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

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Date:	Thu, 6 Dec 2007 21:04:00 -0500
From:	Neil Horman <nhorman@...hat.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Vivek Goyal <vgoyal@...hat.com>,
	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

On Thu, Dec 06, 2007 at 05:33:31PM -0700, Eric W. Biederman wrote:
> 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

Copy that.  Thus far, I've tested it on a pure AMD engineering sample, an intel
x86_64 box, and the affected system, a quad socket dual core AMD system with an
nvidia chipset.  That last system is currently the only system that this patch
will check/enable the broadcast flag on.  I did try a variant of the patch on
the AMD engineering sample where it enabled the bit unconditionally.
Interestingly enough, the bit was already turned on on that system.  I'm
wondering if most systems don't already have this bit turned on.  You should be
able to universally enable this bit, by moving the call to
check_hypertransport_config to the top of early_quirks()

Regards
Neil

-- 
/***************************************************
 *Neil Horman
 *Software Engineer
 *Red Hat, Inc.
 *nhorman@...hat.com
 *gpg keyid: 1024D / 0x92A74FA1
 *http://pgp.mit.edu
 ***************************************************/
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