Re: [RFC 09/10] x86/enter: Create macros to restrict/unrestrict Indirect Branch Speculation

[Ingo Molnar]

* David Woodhouse <dwmw2@xxxxxxxxxxxxx> wrote:

> On Tue, 2018-01-23 at 08:53 +0100, Ingo Molnar wrote:
> > 
> > The patch below demonstrates the principle, it forcibly enables dynamic ftrace 
> > patching (CONFIG_DYNAMIC_FTRACE=y et al) and turns mcount/__fentry__ into a RET:
> > 
> >   ffffffff81a01a40 <__fentry__>:
> >   ffffffff81a01a40:       c3                      retq   
> > 
> > This would have to be extended with (very simple) call stack depth tracking (just 
> > 3 more instructions would do in the fast path I believe) and a suitable SkyLake 
> > workaround (and also has to play nice with the ftrace callbacks).
> > 
> > On non-SkyLake the overhead would be 0 cycles.
> 
> The overhead of forcing CONFIG_DYNAMIC_FTRACE=y is precisely zero
> cycles? That seems a little optimistic. ;)

The overhead of the quick hack patch I sent to show what exact code I mean is 
obviously not zero.

The overhead of using my proposed solution, to utilize the function call callback 
that CONFIG_DYNAMIC_FTRACE=y provides, is exactly zero on non-SkyLake systems 
where the callback is patched out, on typical Linux distros.

The callback is widely enabled on distro kernels:

  Fedora:                    CONFIG_DYNAMIC_FTRACE=y
  Ubuntu:                    CONFIG_DYNAMIC_FTRACE=y
  OpenSuse (default flavor): CONFIG_DYNAMIC_FTRACE=y

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.

Thanks,

	Ingo