mirror of https://github.com/proxmox/mirror_qemu
HACKING: Document vaddr type usage
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>master
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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
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it would not be correct to store an actual guest physical address in a
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it would not be correct to store an actual guest physical address in a
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ram_addr_t.
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ram_addr_t.
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Use target_ulong (or abi_ulong) for CPU virtual addresses, however
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For CPU virtual addresses there are several possible types.
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devices should not need to use target_ulong.
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vaddr is the best type to use to hold a CPU virtual address in
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target-independent code. It is guaranteed to be large enough to hold a
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virtual address for any target, and it does not change size from target
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to target. It is always unsigned.
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target_ulong is a type the size of a virtual address on the CPU; this means
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it may be 32 or 64 bits depending on which target is being built. It should
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therefore be used only in target-specific code, and in some
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performance-critical built-per-target core code such as the TLB code.
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There is also a signed version, target_long.
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abi_ulong is for the *-user targets, and represents a type the size of
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'void *' in that target's ABI. (This may not be the same as the size of a
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full CPU virtual address in the case of target ABIs which use 32 bit pointers
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on 64 bit CPUs, like sparc32plus.) Definitions of structures that must match
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the target's ABI must use this type for anything that on the target is defined
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to be an 'unsigned long' or a pointer type.
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There is also a signed version, abi_long.
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Of course, take all of the above with a grain of salt. If you're about
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Of course, take all of the above with a grain of salt. If you're about
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to use some system interface that requires a type like size_t, pid_t or
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to use some system interface that requires a type like size_t, pid_t or
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