[PATCH] Documentation about unaligned memory access

[Daniel Drake <dsd@gentoo.org>]

Here's a document I wrote after figuring out what unaligned memory access
is all about. I've tried to cover the information I was looking for when
trying to learn about this, without producing a hopelessly detailed/complex
spew. I hope it is useful to others.

Signed-off-by: Daniel Drake <dsd@gentoo.org>

---

Changes since the v2 draft I posted last week:
 - spelling/grammar fixes
 - fixed get_unaligned/put_unaligned examples
 - clarified the scope of unaligned access

--- /dev/null	2007-12-03 10:17:52.569007801 +0000
+++ linux/Documentation/unaligned-memory-access.txt	2007-12-03 16:04:55.000000000 +0000
@@ -0,0 +1,226 @@
+UNALIGNED MEMORY ACCESSES
+=========================
+
+Linux runs on a wide variety of architectures which have varying behaviour
+when it comes to memory access. This document presents some details about
+unaligned accesses, why you need to write code that doesn't cause them,
+and how to write such code!
+
+
+The definition of an unaligned access
+=====================================
+
+Unaligned memory accesses occur when you try to read N bytes of data starting
+from an address that is not evenly divisible by N (i.e. addr % N != 0).
+For example, reading 4 bytes of data from address 0x10004 is fine, but
+reading 4 bytes of data from address 0x10005 would be an unaligned memory
+access.
+
+The above may seem a little vague, as memory access can happen in different
+ways. The context here is at the machine code level: certain instructions read
+or write a number of bytes to or from memory (e.g. movb, movw, movl in x86
+assembly). As will become clear, it is relatively easy to spot C statements
+which will compile to multiple-byte memory access instructions, namely when
+dealing with types such as u16, u32 and u64.
+
+
+Natural alignment
+=================
+
+The rule mentioned above forms what we refer to as natural alignment:
+When accessing N bytes of memory, the base memory address must be evenly
+divisible by N, i.e. addr % N == 0.
+
+When writing code, assume the target architecture has natural alignment
+requirements.
+
+In reality, only a few architectures require natural alignment on all sizes
+of memory access. However, we must consider ALL supported architectures;
+writing code that satisfies natural alignment requirements is the easiest way
+to achieve full portability.
+
+
+Why unaligned access is bad
+===========================
+
+The effects of performing an unaligned memory access vary from architecture
+to architecture. It would be easy to write a whole document on the differences
+here; a summary of the common scenarios is presented below:
+
+ - Some architectures are able to perform unaligned memory accesses
+   transparently, but there is usually a significant performance cost.
+ - Some architectures raise processor exceptions when unaligned accesses
+   happen. The exception handler is able to correct the unaligned access,
+   at significant cost to performance.
+ - Some architectures raise processor exceptions when unaligned accesses
+   happen, but the exceptions do not contain enough information for the
+   unaligned access to be corrected.
+ - Some architectures are not capable of unaligned memory access, but will
+   silently perform a different memory access to the one that was requested,
+   resulting a a subtle code bug that is hard to detect!
+
+It should be obvious from the above that if your code causes unaligned
+memory accesses to happen, your code will not work correctly on certain
+platforms and will cause performance problems on others.
+
+
+Code that does not cause unaligned access
+=========================================
+
+At first, the concepts above may seem a little hard to relate to actual
+coding practice. After all, you don't have a great deal of control over
+memory addresses of certain variables, etc.
+
+Fortunately things are not too complex, as in most cases, the compiler
+ensures that things will work for you. For example, take the following
+structure:
+
+	struct foo {
+		u16 field1;
+		u32 field2;
+		u8 field3;
+	};
+
+Let us assume that an instance of the above structure resides in memory
+starting at address 0x10000. With a basic level of understanding, it would
+not be unreasonable to expect that accessing field2 would cause an unaligned
+access. You'd be expecting field2 to be located at offset 2 bytes into the
+structure, i.e. address 0x10002, but that address is not evenly divisible
+by 4 (remember, we're reading a 4 byte value here).
+
+Fortunately, the compiler understands the alignment constraints, so in the
+above case it would insert 2 bytes of padding in between field1 and field2.
+Therefore, for standard structure types you can always rely on the compiler
+to pad structures so that accesses to fields are suitably aligned (assuming
+you do not cast the field to a type of different length).
+
+Similarly, you can also rely on the compiler to align variables and function
