106 lines
3.8 KiB
Plaintext
106 lines
3.8 KiB
Plaintext
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===============================================
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Power Architecture 64-bit Linux system call ABI
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===============================================
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syscall
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=======
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syscall calling sequence[*] matches the Power Architecture 64-bit ELF ABI
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specification C function calling sequence, including register preservation
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rules, with the following differences.
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[*] Some syscalls (typically low-level management functions) may have
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different calling sequences (e.g., rt_sigreturn).
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Parameters and return value
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---------------------------
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The system call number is specified in r0.
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There is a maximum of 6 integer parameters to a syscall, passed in r3-r8.
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Both a return value and a return error code are returned. cr0.SO is the return
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error code, and r3 is the return value or error code. When cr0.SO is clear,
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the syscall succeeded and r3 is the return value. When cr0.SO is set, the
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syscall failed and r3 is the error code that generally corresponds to errno.
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Stack
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-----
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System calls do not modify the caller's stack frame. For example, the caller's
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stack frame LR and CR save fields are not used.
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Register preservation rules
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---------------------------
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Register preservation rules match the ELF ABI calling sequence with the
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following differences:
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r0: Volatile. (System call number.)
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r3: Volatile. (Parameter 1, and return value.)
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r4-r8: Volatile. (Parameters 2-6.)
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cr0: Volatile (cr0.SO is the return error condition)
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cr1, cr5-7: Nonvolatile.
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lr: Nonvolatile.
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All floating point and vector data registers as well as control and status
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registers are nonvolatile.
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Invocation
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----------
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The syscall is performed with the sc instruction, and returns with execution
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continuing at the instruction following the sc instruction.
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Transactional Memory
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--------------------
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Syscall behavior can change if the processor is in transactional or suspended
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transaction state, and the syscall can affect the behavior of the transaction.
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If the processor is in suspended state when a syscall is made, the syscall
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will be performed as normal, and will return as normal. The syscall will be
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performed in suspended state, so its side effects will be persistent according
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to the usual transactional memory semantics. A syscall may or may not result
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in the transaction being doomed by hardware.
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If the processor is in transactional state when a syscall is made, then the
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behavior depends on the presence of PPC_FEATURE2_HTM_NOSC in the AT_HWCAP2 ELF
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auxiliary vector.
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- If present, which is the case for newer kernels, then the syscall will not
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be performed and the transaction will be doomed by the kernel with the
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failure code TM_CAUSE_SYSCALL | TM_CAUSE_PERSISTENT in the TEXASR SPR.
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- If not present (older kernels), then the kernel will suspend the
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transactional state and the syscall will proceed as in the case of a
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suspended state syscall, and will resume the transactional state before
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returning to the caller. This case is not well defined or supported, so this
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behavior should not be relied upon.
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vsyscall
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========
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vsyscall calling sequence matches the syscall calling sequence, with the
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following differences. Some vsyscalls may have different calling sequences.
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Parameters and return value
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---------------------------
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r0 is not used as an input. The vsyscall is selected by its address.
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Stack
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-----
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The vsyscall may or may not use the caller's stack frame save areas.
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Register preservation rules
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---------------------------
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r0: Volatile.
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cr1, cr5-7: Volatile.
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lr: Volatile.
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Invocation
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----------
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The vsyscall is performed with a branch-with-link instruction to the vsyscall
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function address.
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Transactional Memory
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--------------------
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vsyscalls will run in the same transactional state as the caller. A vsyscall
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may or may not result in the transaction being doomed by hardware.
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