type. This GCC-specific attribute tells the compiler never to
132insert any padding within structures, useful when you want to use a C struct
133to represent some data that comes in a fixed arrangement 'off the wire'.
134
135You might be inclined to believe that usage of this attribute can easily
136lead to unaligned accesses when accessing fields that do not satisfy
137architectural alignment requirements. However, again, the compiler is aware
138of the alignment constraints and will generate extra instructions to perform
139the memory access in a way that does not cause unaligned access. Of course,
140the extra instructions obviously cause a loss in performance compared to the
141non-packed case, so the packed attribute should only be used when avoiding
142structure padding is of importance.
143
144
145Code that causes unaligned access
146=================================
147
148With the above in mind, let's move onto a real life example of a function
149that can cause an unaligned memory access. The following function taken
150from include/linux/etherdevice.h is an optimized routine to compare two
151ethernet 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
167In the above function, when the hardware has efficient unaligned access
168capability, there is no issue with this code.  But when the hardware isn't
169able to access memory on arbitrary boundaries, the reference to a[0] causes
1702 bytes (16 bits) to be read from memory starting at address addr1.
171
172Think about what would happen if addr1 was an odd address such as 0x10003.
173(Hint: it'd be an unaligned access.)
174
175Despite the potential unaligned access problems with the above function, it
176is included in the kernel anyway but is understood to only work normally on
17716-bit-aligned addresses. It is up to the caller to ensure this alignment or
178not use this function at all. This alignment-unsafe function is still useful
179as it is a decent optimization for the cases when you can ensure alignment,
180which is true almost all of the time in ethernet networking context.
181
182
183Here 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
192This code will cause unaligned accesses every time the data parameter points
193to an address that is not evenly divisible by 4.
194
195In summary, the 2 main scenarios where you may run into unaligned access
196problems 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
202Avoiding unaligned accesses
203===========================
204
205The easiest way to avoid unaligned access is to use the get_unaligned() and
206put_unaligned() macros provided by the <asm/unaligned.h> header file.
207
208Going back to an earlier example of code that potentially causes unaligned
209access::
210
211	void myfunc(u8 *data, u32 value)
212	{
213		[...]
214		*((u32 *) data) = cpu_to_le32(value);
215		[...]
216	}
217
218To 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
228The get_unaligned() macro works similarly. Assuming 'data' is a pointer to
229memory and you wish to avoid unaligned access, its usage is as follows::
230
231	u32 value = get_unaligned((u32 *) data);
232
233These macros work for memory accesses of any length (not just 32 bits as
234in the examples above). Be aware that when compared to standard access of
235aligned memory, using these macros to access unaligned memory can be costly in
236terms of performance.
237
238If use of such macros is not convenient, another option is to use memcpy(),
239where the source or destination (or both) are of type u8* or unsigned char*.
240Due to the byte-wise nature of this operation, unaligned accesses are avoided.
241
242
243Alignment vs. Networking
244========================
245
246On architectures that require aligned loads, networking requires that the IP
247header is aligned on a four-byte boundary to optimise the IP stack. For
248regular ethernet hardware, the constant NET_IP_ALIGN is used. On most
249architectures this constant has the value 2 because the normal ethernet
250header is 14 bytes long, so in order to get proper alignment one needs to
251DMA to an address which can be expressed as 4*n + 2. One notable exception
252here is powerpc which defines NET_IP_ALIGN to 0 because DMA to unaligned
253addresses can be very expensive and dwarf the cost of unaligned loads.
254
255For some ethernet hardware that cannot DMA to unaligned addresses like
2564*n+2 or non-ethernet hardware, this can be a problem, and it is then
257required to copy the incoming frame into an aligned buffer. Because this is
258unnecessary on architectures that can do unaligned accesses, the code can be
259made 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
266