Documentation about unaligned memory access

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