[PATCH 0/2][concept RFC] x86: BIOS-save kernel log to disk upon panic

Ahmed S. Darwish darwish.07 at gmail.com
Wed Jan 26 08:34:00 EST 2011

On Wed, Jan 26, 2011 at 01:58:13PM +0100, Ingo Molnar wrote:
> * Ahmed S. Darwish <darwish.07 at gmail.com> wrote:
> > - The latest approach (proposed by Linus) is to forget the disk: jump to
> >   real-mode, but display the kernel log in a fancy format (with scroll
> >   ups and downs) instead.
> > 
> >   Will re-initializing the VGA registers to their POST state be possible?
> >   If not, what about a "fail-safe" VGA driver?
> > 
> > I'm most likely going to implement either the second or the third point,
> > so I'd really appreciate some input.
> The third one suggested by Linus is the most useful and most direct one IMO. Such a 
> 'oops mode' screen would be useful for regular kernel crashes as well.

Yes, it is the safest approach.

If I can make kexec and kdump work on early panics using some help from
bootloaders though, that can possibly give us the feature of saving kernel
logs to disk upon panics _without_ danger. I did not look at the kexec and
kdump code bases yet, so this might be a pipe dream (respective developers
are CCed -- input appreciated).

> Also, have you tried BIOS warm reset vector, which is supposed to reboot without 
> clearing RAM contents - how well does it work in practice on typical laptops? If on 
> crash we could reboot without memory getting cleared that would open up a vast area 
> of space to store the kernel log into (RAM).

AFAIK, the lmode->rmode transition is more forward-compatible.

It seems the only place warm boot was documented is in the Intel MP spec,
a 12-years old document long obsoleted by ACPI. Meanwhile, the real-mode
transition is rigorously documented in the current Intel and AMD manuals,
albeit in kind of a holier-than-thou approach.

I'll investigate this further though.



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