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Ryan Houdek
c4c31d4e80
When originally written I was testing symbol loading on an x86 host
assuming that the vdso symbol names were the same between x86 and
aarch64.
Turns out this is not true and aarch64 uses `__kernel_` prefix instead
of x86's `__vdso_`.
AArch64 VDSO only provides us with four symbols right now
- __kernel_rt_sigreturn
- __kernel_gettimeofday
- __kernel_clock_gettime
- __kernel_clock_getres
This means that currently AArch64 is missing getcpu and time.
time is unlikely to ever be implemented so the vdso scanning for that is
unlikely to ever get a pointer and will need to go down the glibc path.
getcpu is likely to get an implementation at some point. There's even an
old patch series that implemented it[1]. Currently the patch has been
dropped on the floor for whatever reason but hopefully they will come
back to it.
Benchmarks on Cortex-X1C:
- Native AArch64 clock_gettime bench
- VDSO: 33.3ns per call
- glibc (which uses vdso): 33.3ns per call
- Roughly equivalent
- Emulated x86_64 clock_gettime bench
- Prior to fix:
- 49ns per call for both VDSO and glibc
- After fix:
- VDSO: ~46ns per call
- glibc: ~49ns per call
- Emulated i386 clock_gettime bench
- Only tested after fix
- VDSO: 50ns per call
- glibc: 65ns per call
The only way to improve this further would be to optimize thunks to push
certain function signature arguments in registers instead of stack.
Which is a future optimization as time goes on.
[1] https://patchwork.kernel.org/project/linux-kselftest/list/?series=335203