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 the
50following differences:
51
52--- For the sc instruction, differences with the ELF ABI ---
53=========== ============= ========================================
54r0          Volatile      (System call number.)
55r3          Volatile      (Parameter 1, and return value.)
56r4-r8       Volatile      (Parameters 2-6.)
57cr0         Volatile      (cr0.SO is the return error condition.)
58cr1, cr5-7  Nonvolatile
59lr          Nonvolatile
60=========== ============= ========================================
61
62--- For the scv 0 instruction, differences with the ELF ABI ---
63=========== ============= ========================================
64r0          Volatile      (System call number.)
65r3          Volatile      (Parameter 1, and return value.)
66r4-r8       Volatile      (Parameters 2-6.)
67=========== ============= ========================================
68
69All floating point and vector data registers as well as control and status
70registers are nonvolatile.
71
72Transactional Memory
73--------------------
74Syscall behavior can change if the processor is in transactional or suspended
75transaction state, and the syscall can affect the behavior of the transaction.
76
77If the processor is in suspended state when a syscall is made, the syscall
78will be performed as normal, and will return as normal. The syscall will be
79performed in suspended state, so its side effects will be persistent according
80to the usual transactional memory semantics. A syscall may or may not result
81in the transaction being doomed by hardware.
82
83If the processor is in transactional state when a syscall is made, then the
84behavior depends on the presence of PPC_FEATURE2_HTM_NOSC in the AT_HWCAP2 ELF
85auxiliary vector.
86
87- If present, which is the case for newer kernels, then the syscall will not
88  be performed and the transaction will be doomed by the kernel with the
89  failure code TM_CAUSE_SYSCALL | TM_CAUSE_PERSISTENT in the TEXASR SPR.
90
91- If not present (older kernels), then the kernel will suspend the
92  transactional state and the syscall will proceed as in the case of a
93  suspended state syscall, and will resume the transactional state before
94  returning to the caller. This case is not well defined or supported, so this
95  behavior should not be relied upon.
96
97scv 0 syscalls will always behave as PPC_FEATURE2_HTM_NOSC.
98
99vsyscall
100========
101
102vsyscall calling sequence matches the syscall calling sequence, with the
103following differences. Some vsyscalls may have different calling sequences.
104
105Parameters and return value
106---------------------------
107r0 is not used as an input. The vsyscall is selected by its address.
108
109Stack
110-----
111The vsyscall may or may not use the caller's stack frame save areas.
112
113Register preservation rules
114---------------------------
115
116=========== ========
117r0          Volatile
118cr1, cr5-7  Volatile
119lr          Volatile
120=========== ========
121
122Invocation
123----------
124The vsyscall is performed with a branch-with-link instruction to the vsyscall
125function address.
126
127Transactional Memory
128--------------------
129vsyscalls will run in the same transactional state as the caller. A vsyscall
130may or may not result in the transaction being doomed by hardware.
131