How to get better debugging for the kernel.

[Gordon Ross]

I heard a rumor someone might be working on a port of illumos "mdb".
Anyone know if that's true, and how far along it went?

We depend heavily upon this tool for production support;
so much that I'm not sure how we'd live without it.


On Fri, Aug 12, 2016 at 4:39 PM, Samy Bahra <sbahra at backtrace.io> wrote:
> 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
>>
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