what does "boot in safe mode" enable / disable?

[Brandon Gooch]

On Mon, Apr 12, 2010 at 6:11 PM, Henrik Hudson <lists at rhavenn.net> wrote:
> I emailed a few days back, to -stable,  about having issues running 8-STABLE in a
> Xen environment. I solved this, sort of. However, I didn't get any
> bites over there.
>
> I have a XEN HVM environment and it was given to me running 8-REL-p2
> and it was working fine. I moved up to -STABLE and GENERIC wouldn't
> boot. After some fiddling, etc.. I tried booting it in "safe mode"
> and viola it boots fine.
>
> The system doesn't show any errors it just stops on "trying to mount
> disk: /dev/xxxx". Enabling "logging mode" doesn't show any
> difference between the stable kernel and the rel kernel when
> booting.
>
> So, I'm guessing there was some sort of regression, but I'm trying
> to isolate this. What does "booting in safe mode" enable / disable
> that would allow the system to boot successfully?
>
> Thanks.
>
> Henrik
> --
> Henrik Hudson
> lists at rhavenn.net

I've wondered this myself, having never actually booted into "safe
mode". Here is a snippet from a reply to a related question from the
archives:

"Safe mode forces PIO mode (disabling the use of DMA) and disables write
caching for all IDE hard drives and CD ROM drives, disables the probing
of EISA slots (as very few systems have them), and in i386 it also
disables the use of ACPI and the APICs."

from "What is FreeBSD 5.3 Safe Mode?"
http://lists.freebsd.org/pipermail/freebsd-current/2005-February/046882.html

-Brandon