Early kernel boot log?

[Konstantin Belousov]

On Thu, Aug 09, 2018 at 10:26:06AM +0100, Johannes Lundberg wrote:
> On Thu, Aug 9, 2018 at 9:29 AM Konstantin Belousov <kostikbel at gmail.com>
> wrote:
> 
> > On Thu, Aug 09, 2018 at 08:54:31AM +0100, Johannes Lundberg wrote:
> > > Hi
> > >
> > > So I believe the reason I'm not seeing and printf output in dmesg is that
> > > it is too early in some functions.
> > > For example
> > > machdep.s
> > >  getmemsize()
> > >  add_efi_map_entries()
> > >  etc
> > >
> > > However, these functions do contain debug printf statements so if they're
> > > logging to somewhere, where/how can I see this?
> > >
> > > I also tried booting in bhyve too see if I could get any output via
> > serial
> > > console but nothing there either.
> > Disable efi console, only leaving comconsole around, then set
> > debug.late_console=0
> > in loader.
> >
> 
> Thanks for the tip. I found the comment in machdep.c that explains this
> now.
> However, running in bhyve with
> console="comconsole" (not needed in bhyve I guess?)
> debug.late_console=0
> 
> Boot hangs after
> Booting...
> output.
> Caused by late_console=0.

That early hangs are typically due to an exception occuring before
IDT is set up and trap machinery operational.  Double-check that
there is no any early framebuffer access, as a drastic measure remove
all framebuffer drivers from your kernel config.

I do not remember, where gdb stubs added to bhyve ?  Is there a way
to inspect the vm guest state in bhyve by other means ?