+parameters to a naturally aligned scheme, based on the size of the type of
+the variable.
+
+At this point, it should be clear that accessing a single byte (u8 or char)
+will never cause an unaligned access, because all memory addresses are evenly
+divisible by one.
+
+On a related topic, with the above considerations in mind you may observe
+that you could reorder the fields in the structure in order to place fields
+where padding would otherwise be inserted, and hence reduce the overall
+resident memory size of structure instances. The optimal layout of the
+above example is:
+
+	struct foo {
+		u32 field2;
+		u16 field1;
+		u8 field3;
+	};
+
+For a natural alignment scheme, the compiler would only have to add a single
+byte of padding at the end of the structure. This padding is added in order
+to satisfy alignment constraints for arrays of these structures.
+
+Another point worth mentioning is the use of __attribute__((packed)) on a
+structure type. This GCC-specific attribute tells the compiler never to
+insert any padding within structures, useful when you want to use a C struct
+to represent some data that comes in a fixed arrangement 'off the wire'.
+
+You might be inclined to believe that usage of this attribute can easily
+lead to unaligned accesses when accessing fields that do not satisfy
+architectural alignment requirements. However, again, the compiler is aware
+of the alignment constraints and will generate extra instructions to perform
+the memory access in a way that does not cause unaligned access. Of course,
+the extra instructions obviously cause a loss in performance compared to the
+non-packed case, so the packed attribute should only be used when avoiding
+structure padding is of importance.
+
+
+Code that causes unaligned access
+=================================
+
+With the above in mind, let's move onto a real life example of a function
+that can cause an unaligned memory access. The following function adapted
+from include/linux/etherdevice.h is an optimized routine to compare two
+ethernet MAC addresses for equality.
+
+unsigned int compare_ether_addr(const u8 *addr1, const u8 *addr2)
+{
+	const u16 *a = (const u16 *) addr1;
+	const u16 *b = (const u16 *) addr2;
+	return ((a[0] ^ b[0]) | (a[1] ^ b[1]) | (a[2] ^ b[2])) != 0;
+}
+
+In the above function, the reference to a[0] causes 2 bytes (16 bits) to
+be read from memory starting at address addr1. Think about what would happen
+if addr1 was an odd address such as 0x10003. (Hint: it'd be an unaligned
+access.)
+
+Despite the potential unaligned access problems with the above function, it
+is included in the kernel anyway but is understood to only work on
+16-bit-aligned addresses. It is up to the caller to ensure this alignment or
+not use this function at all. This alignment-unsafe function is still useful
+as it is a decent optimization for the cases when you can ensure alignment,
+which is true almost all of the time in ethernet networking context.
+
+
+Here is another example of some code that could cause unaligned accesses:
+	void myfunc(u8 *data, u32 value)
+	{
+		[...]
+		*((u32 *) data) = cpu_to_le32(value);
+		[...]
+	}
+
+This code will cause unaligned accesses every time the data parameter points
+to an address that is not evenly divisible by 4.
+
+In summary, the 2 main scenarios where you may run into unaligned access
+problems involve:
+ 1. Casting variables to types of different lengths
+ 2. Pointer arithmetic followed by access to at least 2 bytes of data
+
+
+Avoiding unaligned accesses
+===========================
+
+The easiest way to avoid unaligned access is to use the get_unaligned() and
+put_unaligned() macros provided by the <asm/unaligned.h> header file.
+
+Going back to an earlier example of code that potentially causes unaligned
+access:
+
+	void myfunc(u8 *data, u32 value)
+	{
+		[...]
+		*((u32 *) data) = cpu_to_le32(value);
+		[...]
+	}
+
+To avoid the unaligned memory access, you would rewrite it as follows:
+
+	void myfunc(u8 *data, u32 value)
+	{
+		[...]
+		value = cpu_to_le32(value);
+		put_unaligned(value, (u32 *) data);
+		[...]
+	}
+
+The get_unaligned() macro works similarly. Assuming 'data' is a pointer to
+memory and you wish to avoid unaligned access, its usage is as follows:
+
+	u32 value = get_unaligned((u32 *) data);
+
+These macros work work for memory accesses of any length (not just 32 bits as
+in the examples above). Be aware that when compared to standard access of
+aligned memory, using these macros to access unaligned memory can be costly in
+terms of performance.
+
+If use of such macros is not convenient, another option is to use memcpy(),
+where the source or destination (or both) are of type u8* or unsigned char*.
+Due to the byte-wise nature of this operation, unaligned accesses are avoided.
+
+--
+Author: Daniel Drake <dsd@gentoo.org>
+With help from: Alan Cox, Avuton Olrich, Heikki Orsila, Jan Engelhardt,
+Johannes Berg, Kyle McMartin, Kyle Moffett, Randy Dunlap, Robert Hancock,
+Uli Kunitz, Vadim Lobanov
+
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