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

From: Ahmed S. Darwish
Date: Tue Jan 25 2011 - 12:05:47 EST


On Tue, Jan 25, 2011 at 11:02:35AM -0500, James Bottomley wrote:
> On Tue, 2011-01-25 at 17:36 +0200, Ahmed S. Darwish wrote:
> > The complete __roadblock__ I'm currently facing though is restoring the disk
> > controllers to the state originally setup by the BIOS Power-on self-test (POST).
> > I hope such re-initialization is even technically feasible.
> >
> > Without such re-initialization, we'll just be risking the BIOS code exploding.
> > That was the case in the 5-minute hang described in the cover sheet (PATCH #0).
> So this is the bit that's not really technically feasible. BIOS tends
> to run storage devices in a very primitive way (so it takes basic
> settings, for example and sets the device up for one particular channel
> of access). When preparing the device for an operating system, we have
> to blow away all the bios stuff and put it into a more generally
> performant mode (this isn't just the storage per se, it's also the
> interrupt and routing). Unfortunately, currently, we don't bother to
> save the settings the BIOS was using, so there's no way to reinitialise
> the device back to bios without an effective reboot.

My current x86 laptop includes the very common ATA PIIX controller. If I
dumped the PIC, IOAPIC, and disk controller state in a register file in a
safe area, and re-initialized these before giving control to the BIOS, can
this move the solution space from being not really technically feasible to
"quite possible"?

I have to admit that my knowledge of disk controllers is nil. The final
result though can really be worth the effort (as stated by Ingo's mail),
so I'm _willing_ to explore and prototype different paths.

> Most BIOS doesn't seem to contain storage re-POST code that's usable
> (it's all embedded in the boot sequence).

There's this arcane `Warm boot with partial re-initialization' BIOS feature
using the CMOS register 0xf, but I don't want to to depend on it either.

> James


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