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The most reliable way to reduce syscall cost on Linux Power systems is to avoid entering the kernel when a supported vDSO function can do the work. For calls that must enter the kernel, the Power ABI permits scv 0 as an alternative to sc when the processor advertises PPC_FEATURE2_SCV, but the ABI says it may perform better—not that it always does. Check what your libc and workload actually use, then benchmark on the target machine before changing code.
First determine whether the call enters the kernel
Not every function commonly described as a system call requires a kernel transition on every invocation. The Linux vDSO is a shared library that the kernel maps into dynamically linked programs. Glibc detects the available vDSO functions and can use them instead of making a conventional syscall.
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IBM documents vDSO implementations for gettimeofday, clock_getres, and clock_gettime. Whether a call takes that path depends on the function, system support, and runtime setup; do not assume every clock-related operation or every libc build avoids the kernel.
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- Identify whether the hot operation is one of the calls supported through the vDSO on your system.
- Confirm that the process is dynamically linked and that the relevant libc function is being called; IBM describes glibc as detecting and using vDSO functions.
- Keep the default vDSO setting unless you have observed a compatibility problem. IBM documents vDSO as enabled by default with
vdso=1orvdso=on, and recommends disabling it only when a real compatibility issue is present.
If the function already uses the vDSO, changing the kernel-entry instruction cannot make that call skip an entry it was not making. If it must perform a real kernel operation—such as a file or socket operation—the vDSO is not a general replacement for that syscall.
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How Power Linux makes a syscall
The Linux Power Architecture 64-bit syscall ABI specifies the sc instruction for making a syscall. It places the syscall number in register r0 and permits up to six integer arguments in r3 through r8. Execution resumes at the instruction following sc when the syscall returns.
The same ABI documents scv 0 as an alternative when PPC_FEATURE2_SCV appears in the AT_HWCAP2 ELF auxiliary vector. Its wording is deliberately conditional: the instruction “may provide better performance.” That is not a guarantee of a speedup on every processor, kernel, libc, or call.
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Do not assume the hardware feature is available
Check the deployed system’s auxiliary-vector feature flags and confirm that PPC_FEATURE2_SCV is exposed before attempting to use scv 0. The presence of a Power processor generation alone is not a substitute for checking the feature flag. If the flag is absent, use the supported syscall path rather than issuing an unsupported instruction.
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Start with the application’s normal libc call. If syscall entry itself is proven to matter, compare that path with a carefully controlled direct-sc implementation and, only when the feature is advertised, a direct-scv 0 implementation. A hand-written path must preserve the platform ABI’s argument, return, and error-handling conventions; otherwise, the result may be incorrect even if a microbenchmark appears faster.
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Compare the available approaches
| Approach | Kernel entry | When it applies | Main trade-off |
|---|---|---|---|
| Libc call using vDSO | Can avoid a kernel entry for a supported vDSO operation | For functions implemented through the vDSO on that system, including documented clock and time calls | Availability and selection depend on the operation and runtime; verify the actual path |
| Libc call requiring a syscall | Enters the kernel | For operations that need kernel services, such as file or socket work | Simple and ABI-aware, but pays the cost of the kernel transition and operation |
Direct sc |
Enters the kernel | When implementing the Power 64-bit syscall ABI directly | Can bypass some library handling; correctness and error handling become the caller’s responsibility |
Direct scv 0 |
Enters the kernel | Only when PPC_FEATURE2_SCV is advertised and the kernel supports the path |
Potential performance difference is workload- and system-dependent; measure rather than assume |
| Seccomp BPF filtering | Does not eliminate the syscall entry | When restricting which syscall numbers or arguments a process may use | Improves security boundaries, not syscall latency by design; filter evaluation can add work |
| Syscall-user-dispatch | Can redirect calls from compatibility-layer regions to userspace; native regions can make syscalls directly | For compatibility designs that distinguish emulated from native code | Adds dispatch behavior for the compatibility boundary and is not a blanket optimization; vDSO trampolines are not intercepted |
Measure on the system that will run the workload
No portable numeric speedup for scv over sc is established by the cited ABI and documentation. A result from one Power system should not be generalized to another: processor generation, kernel, libc, call type, and runtime instrumentation can all change the outcome.
Record the test conditions
- Processor generation and model, kernel release and configuration, libc version, and compiler or toolchain used for any direct-syscall code.
- Whether
PPC_FEATURE2_SCVis present in theAT_HWCAP2auxiliary vector. - Whether seccomp filters, tracing, virtualization, or syscall-user-dispatch are active for the process.
- The exact function or syscall, arguments, error behavior, and workload that the test represents.
Benchmark distinct kinds of work
- Test a vDSO-eligible call separately. Use a supported time or clock function and determine whether libc takes the vDSO path. Do not treat this result as a measurement of a kernel-entering syscall.
- Test a real kernel operation. Use a representative file or socket operation so the benchmark includes the syscall and the work the application actually needs.
- Test the application’s call pattern. Compare isolated calls with the repeated or batched workload where syscall frequency is believed to limit performance. Reducing the number of calls through batching may matter more than changing the entry instruction.
- Compare paths under matched conditions. Where supported, compare the normal libc path, direct
sc, and feature-checkedscv 0. Keep tracing, filters, virtualization, and compatibility dispatch consistent between runs. - Repeat and report distributions. Run repeated trials and report the median and tail percentiles. Also measure end-to-end throughput and latency; isolated instruction timings do not establish an application-level benefit.
Use seccomp, eBPF, and syscall-user-dispatch for their intended jobs
Seccomp is a security control, not a speed switch
Seccomp BPF can filter syscall numbers and arguments to reduce a process’s available kernel attack surface. It does not remove the kernel entry for an allowed syscall, and evaluating a filter can add work. Enable it to enforce a security policy, then include its presence in performance tests rather than treating it as a latency optimization.
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Power support for JITs does not make filtering faster by definition
The PowerPC feature table marks eBPF-JIT, cBPF-JIT, and seccomp-filter support as available. Whether a particular distribution enables the relevant kernel configuration is a separate question. JIT support can matter to how filtering or observability programs execute, but it does not establish that adding a filter or tracing program improves syscall latency.
Syscall-user-dispatch is for compatibility boundaries
The kernel administrator guide describes syscall-user-dispatch as a way to redirect calls from compatibility-layer regions to userspace while native regions execute syscalls directly. The guide also notes that vDSO trampolines are not intercepted. Use this mechanism when designing a compatibility boundary, not as a general substitute for vDSO or as an assumed faster syscall path.
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Practical optimization order
- Find the actual hot operation and determine whether its libc call can use the vDSO.
- Reduce avoidable calls through batching or by changing the application’s call pattern where that preserves its behavior.
- For calls that still enter the kernel, verify the
PPC_FEATURE2_SCVfeature flag before testingscv 0. - Benchmark the real workload across supported paths on the target processor and software stack, including the security and tracing configuration used in production.
- Keep the lower-level path only if repeated measurements show an end-to-end improvement without breaking ABI correctness, portability, or error handling.
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