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HACKING: Document vaddr type usage

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Also extend documentation of target_ulong and abi_ulong.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Andreas Färber <afaerber@suse.de>
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Peter Maydell 2013-07-22 18:34:35 +02:00 committed by Andreas Färber
parent 577f42c0e1
commit 2be8d45098
1 changed files with 17 additions and 2 deletions
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HACKING
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@ -40,8 +40,23 @@ speaking, the size of guest memory can always fit into ram_addr_t but
it would not be correct to store an actual guest physical address in a
ram_addr_t.
Use target_ulong (or abi_ulong) for CPU virtual addresses, however
devices should not need to use target_ulong.
For CPU virtual addresses there are several possible types.
vaddr is the best type to use to hold a CPU virtual address in
target-independent code. It is guaranteed to be large enough to hold a
virtual address for any target, and it does not change size from target
to target. It is always unsigned.
target_ulong is a type the size of a virtual address on the CPU; this means
it may be 32 or 64 bits depending on which target is being built. It should
therefore be used only in target-specific code, and in some
performance-critical built-per-target core code such as the TLB code.
There is also a signed version, target_long.
abi_ulong is for the *-user targets, and represents a type the size of
'void *' in that target's ABI. (This may not be the same as the size of a
full CPU virtual address in the case of target ABIs which use 32 bit pointers
on 64 bit CPUs, like sparc32plus.) Definitions of structures that must match
the target's ABI must use this type for anything that on the target is defined
to be an 'unsigned long' or a pointer type.
There is also a signed version, abi_long.
Of course, take all of the above with a grain of salt. If you're about
to use some system interface that requires a type like size_t, pid_t or