On Mon, 21 Oct 2019, Christophe Leroy wrote: > This is a tentative to switch powerpc/32 vdso to generic C implementation. > It will likely not work on 64 bits or even build properly at the moment. > > powerpc is a bit special for VDSO as well as system calls in the > way that it requires setting CR SO bit which cannot be done in C. > Therefore, entry/exit and fallback needs to be performed in ASM. > > To allow that, C fallbacks just return -1 and the ASM entry point > performs the system call when the C function returns -1. > > The performance is rather disappoiting. That's most likely all > calculation in the C implementation are based on 64 bits math and > converted to 32 bits at the very end. I guess C implementation should > use 32 bits math like the assembly VDSO does as of today.
> gettimeofday: vdso: 750 nsec/call > > gettimeofday: vdso: 1533 nsec/call The only real 64bit math which can matter is the 64bit * 32bit multiply, i.e. static __always_inline u64 vdso_calc_delta(u64 cycles, u64 last, u64 mask, u32 mult) { return ((cycles - last) & mask) * mult; } Everything else is trivial add/sub/shift, which should be roughly the same in ASM. Can you try to replace that with: static __always_inline u64 vdso_calc_delta(u64 cycles, u64 last, u64 mask, u32 mult) { u64 ret, delta = ((cycles - last) & mask); u32 dh, dl; dl = delta; dh = delta >> 32; res = mul_u32_u32(al, mul); if (ah) res += mul_u32_u32(ah, mul) << 32; return res; } That's pretty much what __do_get_tspec does in ASM. Thanks, tglx