1=============================================== 2Power Architecture 64-bit Linux system call ABI 3=============================================== 4 5syscall 6======= 7 8Invocation 9---------- 10The syscall is made with the sc instruction, and returns with execution 11continuing at the instruction following the sc instruction. 12 13If PPC_FEATURE2_SCV appears in the AT_HWCAP2 ELF auxiliary vector, the 14scv 0 instruction is an alternative that may provide better performance, 15with some differences to calling sequence. 16 17syscall calling sequence\ [1]_ matches the Power Architecture 64-bit ELF ABI 18specification C function calling sequence, including register preservation 19rules, with the following differences. 20 21.. [1] Some syscalls (typically low-level management functions) may have 22 different calling sequences (e.g., rt_sigreturn). 23 24Parameters 25---------- 26The system call number is specified in r0. 27 28There is a maximum of 6 integer parameters to a syscall, passed in r3-r8. 29 30Return value 31------------ 32- For the sc instruction, both a value and an error condition are returned. 33 cr0.SO is the error condition, and r3 is the return value. When cr0.SO is 34 clear, the syscall succeeded and r3 is the return value. When cr0.SO is set, 35 the syscall failed and r3 is the error value (that normally corresponds to 36 errno). 37 38- For the scv 0 instruction, the return value indicates failure if it is 39 -4095..-1 (i.e., it is >= -MAX_ERRNO (-4095) as an unsigned comparison), 40 in which case the error value is the negated return value. 41 42Stack 43----- 44System calls do not modify the caller's stack frame. For example, the caller's 45stack frame LR and CR save fields are not used. 46 47Register preservation rules 48--------------------------- 49Register preservation rules match the ELF ABI calling sequence with some 50differences. 51 52For the sc instruction, the differences from the ELF ABI are as follows: 53 54+--------------+--------------------+-----------------------------------------+ 55| Register | Preservation Rules | Purpose | 56+==============+====================+=========================================+ 57| r0 | Volatile | (System call number.) | 58+--------------+--------------------+-----------------------------------------+ 59| r3 | Volatile | (Parameter 1, and return value.) | 60+--------------+--------------------+-----------------------------------------+ 61| r4-r8 | Volatile | (Parameters 2-6.) | 62+--------------+--------------------+-----------------------------------------+ 63| cr0 | Volatile | (cr0.SO is the return error condition.) | 64+--------------+--------------------+-----------------------------------------+ 65| cr1, cr5-7 | Nonvolatile | | 66+--------------+--------------------+-----------------------------------------+ 67| lr | Nonvolatile | | 68+--------------+--------------------+-----------------------------------------+ 69 70For the scv 0 instruction, the differences from the ELF ABI are as follows: 71 72+--------------+--------------------+-----------------------------------------+ 73| Register | Preservation Rules | Purpose | 74+==============+====================+=========================================+ 75| r0 | Volatile | (System call number.) | 76+--------------+--------------------+-----------------------------------------+ 77| r3 | Volatile | (Parameter 1, and return value.) | 78+--------------+--------------------+-----------------------------------------+ 79| r4-r8 | Volatile | (Parameters 2-6.) | 80+--------------+--------------------+-----------------------------------------+ 81 82All floating point and vector data registers as well as control and status 83registers are nonvolatile. 84 85Transactional Memory 86-------------------- 87Syscall behavior can change if the processor is in transactional or suspended 88transaction state, and the syscall can affect the behavior of the transaction. 89 90If the processor is in suspended state when a syscall is made, the syscall 91will be performed as normal, and will return as normal. The syscall will be 92performed in suspended state, so its side effects will be persistent according 93to the usual transactional memory semantics. A syscall may or may not result 94in the transaction being doomed by hardware. 95 96If the processor is in transactional state when a syscall is made, then the 97behavior depends on the presence of PPC_FEATURE2_HTM_NOSC in the AT_HWCAP2 ELF 98auxiliary vector. 99 100- If present, which is the case for newer kernels, then the syscall will not 101 be performed and the transaction will be doomed by the kernel with the 102 failure code TM_CAUSE_SYSCALL | TM_CAUSE_PERSISTENT in the TEXASR SPR. 103 104- If not present (older kernels), then the kernel will suspend the 105 transactional state and the syscall will proceed as in the case of a 106 suspended state syscall, and will resume the transactional state before 107 returning to the caller. This case is not well defined or supported, so this 108 behavior should not be relied upon. 109 110scv 0 syscalls will always behave as PPC_FEATURE2_HTM_NOSC. 111 112ptrace 113------ 114When ptracing system calls (PTRACE_SYSCALL), the pt_regs.trap value contains 115the system call type that can be used to distinguish between sc and scv 0 116system calls, and the different register conventions can be accounted for. 117 118If the value of (pt_regs.trap & 0xfff0) is 0xc00 then the system call was 119performed with the sc instruction, if it is 0x3000 then the system call was 120performed with the scv 0 instruction. 121 122vsyscall 123======== 124 125vsyscall calling sequence matches the syscall calling sequence, with the 126following differences. Some vsyscalls may have different calling sequences. 127 128Parameters and return value 129--------------------------- 130r0 is not used as an input. The vsyscall is selected by its address. 131 132Stack 133----- 134The vsyscall may or may not use the caller's stack frame save areas. 135 136Register preservation rules 137--------------------------- 138 139=========== ======== 140r0 Volatile 141cr1, cr5-7 Volatile 142lr Volatile 143=========== ======== 144 145Invocation 146---------- 147The vsyscall is performed with a branch-with-link instruction to the vsyscall 148function address. 149 150Transactional Memory 151-------------------- 152vsyscalls will run in the same transactional state as the caller. A vsyscall 153may or may not result in the transaction being doomed by hardware. 154