| Nicholas Piggin | 7b8845a | 2016-09-14 13:21:47 +1000 | [diff] [blame^] | 1 | =============================================== |
| 2 | Power Architecture 64-bit Linux system call ABI |
| 3 | =============================================== |
| 4 | |
| 5 | syscall |
| 6 | ======= |
| 7 | |
| 8 | syscall calling sequence[*] 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 | [*] 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 | r0: Volatile. (System call number.) |
| 37 | r3: Volatile. (Parameter 1, and return value.) |
| 38 | r4-r8: Volatile. (Parameters 2-6.) |
| 39 | cr0: Volatile (cr0.SO is the return error condition) |
| 40 | cr1, cr5-7: Nonvolatile. |
| 41 | lr: Nonvolatile. |
| 42 | |
| 43 | All floating point and vector data registers as well as control and status |
| 44 | registers are nonvolatile. |
| 45 | |
| 46 | Invocation |
| 47 | ---------- |
| 48 | The syscall is performed with the sc instruction, and returns with execution |
| 49 | continuing at the instruction following the sc instruction. |
| 50 | |
| 51 | Transactional Memory |
| 52 | -------------------- |
| 53 | Syscall behavior can change if the processor is in transactional or suspended |
| 54 | transaction state, and the syscall can affect the behavior of the transaction. |
| 55 | |
| 56 | If the processor is in suspended state when a syscall is made, the syscall |
| 57 | will be performed as normal, and will return as normal. The syscall will be |
| 58 | performed in suspended state, so its side effects will be persistent according |
| 59 | to the usual transactional memory semantics. A syscall may or may not result |
| 60 | in the transaction being doomed by hardware. |
| 61 | |
| 62 | If the processor is in transactional state when a syscall is made, then the |
| 63 | behavior depends on the presence of PPC_FEATURE2_HTM_NOSC in the AT_HWCAP2 ELF |
| 64 | auxiliary vector. |
| 65 | |
| 66 | - If present, which is the case for newer kernels, then the syscall will not |
| 67 | be performed and the transaction will be doomed by the kernel with the |
| 68 | failure code TM_CAUSE_SYSCALL | TM_CAUSE_PERSISTENT in the TEXASR SPR. |
| 69 | |
| 70 | - If not present (older kernels), then the kernel will suspend the |
| 71 | transactional state and the syscall will proceed as in the case of a |
| 72 | suspended state syscall, and will resume the transactional state before |
| 73 | returning to the caller. This case is not well defined or supported, so this |
| 74 | behavior should not be relied upon. |
| 75 | |
| 76 | |
| 77 | vsyscall |
| 78 | ======== |
| 79 | |
| 80 | vsyscall calling sequence matches the syscall calling sequence, with the |
| 81 | following differences. Some vsyscalls may have different calling sequences. |
| 82 | |
| 83 | Parameters and return value |
| 84 | --------------------------- |
| 85 | r0 is not used as an input. The vsyscall is selected by its address. |
| 86 | |
| 87 | Stack |
| 88 | ----- |
| 89 | The vsyscall may or may not use the caller's stack frame save areas. |
| 90 | |
| 91 | Register preservation rules |
| 92 | --------------------------- |
| 93 | r0: Volatile. |
| 94 | cr1, cr5-7: Volatile. |
| 95 | lr: Volatile. |
| 96 | |
| 97 | Invocation |
| 98 | ---------- |
| 99 | The vsyscall is performed with a branch-with-link instruction to the vsyscall |
| 100 | function address. |
| 101 | |
| 102 | Transactional Memory |
| 103 | -------------------- |
| 104 | vsyscalls will run in the same transactional state as the caller. A vsyscall |
| 105 | may or may not result in the transaction being doomed by hardware. |