| David Brazdil | 0f672f6 | 2019-12-10 10:32:29 +0000 | [diff] [blame^] | 1 | =============================================== |
| 2 | Power Architecture 64-bit Linux system call ABI |
| 3 | =============================================== |
| 4 | |
| 5 | syscall |
| 6 | ======= |
| 7 | |
| 8 | syscall calling sequence\ [1]_ matches the Power Architecture 64-bit ELF ABI |
| 9 | specification C function calling sequence, including register preservation |
| 10 | rules, with the following differences. |
| 11 | |
| 12 | .. [1] Some syscalls (typically low-level management functions) may have |
| 13 | different calling sequences (e.g., rt_sigreturn). |
| 14 | |
| 15 | Parameters and return value |
| 16 | --------------------------- |
| 17 | The system call number is specified in r0. |
| 18 | |
| 19 | There is a maximum of 6 integer parameters to a syscall, passed in r3-r8. |
| 20 | |
| 21 | Both a return value and a return error code are returned. cr0.SO is the return |
| 22 | error code, and r3 is the return value or error code. When cr0.SO is clear, |
| 23 | the syscall succeeded and r3 is the return value. When cr0.SO is set, the |
| 24 | syscall failed and r3 is the error code that generally corresponds to errno. |
| 25 | |
| 26 | Stack |
| 27 | ----- |
| 28 | System calls do not modify the caller's stack frame. For example, the caller's |
| 29 | stack frame LR and CR save fields are not used. |
| 30 | |
| 31 | Register preservation rules |
| 32 | --------------------------- |
| 33 | Register preservation rules match the ELF ABI calling sequence with the |
| 34 | following differences: |
| 35 | |
| 36 | =========== ============= ======================================== |
| 37 | r0 Volatile (System call number.) |
| 38 | r3 Volatile (Parameter 1, and return value.) |
| 39 | r4-r8 Volatile (Parameters 2-6.) |
| 40 | cr0 Volatile (cr0.SO is the return error condition) |
| 41 | cr1, cr5-7 Nonvolatile |
| 42 | lr Nonvolatile |
| 43 | =========== ============= ======================================== |
| 44 | |
| 45 | All floating point and vector data registers as well as control and status |
| 46 | registers are nonvolatile. |
| 47 | |
| 48 | Invocation |
| 49 | ---------- |
| 50 | The syscall is performed with the sc instruction, and returns with execution |
| 51 | continuing at the instruction following the sc instruction. |
| 52 | |
| 53 | Transactional Memory |
| 54 | -------------------- |
| 55 | Syscall behavior can change if the processor is in transactional or suspended |
| 56 | transaction state, and the syscall can affect the behavior of the transaction. |
| 57 | |
| 58 | If the processor is in suspended state when a syscall is made, the syscall |
| 59 | will be performed as normal, and will return as normal. The syscall will be |
| 60 | performed in suspended state, so its side effects will be persistent according |
| 61 | to the usual transactional memory semantics. A syscall may or may not result |
| 62 | in the transaction being doomed by hardware. |
| 63 | |
| 64 | If the processor is in transactional state when a syscall is made, then the |
| 65 | behavior depends on the presence of PPC_FEATURE2_HTM_NOSC in the AT_HWCAP2 ELF |
| 66 | auxiliary vector. |
| 67 | |
| 68 | - If present, which is the case for newer kernels, then the syscall will not |
| 69 | be performed and the transaction will be doomed by the kernel with the |
| 70 | failure code TM_CAUSE_SYSCALL | TM_CAUSE_PERSISTENT in the TEXASR SPR. |
| 71 | |
| 72 | - If not present (older kernels), then the kernel will suspend the |
| 73 | transactional state and the syscall will proceed as in the case of a |
| 74 | suspended state syscall, and will resume the transactional state before |
| 75 | returning to the caller. This case is not well defined or supported, so this |
| 76 | behavior should not be relied upon. |
| 77 | |
| 78 | |
| 79 | vsyscall |
| 80 | ======== |
| 81 | |
| 82 | vsyscall calling sequence matches the syscall calling sequence, with the |
| 83 | following differences. Some vsyscalls may have different calling sequences. |
| 84 | |
| 85 | Parameters and return value |
| 86 | --------------------------- |
| 87 | r0 is not used as an input. The vsyscall is selected by its address. |
| 88 | |
| 89 | Stack |
| 90 | ----- |
| 91 | The vsyscall may or may not use the caller's stack frame save areas. |
| 92 | |
| 93 | Register preservation rules |
| 94 | --------------------------- |
| 95 | |
| 96 | =========== ======== |
| 97 | r0 Volatile |
| 98 | cr1, cr5-7 Volatile |
| 99 | lr Volatile |
| 100 | =========== ======== |
| 101 | |
| 102 | Invocation |
| 103 | ---------- |
| 104 | The vsyscall is performed with a branch-with-link instruction to the vsyscall |
| 105 | function address. |
| 106 | |
| 107 | Transactional Memory |
| 108 | -------------------- |
| 109 | vsyscalls will run in the same transactional state as the caller. A vsyscall |
| 110 | may or may not result in the transaction being doomed by hardware. |