stapstd elf note flags

[Frank Ch. Eigler]

Hi, Andrii -


> > > That works well for apps running on new kernel that allows to optimize
> > > nop5, but it will regress on old kernel, which needs to do single
> > > stepping of the nop5 instruction when the USDT probe is hit.
> >
> > Can you explain the regression exactly?  Would and old kernel tripping
> > across a nop5 cause more single-stepping ops than if it had come
> > across a nop1?  Is that a big problem?
> 
> Yes, it takes one extra userspace->kernel->userspace trip (through
> interrupt), which basically slows USDTs 2x, so that's not good
> (especially that one of the main concern with uprobes and USDTs are
> that they are relatively slow compared to kernel-side tracepoints).

Just one more bit of detail please: what NOP5 instruction is it that
would require two interrupts through an older kernel?  There doesn't
exist another 5-byte nop that (how much?) older kernels recognize as
one instruction?

(What kind/size of nop a NT_STAPSDT refers to is not actually limited
by the spec.)

- FChE