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

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Date:	Thu, 06 Dec 2007 17:33:31 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	Neil Horman <nhorman@...driver.com>,
	Neil Horman <nhorman@...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

Vivek Goyal <vgoyal@...hat.com> writes:

> On Thu, Dec 06, 2007 at 04:39:51PM -0500, Neil Horman wrote:
>> On Fri, Nov 30, 2007 at 09:51:31AM -0500, Neil Horman wrote:
>> > On Fri, Nov 30, 2007 at 09:42:50AM -0500, Vivek Goyal wrote:
>> <snip>
>> > 
>> > Thats what I'm doing at the moment.  I'm working on a RHEL5 patch at the
> moment
>> > (since thats whats on the production system thats failing), and will forward
>> > port it once its working
>> > 
>> > And not to split hairs, but techically thats not our _only_ choice.  We
> could
>> > force kdump boots on cpu0 as well ;)
>> > 
>> > Thanks
>> > Neil
>> > 
>> > > Thanks
>> > > Vivek
>> > 
>> 
>> 
>> Sorry to have been quiet on this issue for a few days. Interesting news to
>> report, though.  So I was working on a patch to do early apic enabling on
>> x86_64, and had something working for the old 2.6.18 kernel that we were
>> origionally testing on.  Unfortunately while it worked on 2.6.18 it failed
>> miserably on 2.6.24-rc3-mm2, causing check_timer to consistently report that
> the
>> timer interrupt wasn't getting received (even though we could successfully run
>> calibrate_delay).  Vivek and I were digging into this, when I ran accross the
>> description of the hypertransport configuration register in the opteron
>> specification.  It contains a bit that, suprise, configures the ht bus to
> either
>> unicast interrupts delivered accross the ht bus to a single cpu, or to
> broadcast
>> it to all cpus.  Since it seemed more likely that the 8259 in the nvidia
>> southbridge was transporting legacy mode interrupts over the ht bus than
>> directly to cpu0 via an actual wire, I wrote the attached patch to add a quirk
>> for nvidia chipsets, which scanned for hypertransport controllers, and ensured
>> that that broadcast bit was set.  Test results indicate that this solves the
>> problem, and kdump kernels boot just fine on the affected system.
>> 
>
> Hi Neil,
>
> Should we disable this broadcasting feature once we are through? Otherwise
> in normal systems it might mean extra traffic on hypertransport. There
> is no need for every interrupt to be broadcasted in normal systems?

My feel is that if it is for legacy interrupts only it should not be a problem.
Let's investigate and see if we can unconditionally enable this quirk
for all opteron systems.

Eric
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