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 some
50differences.
51
52For 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
70For 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
82All floating point and vector data registers as well as control and status
83registers are nonvolatile.
84
85Transactional Memory
86--------------------
87Syscall behavior can change if the processor is in transactional or suspended
88transaction state, and the syscall can affect the behavior of the transaction.
89
90If the processor is in suspended state when a syscall is made, the syscall
91will be performed as normal, and will return as normal. The syscall will be
92performed in suspended state, so its side effects will be persistent according
93to the usual transactional memory semantics. A syscall may or may not result
94in the transaction being doomed by hardware.
95
96If the processor is in transactional state when a syscall is made, then the
97behavior depends on the presence of PPC_FEATURE2_HTM_NOSC in the AT_HWCAP2 ELF
98auxiliary 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
110scv 0 syscalls will always behave as PPC_FEATURE2_HTM_NOSC.
111
112vsyscall
113========
114
115vsyscall calling sequence matches the syscall calling sequence, with the
116following differences. Some vsyscalls may have different calling sequences.
117
118Parameters and return value
119---------------------------
120r0 is not used as an input. The vsyscall is selected by its address.
121
122Stack
123-----
124The vsyscall may or may not use the caller's stack frame save areas.
125
126Register preservation rules
127---------------------------
128
129=========== ========
130r0          Volatile
131cr1, cr5-7  Volatile
132lr          Volatile
133=========== ========
134
135Invocation
136----------
137The vsyscall is performed with a branch-with-link instruction to the vsyscall
138function address.
139
140Transactional Memory
141--------------------
142vsyscalls will run in the same transactional state as the caller. A vsyscall
143may or may not result in the transaction being doomed by hardware.
144