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diff --git a/Documentation/powerpc/syscall64-abi.rst b/Documentation/powerpc/syscall64-abi.rst new file mode 100644 index 000000000000..e49f69f941b9 --- /dev/null +++ b/Documentation/powerpc/syscall64-abi.rst @@ -0,0 +1,110 @@ +=============================================== +Power Architecture 64-bit Linux system call ABI +=============================================== + +syscall +======= + +syscall calling sequence\ [1]_ matches the Power Architecture 64-bit ELF ABI +specification C function calling sequence, including register preservation +rules, with the following differences. + +.. [1] 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. |