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 in 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 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.

  
  Alignment vs. Networking
  ========================
  
  On architectures that require aligned loads, networking requires that the IP
  header is aligned on a four-byte boundary to optimise the IP stack. For
  regular ethernet hardware, the constant NET_IP_ALIGN is used. On most
  architectures this constant has the value 2 because the normal ethernet
  header is 14 bytes long, so in order to get proper alignment one needs to
  DMA to an address which can be expressed as 4*n + 2. One notable exception
  here is powerpc which defines NET_IP_ALIGN to 0 because DMA to unaligned
  addresses can be very expensive and dwarf the cost of unaligned loads.
  
  For some ethernet hardware that cannot DMA to unaligned addresses like
  4*n+2 or non-ethernet hardware, this can be a problem, and it is then
  required to copy the incoming frame into an aligned buffer. Because this is
  unnecessary on architectures that can do unaligned accesses, the code can be
  made dependent on CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS like so:
  
  #ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
  	skb = original skb
  #else
  	skb = copy skb
  #endif

  --

  Authors: Daniel Drake <dsd@gentoo.org>,
           Johannes Berg <johannes@sipsolutions.net>

  With help from: Alan Cox, Avuton Olrich, Heikki Orsila, Jan Engelhardt,

  Kyle McMartin, Kyle Moffett, Randy Dunlap, Robert Hancock, Uli Kunitz,
  Vadim Lobanov