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153 lines
6.4 KiB
153 lines
6.4 KiB
=============================================== |
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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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Invocation |
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---------- |
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The syscall is made with the sc instruction, and returns with execution |
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continuing at the instruction following the sc instruction. |
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If PPC_FEATURE2_SCV appears in the AT_HWCAP2 ELF auxiliary vector, the |
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scv 0 instruction is an alternative that may provide better performance, |
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with some differences to calling sequence. |
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syscall calling sequence\ [1]_ 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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.. [1] 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 |
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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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Return value |
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------------ |
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- For the sc instruction, both a value and an error condition are returned. |
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cr0.SO is the error condition, and r3 is the return value. When cr0.SO is |
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clear, the syscall succeeded and r3 is the return value. When cr0.SO is set, |
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the syscall failed and r3 is the error value (that normally corresponds to |
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errno). |
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- For the scv 0 instruction, the return value indicates failure if it is |
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-4095..-1 (i.e., it is >= -MAX_ERRNO (-4095) as an unsigned comparison), |
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in which case the error value is the negated return value. |
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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 some |
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differences. |
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For the sc instruction, the differences from the ELF ABI are as follows: |
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+--------------+--------------------+-----------------------------------------+ |
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| Register | Preservation Rules | Purpose | |
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+==============+====================+=========================================+ |
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| r0 | Volatile | (System call number.) | |
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+--------------+--------------------+-----------------------------------------+ |
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| r3 | Volatile | (Parameter 1, and return value.) | |
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+--------------+--------------------+-----------------------------------------+ |
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| r4-r8 | Volatile | (Parameters 2-6.) | |
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+--------------+--------------------+-----------------------------------------+ |
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| cr0 | Volatile | (cr0.SO is the return error condition.) | |
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+--------------+--------------------+-----------------------------------------+ |
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| cr1, cr5-7 | Nonvolatile | | |
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+--------------+--------------------+-----------------------------------------+ |
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| lr | Nonvolatile | | |
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+--------------+--------------------+-----------------------------------------+ |
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For the scv 0 instruction, the differences from the ELF ABI are as follows: |
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+--------------+--------------------+-----------------------------------------+ |
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| Register | Preservation Rules | Purpose | |
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+==============+====================+=========================================+ |
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| r0 | Volatile | (System call number.) | |
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+--------------+--------------------+-----------------------------------------+ |
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| r3 | Volatile | (Parameter 1, and return value.) | |
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+--------------+--------------------+-----------------------------------------+ |
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| r4-r8 | Volatile | (Parameters 2-6.) | |
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+--------------+--------------------+-----------------------------------------+ |
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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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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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scv 0 syscalls will always behave as PPC_FEATURE2_HTM_NOSC. |
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ptrace |
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------ |
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When ptracing system calls (PTRACE_SYSCALL), the pt_regs.trap value contains |
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the system call type that can be used to distinguish between sc and scv 0 |
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system calls, and the different register conventions can be accounted for. |
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If the value of (pt_regs.trap & 0xfff0) is 0xc00 then the system call was |
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performed with the sc instruction, if it is 0x3000 then the system call was |
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performed with the scv 0 instruction. |
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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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=========== ======== |
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r0 Volatile |
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cr1, cr5-7 Volatile |
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lr Volatile |
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=========== ======== |
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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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