Re: [PATCH] kexec: force x86_64 arches to boot kdump kernels on boot cpu

Neil Horman
2007-11-28T16:07:52Z

On Wed, Nov 28, 2007 at 10:36:49AM -0500, Vivek Goyal wrote:
> On Tue, Nov 27, 2007 at 03:24:35PM -0800, Ben Woodard wrote:
> > Andi Kleen wrote:
> >>> Are we putting the system back in PIC mode or virtual wire mode? I have
> >>> not seen systems which support PIC mode. All latest systems seems
> >>> to be having virtual wire mode. I think in case of PIC mode, interrupts
> >>
> >> Yes it's probably virtual wire. For real PIC mode we would need really
> >> old systems without APIC.
> >>
> >>> can be delivered to cpu0 only. In virt wire mode, one can program IOAPIC
> >>> to deliver interrupt to any of the cpus and that's what we have been
> >>
> >> The code doesn't try to program anything specific, it just restores the state
> >> that was left over originally by the BIOS.
> >>
> >
> > So if the BIOS originally left the IOAPIC in a state where the timer
> > interrupts were only going to CPU0 then by restoring that state we could be
> > bringing this problem upon ourselves when we restore that state.
> >
>
> Hi Ben,
>
> Apart from restoring the original state (Bring APICS back to virtual wire
> mode), we also reprogram IOAPIC so that timer interrupt can go to crashing
> cpu (and not necessarily cpu0). Look at following code in disable_IO_APIC.
>
> entry.dest.physical.physical_dest =
> GET_APIC_ID(apic_read(APIC_ID));
>
> Here we read the apic id of crashing cpu and program IOAPIC accordingly.
> This will make sure that even in virtual wire mode, timer interrupts
> will be delivered to crashing cpu APIC.
>
Yes, but according to Bens last debug effort, the APIC printout regarding the
timer setup, indicates that ioapic_i8259.pin == -1, meaning that the 8259 is not
routed through the ioapic. In those cases, disable_IO_APIC does not take us
through the path you reference above, and does not revert to virtual wire mode.
Instead, it simply disables legacy vector 0, which if I understand this
correctly, simply tells the ioapic to not handle timer interrupts, trusting that
the 8259 in the system will deliver that interrupt where it needs to be. If the
8259 is wired to deliver timer interrupts to cpu0 only, then you get the problem
that we have, do you?

Regards
Neil

> I think we need to go deeper and compare the state of system (APICS,
> timer etc) during normal boot and kdump boot and see where is the
> difference. This is how I solved some of the timer interrupt related
> issues in the past.
>
> Thanks
> Vivek

--
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*Neil Horman
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*Red Hat, Inc.
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