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7b8845a2 NP |
1 | =============================================== |
2 | Power Architecture 64-bit Linux system call ABI | |
3 | =============================================== | |
4 | ||
5 | syscall | |
6 | ======= | |
7 | ||
7fa95f9a NP |
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 | ||
4d2e26a3 | 17 | syscall calling sequence\ [1]_ matches the Power Architecture 64-bit ELF ABI |
7b8845a2 NP |
18 | specification C function calling sequence, including register preservation |
19 | rules, with the following differences. | |
20 | ||
4d2e26a3 MCC |
21 | .. [1] Some syscalls (typically low-level management functions) may have |
22 | different calling sequences (e.g., rt_sigreturn). | |
7b8845a2 | 23 | |
7fa95f9a NP |
24 | Parameters |
25 | ---------- | |
7b8845a2 NP |
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 | ||
7fa95f9a NP |
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. | |
7b8845a2 NP |
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 | --------------------------- | |
36eaf08f AD |
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 | +--------------+--------------------+-----------------------------------------+ | |
7b8845a2 NP |
81 | |
82 | All floating point and vector data registers as well as control and status | |
83 | registers are nonvolatile. | |
84 | ||
7b8845a2 NP |
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 | ||
7fa95f9a | 110 | scv 0 syscalls will always behave as PPC_FEATURE2_HTM_NOSC. |
7b8845a2 | 111 | |
5665bc35 NP |
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 | ||
7b8845a2 NP |
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 | --------------------------- | |
4d2e26a3 MCC |
138 | |
139 | =========== ======== | |
140 | r0 Volatile | |
141 | cr1, cr5-7 Volatile | |
142 | lr Volatile | |
143 | =========== ======== | |
7b8845a2 NP |
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. |