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From 95e9590bfee2df447c8f4c0fd799e8c514beca80 Mon Sep 17 00:00:00 2001
From: Denys Vlasenko <>
Date: Tue, 10 Dec 2013 13:07:35 +0100
Subject: [ABRT PATCH 24/27] doc/MCE_readme.txt: new file - documentation about
 MCE handling

Signed-off-by: Denys Vlasenko <>

Related to rhbz#1032077

Signed-off-by: Jakub Filak <>
 doc/MCE_readme.txt | 86 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 86 insertions(+)
 create mode 100644 doc/MCE_readme.txt

diff --git a/doc/MCE_readme.txt b/doc/MCE_readme.txt
new file mode 100644
index 0000000..ed5b627
--- /dev/null
+++ b/doc/MCE_readme.txt
@@ -0,0 +1,86 @@
+	Background
+MCEs can be fatal (they panic kernel) or not.
+Fatal MCE are delivered as exception#18.
+Non-fatal ones sometimes are delivered as exception#18; other times
+they are silently recorded in magic MSRs, CPU is not alerted.
+Linux kernel periodically (up to 5 mins interval) reads those MSRs
+and if MCE is seen there, it is piped in binary form through
+/dev/mcelog to whoever listens on it. (Such as mcelog tool in
+--daemon mode; but cat </dev/mcelog would do too).
+"Machine Check Exception:" message is printed *only* by fatal MCEs.
+It will be caught as vmcore if kdump is configured.
+Non-fatal MCEs have "[Hardware Error]: Machine check events logged"
+message in kernel log.
+When /dev/mcelog is read, *no additional kernel log messages appear*.
+> Are those magic MSR registers cleared when read via /dev/mcelog?
+> Without mcelog utility, we can directly read only binary form, right?
+> Not nice, but still useful, right?
+> (could be transferred to nice text form on other machine).
+No, raw /dev/mcelog data is not easy to interpret on other machine.
+In fact, it can't be used by mcelog tool even on the same machine.
+Technical reason is that mcelog uses an obscure ioctl on /dev/mcelog
+in order to know the size of binary blob with MCE information.
+When run on a file, ioctl fails, and mcelog bombs out.
+Looks like without mcelog running and processing /dev/mcelog data,
+non-fatal MCE's can't be easily decoded with currently existing tools.
+mcelog tool can be configured to write log to /var/log/mcelog
+(RHEL6 does that) or to syslog (RHEL7 does that).
+	How ABRT catches MCEs
+Fatal MCEs are caught as any fatal kernel panic is caught - as a vmcore.
+The oops text, which goes to "backtrace" element, will be the decoded
+MCE message from kernel log buffer.
+Non-fatal MCEs are caught as kernel oopses.
+If "Machine check events logged" message is seen in "dmesg" element,
+we assume it's a MCE, and create "not-reportable" element with suitable
+Then we check whether /var/log/mcelog exists,
+or whether system log contains "mcelog: Hardware event",
+and create a "comment" element with explanatory text, followed by
+last 20 lines from either of those files.
+	How to test MCEs
+There is an MCE injection tool and a kernel module, both named mce-inject.
+(The tool comes from mce-test project, may be found in ras-utils RHEL7 package).
+The script I used is:
+modprobe mce-inject
+sync &
+sleep 1
+# This can crash the machine:
+echo "Injecting MCE from file $1"
+mce-inject "$1"
+echo "Exitcode:$?"
+It requires files which describe MCE to simulate. I grabbed a few examples
+from mce-test.tar.gz (source tarball of mce-test project).
+I used this this file to cause a non-fatal MCE:
+And this one to cause a fatal one:
+RIP 12343434
+MISC 11
+(Not sure what failures exactly they imitate, maybe there are better examples).