FreeBSD/Linux Kernel Cross Reference
sys/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.
    
    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
   

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