Commit 7b8845a2 authored by Nicholas Piggin's avatar Nicholas Piggin Committed by Michael Ellerman

powerpc/64: Document the syscall ABI

Add some documentation for the 64-bit syscall ABI, which doesn't seem
to be documented elsewhere.

This attempts to document existing practice. The only small discrepancy
is glibc clobbers not quite matching the kernel (e.g., xer, some
vsyscalls trash cr1 whereas glibc only clobbers cr0). These will be
resolved after this document is merged.
Signed-off-by: default avatarNicholas Piggin <>
Signed-off-by: default avatarMichael Ellerman <>
parent e0e0d6b7
Power Architecture 64-bit Linux system call ABI
syscall calling sequence[*] matches the Power Architecture 64-bit ELF ABI
specification C function calling sequence, including register preservation
rules, with the following differences.
[*] Some syscalls (typically low-level management functions) may have
different calling sequences (e.g., rt_sigreturn).
Parameters and return value
The system call number is specified in r0.
There is a maximum of 6 integer parameters to a syscall, passed in r3-r8.
Both a return value and a return error code are returned. cr0.SO is the return
error code, and r3 is the return value or error code. 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 code that generally corresponds to errno.
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:
r0: Volatile. (System call number.)
r3: Volatile. (Parameter 1, and return value.)
r4-r8: Volatile. (Parameters 2-6.)
cr0: Volatile (cr0.SO is the return error condition)
cr1, cr5-7: Nonvolatile.
lr: Nonvolatile.
All floating point and vector data registers as well as control and status
registers are nonvolatile.
The syscall is performed with the sc instruction, and returns with execution
continuing at the instruction following the sc instruction.
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
- 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.
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.
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.
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.
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