Re: why kexec (was Re: [PATCH 2/2] ACPI: PCI Interrupt Links -- disable when unused)

[Len Brown <lenb@kernel.org>]

> On Thu, Jan 8, 2009 at 10:55 PM, Len Brown <lenb@kernel.org> wrote:
> >> >> maybe could have one switch in /proc so could not disable that for
> >> >> some kexec path...
> >> >
> >> > I fail to comprehend the benefits of kexec,
> >> > and the requirements that kexec puts on the kernel, other than:
> >> >
> >> > # CONFIG_KEXEC is not set
> >> >
> >>
> >> for me: it is a tools that i could use to make sure root-cause is in FW, and BIOS guys will not kick back the ball to os team.
> >> after modifying table or hw reg in first kernel, and kexec even stock kernel, everything will work well.
> >
> > For ACPI, we already have the ability to override the BIOS tables
> > upon the 1st boot.
> >
> > I don't know which BIOS guys you refer to,
> > but when we find a BIOS bug and have access to
> > the associated BIOS developer, we've never needed
> > to do such a demonstration to convince them they have a bug.
> 
> Your BIOS developer must have enough time or have bunch of BIOS engineers.

Reasonable engineers do not need their own bugs
handed to them on a silver platter.

> > kexec seems like a science project that has a chance of working
> > only under extremely controlled conditions.
> > I have no problem with that, as long as it is not built into my kernel.
> 
> could reduce boot time.

Effort would be better spent on reducing boot time w/ kexec.

> or some system has problem with reset...
> 
> or linuxbios + tinykernel to kexec final production kernel.
> here linuxbios only init ram and basic pci resource allocation ...

My understanding is that the linuxbios people realized
that creating a BIOS that boots Linux (tiny or not)
only to then boot Linux again, was sort of subgenius.
So they re-named linuxbios to be coreboot and now
they talk about loading a "payload"....

> > But say kexec is useful to somebody out there -- what are
> > the requirements that kexec puts on the kernel?
> 
> the support got into the mainline kernel several years ago... from 2.6.16?

I'm afraid I'm not being clear.
This conversation started when you suggested that we should
put a hook in to ask ACPI to manipulate Link state
for the benefit of kexec.

My question is, what else does kexec need to properly
manage the state of the sytem?

I don't think that question has an answer.
I don't think it will ever have an answer.
I think that kexec is a viable concept only
under conditions I don't care about.

-Len