* David Woodhouse <dw...@infradead.org> wrote:
> On Tue, 2018-01-23 at 11:15 +0100, Ingo Molnar wrote:
> >
> > BTW., the reason this is enabled on all distro kernels is because the
> > overhead
> > is a single patched-in NOP instruction in the function epilogue, when
> > tracing
> > is disabled. So it's not even a CALL+RET - it's a patched in NOP.
>
> Hm? We still have GCC emitting 'call __fentry__' don't we? Would be nice to
> get
> to the point where we can patch *that* out into a NOP... or are you saying we
> already can?
Yes, we already can and do patch the 'call __fentry__/ mcount' call site into a
NOP today - all 50,000+ call sites on a typical distro kernel.
We did so for a long time - this is all a well established, working mechanism.
> But this is a digression. I was being pedantic about the "0 cycles" but sure,
> this would be perfectly tolerable.
It's not a digression in two ways:
- I wanted to make it clear that for distro kernels it _is_ a zero cycles
overhead
mechanism for non-SkyLake CPUs, literally.
- I noticed that Meltdown and the CR3 writes for PTI appears to have
established a
kind of ... insensitivity and numbness to kernel micro-costs, which peaked
with
the per-syscall MSR write nonsense patch of the SkyLake workaround.
That attitude is totally unacceptable to me as x86 maintainer and yes, still
every cycle counts.
Thanks,
Ingo
