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

From: "Ahmed S. Darwish" <darwish.07@gmail.com>
To: Ingo Molnar <mingo@elte.hu>
Cc: "Willy Tarreau" <wtarreau@hera.kernel.org>,
	"Dirk Hohndel" <hohndel@infradead.org>,
	"Frédéric Weisbecker" <fweisbec@gmail.com>,
	FBDEV-ML <linux-fbdev@vger.kernel.org>,
	IDE-ML <linux-ide@vger.kernel.org>,
	"Randy Dunlap" <rdunlap@xenotime.net>,
	"James Bottomley" <James.Bottomley@suse.de>,
	"H. Peter Anvin" <hpa@zytor.com>, X86-ML <x86@kernel.org>,
	"Ingo Molnar" <mingo@redhat.com>,
	"Jeff Garzik" <jgarzik@pobox.com>,
	"Vivek Goyal" <vgoyal@redhat.com>,
	"Peter Zijlstra" <a.p.zijlstra@chello.nl>,
	Dirk.Hohndel@intel.com, "Borislav Petkov" <bp@alien8.de>,
	"Dave Jones" <davej@redhat.com>,
	"Thomas Gleixner" <tglx@linutronix.de>,
	"Andrew Morton" <akpm@linux-foundation.org>,
	"Arjan van de Ven" <arjan@infradead.org>,
	"Tony Luck" <tony.luck@intel.com>,
	KEXEC-ML <kexec@lists.infradead.org>,
	LKML <linux-kernel@vger.kernel.org>,
	"Haren Myneni" <hbabu@us.ibm.com>,
	"Eric Biederman" <ebiederm@xmission.com>,
	"Tejun Heo" <tj@kernel.org>, "Mark Lord" <kernel@teksavvy.com>,
	"Linus Torvalds" <torvalds@linux-foundation.org>,
	"Willy Tarreau" <w@1wt.eu>
Subject: Re: [PATCH 0/2][concept RFC] x86: BIOS-save kernel log to disk upon panic
Date: Wed, 26 Jan 2011 15:34:00 +0200	[thread overview]
Message-ID: <20110126133400.GA3399@laptop> (raw)
In-Reply-To: <20110126125813.GA26952@elte.hu>

On Wed, Jan 26, 2011 at 01:58:13PM +0100, Ingo Molnar wrote:
> 
> * Ahmed S. Darwish <darwish.07@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.

thanks,

-- 
Darwish
http://darwish.07.googlepages.com

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