[RFC v2] Documentation about unaligned memory access

[Daniel Drake <dsd@gentoo.org>]

New version of the unaligned access document I posted recently. Thanks for
all the feedback, I've learned a lot!

Changes:
- 'in between' spelling fix
- shortened example addresses for readability
- better summarised the common architectural differences in handling
  unaligned access under "Why unaligned access is bad"
- expanded the notes on structure field ordering vs memory usage (trying not
  to go too far off topic though)
- correction regarding packed attribute: compiler will generate extra
  instructions, so accessing __attribute__((packed)) structures in standard
  ways will never cause unaligned access
- natural alignment is defined earlier in the document
- memcpy is now the alternative suggestion, put_unaligned/get_unaligned is
  the encouraged solution

There were some suggestions I didn't include. I'd like this document to
remain as a concise and general overview of the problems, not focusing on
too many details. In other words I'm trying to produce a document that *I*
would have found useful to write generic portable code. For example the fact
that mc68020+ has different alignment requirements from mc68000 isn't of
much value here, I just want to know the fundamentals of writing code that
works everywhere.

On the other hand I can see why such information would be useful for other
scenarios, so maybe someone with a good understanding should collect all the
fine details into an 'advanced unaligned memory access topics' document.
Here's a list of the suggestions/discussions I excluded:

- table of alignment requirements for architectures
- details of performance costs of unaligned accesses on different processors
- memcpy discussion

Assuming there aren't too many comments/suggestions on this revision, the
next version will be submitted for inclusion as
Documentation/unaligned_memory_access.txt
  




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.


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 transparently perform unaligned memory
   accesses, 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, 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(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 costy 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 Moffett, Robert Hancock, Uli Kunitz, Vadim Lobanov

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