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-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
-