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-===============================================
-Power Architecture 64-bit Linux system call ABI
-===============================================
-
-syscall
-=======
-
-syscall calling sequence[*] matches the Power Architecture 64-bit ELF ABI
-specification C function calling sequence, including register preservation
-rules, with the following differences.
-
-[*] Some syscalls (typically low-level management functions) may have
- different calling sequences (e.g., rt_sigreturn).
-
-Parameters and return value
----------------------------
-The system call number is specified in r0.
-
-There is a maximum of 6 integer parameters to a syscall, passed in r3-r8.
-
-Both a return value and a return error code are returned. cr0.SO is the return
-error code, and r3 is the return value or error code. When cr0.SO is clear,
-the syscall succeeded and r3 is the return value. When cr0.SO is set, the
-syscall failed and r3 is the error code that generally corresponds to errno.
-
-Stack
------
-System calls do not modify the caller's stack frame. For example, the caller's
-stack frame LR and CR save fields are not used.
-
-Register preservation rules
----------------------------
-Register preservation rules match the ELF ABI calling sequence with the
-following differences:
-
-r0: Volatile. (System call number.)
-r3: Volatile. (Parameter 1, and return value.)
-r4-r8: Volatile. (Parameters 2-6.)
-cr0: Volatile (cr0.SO is the return error condition)
-cr1, cr5-7: Nonvolatile.
-lr: Nonvolatile.
-
-All floating point and vector data registers as well as control and status
-registers are nonvolatile.
-
-Invocation
-----------
-The syscall is performed with the sc instruction, and returns with execution
-continuing at the instruction following the sc instruction.
-
-Transactional Memory
---------------------
-Syscall behavior can change if the processor is in transactional or suspended
-transaction state, and the syscall can affect the behavior of the transaction.
-
-If the processor is in suspended state when a syscall is made, the syscall
-will be performed as normal, and will return as normal. The syscall will be
-performed in suspended state, so its side effects will be persistent according
-to the usual transactional memory semantics. A syscall may or may not result
-in the transaction being doomed by hardware.
-
-If the processor is in transactional state when a syscall is made, then the
-behavior depends on the presence of PPC_FEATURE2_HTM_NOSC in the AT_HWCAP2 ELF
-auxiliary vector.
-
-- If present, which is the case for newer kernels, then the syscall will not
- be performed and the transaction will be doomed by the kernel with the
- failure code TM_CAUSE_SYSCALL | TM_CAUSE_PERSISTENT in the TEXASR SPR.
-
-- If not present (older kernels), then the kernel will suspend the
- transactional state and the syscall will proceed as in the case of a
- suspended state syscall, and will resume the transactional state before
- returning to the caller. This case is not well defined or supported, so this
- behavior should not be relied upon.
-
-
-vsyscall
-========
-
-vsyscall calling sequence matches the syscall calling sequence, with the
-following differences. Some vsyscalls may have different calling sequences.
-
-Parameters and return value
----------------------------
-r0 is not used as an input. The vsyscall is selected by its address.
-
-Stack
------
-The vsyscall may or may not use the caller's stack frame save areas.
-
-Register preservation rules
----------------------------
-r0: Volatile.
-cr1, cr5-7: Volatile.
-lr: Volatile.
-
-Invocation
-----------
-The vsyscall is performed with a branch-with-link instruction to the vsyscall
-function address.
-
-Transactional Memory
---------------------
-vsyscalls will run in the same transactional state as the caller. A vsyscall
-may or may not result in the transaction being doomed by hardware.