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Based on kernel version 4.13.3. Page generated on 2017-09-23 13:56 EST.

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 <asm/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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