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

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Date:	Wed, 28 Nov 2007 10:36:12 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Neil Horman <nhorman@...hat.com>
Cc:	Vivek Goyal <vgoyal@...hat.com>, 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
Subject: Re: [PATCH] kexec: force x86_64 arches to boot kdump kernels on boot cpu

Neil Horman <nhorman@...hat.com> writes:

> 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?

Exactly.

It is still interesting to test to see what happens if you plugin the
normal values into ioapic_i8259 for .pin and .apic (.pin is 0 or 2 and .apic is 0)
and see what happens.

Having a command line parameter that could do that would be a cheap temporary
solution.

But this is the most likely reason why the timer interrupt is not working.

Eric
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