1 =============================================== 2 Power Architecture 64-bit Linux system call ABI 3 =============================================== 4 5 syscall 6 ======= 7 8 Invocation 9 ---------- 10 The syscall is made with the sc instruction, and returns with execution 11 continuing at the instruction following the sc instruction. 12 13 If PPC_FEATURE2_SCV appears in the AT_HWCAP2 ELF auxiliary vector, the 14 scv 0 instruction is an alternative that may provide better performance, 15 with some differences to calling sequence. 16 17 syscall calling sequence\ [1]_ matches the Power Architecture 64-bit ELF ABI 18 specification C function calling sequence, including register preservation 19 rules, 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 24 Parameters 25 ---------- 26 The system call number is specified in r0. 27 28 There is a maximum of 6 integer parameters to a syscall, passed in r3-r8. 29 30 Return 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 42 Stack 43 ----- 44 System calls do not modify the caller's stack frame. For example, the caller's 45 stack frame LR and CR save fields are not used. 46 47 Register preservation rules 48 --------------------------- 49 Register preservation rules match the ELF ABI calling sequence with some 50 differences. 51 52 For 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 70 For 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 82 All floating point and vector data registers as well as control and status 83 registers are nonvolatile. 84 85 Transactional Memory 86 -------------------- 87 Syscall behavior can change if the processor is in transactional or suspended 88 transaction state, and the syscall can affect the behavior of the transaction. 89 90 If the processor is in suspended state when a syscall is made, the syscall 91 will be performed as normal, and will return as normal. The syscall will be 92 performed in suspended state, so its side effects will be persistent according 93 to the usual transactional memory semantics. A syscall may or may not result 94 in the transaction being doomed by hardware. 95 96 If the processor is in transactional state when a syscall is made, then the 97 behavior depends on the presence of PPC_FEATURE2_HTM_NOSC in the AT_HWCAP2 ELF 98 auxiliary 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. 109 110 scv 0 syscalls will always behave as PPC_FEATURE2_HTM_NOSC. 111 112 ptrace 113 ------ 114 When ptracing system calls (PTRACE_SYSCALL), the pt_regs.trap value contains 115 the system call type that can be used to distinguish between sc and scv 0 116 system calls, and the different register conventions can be accounted for. 117 118 If the value of (pt_regs.trap & 0xfff0) is 0xc00 then the system call was 119 performed with the sc instruction, if it is 0x3000 then the system call was 120 performed with the scv 0 instruction. 121 122 vsyscall 123 ======== 124 125 vsyscall calling sequence matches the syscall calling sequence, with the 126 following differences. Some vsyscalls may have different calling sequences. 127 128 Parameters and return value 129 --------------------------- 130 r0 is not used as an input. The vsyscall is selected by its address. 131 132 Stack 133 ----- 134 The vsyscall may or may not use the caller's stack frame save areas. 135 136 Register preservation rules 137 --------------------------- 138 139 =========== ======== 140 r0 Volatile 141 cr1, cr5-7 Volatile 142 lr Volatile 143 =========== ======== 144 145 Invocation 146 ---------- 147 The vsyscall is performed with a branch-with-link instruction to the vsyscall 148 function address. 149 150 Transactional Memory 151 -------------------- 152 vsyscalls will run in the same transactional state as the caller. A vsyscall 153 may or may not result in the transaction being doomed by hardware.
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