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

On Thu, Dec 06, 2007 at 05:11:43PM -0500, Vivek Goyal wrote:
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?

Thanks
Vivek

No, I don't think thats necessecary. Once the apics are enabled, interrupts
shouldn't travel accross the hypertransport bus anyway, opting instead to use
the dedicated apic bus (at least thats my understanding). The only systems what
you are suggesting would help with are systems that have no apic at all, which I
can only imagine on 64 bit systems is rare, to say the least. The affected
domain is further reduced by the fact that this quirk is only currently being
applied to systems with nvidia PCI bridges, since those are the only systems
that this problem has manifested on. That seems like a rather small subset, if
it exists at all. I suppose we could only optionally enable the quirk if we
are booting a kdump kernel (implying that we would need to do something like
detect the reset_devices command line option), but I think given the limited
affect this patch, its not really needed.

Regards
Neil

--
/***************************************************
*Neil Horman
*Software Engineer
*Red Hat, Inc.
*nhorman@xxxxxxxxxx
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*http://pgp.mit.edu
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