xref: /kernel/linux/linux-5.10/Documentation/powerpc/syscall64-abi.rst

===============================================
Power Architecture 64-bit Linux system call ABI
===============================================

syscall
=======

Invocation
----------
The syscall is made with the sc instruction, and returns with execution
continuing at the instruction following the sc instruction.

If PPC_FEATURE2_SCV appears in the AT_HWCAP2 ELF auxiliary vector, the
scv 0 instruction is an alternative that may provide better performance,
with some differences to calling sequence.

syscall calling sequence\ [1]_ matches the Power Architecture 64-bit ELF ABI
specification C function calling sequence, including register preservation
rules, with the following differences.

.. [1] Some syscalls (typically low-level management functions) may have
       different calling sequences (e.g., rt_sigreturn).

Parameters
----------
The system call number is specified in r0.

There is a maximum of 6 integer parameters to a syscall, passed in r3-r8.

Return value
------------
- For the sc instruction, both a value and an error condition are returned.
  cr0.SO is the error condition, and r3 is the return value. When cr0.SO is
  clear, the syscall succeeded and r3 is the return value. When cr0.SO is set,
  the syscall failed and r3 is the error value (that normally corresponds to
  errno).

- For the scv 0 instruction, the return value indicates failure if it is
  -4095..-1 (i.e., it is >= -MAX_ERRNO (-4095) as an unsigned comparison),
  in which case the error value is the negated return value.

Stack
-----
System calls do not modify the caller's stack frame. For example, the caller's
stack frame LR and CR save fields are not used.

Register preservation rules
---------------------------
Register preservation rules match the ELF ABI calling sequence with the
following differences:

+------------------------------------------------------------------------+
|        For the sc instruction, differences with the ELF ABI		 |
+--------------+--------------+------------------------------------------+
| r0           | Volatile     | (System call number.)			 |
| rr3          | Volatile     | (Parameter 1, and return value.)	 |
| rr4-r8       | Volatile     | (Parameters 2-6.)			 |
| rcr0         | Volatile     | (cr0.SO is the return error condition.)	 |
| rcr1, cr5-7  | Nonvolatile  |						 |
| rlr          | Nonvolatile  |						 |
+--------------+--------------+------------------------------------------+
|      For the scv 0 instruction, differences with the ELF ABI		 |
+--------------+--------------+------------------------------------------+
| r0           | Volatile     | (System call number.)			 |
| r3           | Volatile     | (Parameter 1, and return value.)	 |
| r4-r8        | Volatile     | (Parameters 2-6.)			 |
+--------------+--------------+------------------------------------------+

All floating point and vector data registers as well as control and status
registers are nonvolatile.

Transactional Memory
--------------------
Syscall behavior can change if the processor is in transactional or suspended
transaction state, and the syscall can affect the behavior of the transaction.

If the processor is in suspended state when a syscall is made, the syscall
will be performed as normal, and will return as normal. The syscall will be
performed in suspended state, so its side effects will be persistent according
to the usual transactional memory semantics. A syscall may or may not result
in the transaction being doomed by hardware.

If the processor is in transactional state when a syscall is made, then the
behavior depends on the presence of PPC_FEATURE2_HTM_NOSC in the AT_HWCAP2 ELF
auxiliary vector.

- If present, which is the case for newer kernels, then the syscall will not
  be performed and the transaction will be doomed by the kernel with the
  failure code TM_CAUSE_SYSCALL | TM_CAUSE_PERSISTENT in the TEXASR SPR.

- If not present (older kernels), then the kernel will suspend the
  transactional state and the syscall will proceed as in the case of a
  suspended state syscall, and will resume the transactional state before
  returning to the caller. This case is not well defined or supported, so this
  behavior should not be relied upon.

scv 0 syscalls will always behave as PPC_FEATURE2_HTM_NOSC.

ptrace
------
When ptracing system calls (PTRACE_SYSCALL), the pt_regs.trap value contains
the system call type that can be used to distinguish between sc and scv 0
system calls, and the different register conventions can be accounted for.

If the value of (pt_regs.trap & 0xfff0) is 0xc00 then the system call was
performed with the sc instruction, if it is 0x3000 then the system call was
performed with the scv 0 instruction.

vsyscall
========

vsyscall calling sequence matches the syscall calling sequence, with the
following differences. Some vsyscalls may have different calling sequences.

Parameters and return value
---------------------------
r0 is not used as an input. The vsyscall is selected by its address.

Stack
-----
The vsyscall may or may not use the caller's stack frame save areas.

Register preservation rules
---------------------------

=========== ========
r0          Volatile
cr1, cr5-7  Volatile
lr          Volatile
=========== ========

Invocation
----------
The vsyscall is performed with a branch-with-link instruction to the vsyscall
function address.

Transactional Memory
--------------------
vsyscalls will run in the same transactional state as the caller. A vsyscall
may or may not result in the transaction being doomed by hardware.