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

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Message-ID: <20071128190525.GD3192@redhat.com>
Date:	Wed, 28 Nov 2007 14:05:25 -0500
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Neil Horman <nhorman@...hat.com>
Cc:	Ben Woodard <woodard@...hat.com>, Andi Kleen <andi@...stfloor.org>,
	kexec@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Andi Kleen <ak@...e.de>, hbabu@...ibm.com,
	"Eric W. Biederman" <ebiederm@...ssion.com>
Subject: Re: [PATCH] kexec: force x86_64 arches to boot kdump kernels on
	boot cpu

On Wed, Nov 28, 2007 at 11:02:06AM -0500, Neil Horman wrote:
> 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?
> 

Ok. Got it. So in this case we route the interrupts directly through LAPIC
and put LVT0 in ExtInt mode and IOAPIC is bypassed.

I am looking at Intel Multiprocessor specification v1.4 and as per figure
3-3 on page 3-9, 8259 is connected to LINTIN0 line, which in turn is 
connected to LINTIN0 pin on all processors. If that is the case, even in
this mode, all the CPU should see the timer interrupts (which is coming
from 8259)?

Can you print the LAPIC registers (print_local_APIC) during normal boot
and during kdump boot and paste here?
 
Thanks
Vivek
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