How to get better debugging for the kernel.

[Samy Bahra]

Slides up at: http://backtrace.io/blog/images/bbcon2016-sbahra.pdf

On Fri, Aug 12, 2016 at 4:32 PM John Baldwin <jhb at freebsd.org> wrote:

> On Thursday, August 04, 2016 01:07:39 AM K. Macy wrote:
> > On Wed, Aug 3, 2016 at 12:53 PM, Andriy Gapon <avg at freebsd.org> wrote:
> > > On 03/08/2016 20:14, Ryan Stone wrote:
> > >> Are you using the kgdb from the base system or from ports(it's a part
> of
> > >> devel/gdb)?  The kgdb in ports is significantly better.  If you
> haven't
> > >> tried the version from ports already, definitely do that first.
> > >
> > > kgdb 7.x from ports is certainly more powerful than the old base kgdb,
> > > but clang with O2 optimizations seems to be too much even for it.
> >
> > Samy did a good presentation about this issue. I'm hoping I can get
> > him to put his slides on line. Evidently clang is much more simplistic
> > about how it treats callee saved registers. In essence clang will
> > always err on the side of saying "optimized out" even when it has
> > sufficient state to know otherwise. Gcc, on the other hand will
> > sometimes incorrectly infer that a value is valid when it is in fact
> > not.
> >
> > I have been building some kernels with clang with dwarf4 enabled (and
> > thus needed to use kgdb 7.x from ports). Contrary to what I have heard
> > from some others I have found it to have virtually no added benefit.
>
> My understanding is that dwarf4 will not help with C programs like the
> kernel, that the new idioms in dwarf4 are for declaring more complex
> constructs in C++11, C++14, etc.  I have heard that clang does not update
> debug information during optimization passes causing it to loose track of
> variables that are moved during optimization.  I have not (yet) tried
> using gcc as avg@ describes though I will likely start doing so soon.
>
> --
> John Baldwin
>