linux-kernel - RE: [PATCH v2 1/2] x86: mce: kexec: turn off MCE in kexec

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Date:	Fri, 27 Feb 2015 18:27:16 +0000
From:	"Luck, Tony" <tony.luck@...el.com>
To:	Borislav Petkov <bp@...en8.de>, Prarit Bhargava <prarit@...hat.com>
CC:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Vivek Goyal <vgoyal@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Junichi Nomura <j-nomura@...jp.nec.com>,
	Kiyoshi Ueda <k-ueda@...jp.nec.com>
Subject: RE: [PATCH v2 1/2] x86: mce: kexec: turn off MCE in kexec

> When CR4.MCE=0b and an MCE happens, it will shutdown the system, at
> least on Intel, according to Tony

I checked with the architects ... and I was right. If you clear CR4.MCE you'll still
see the machine check - and you'll pull the big system reset lever.

If you think the other cpus can survive the reset - then the right thing to do is to
have any offline cpus that show up in the machine check handler just clear MCG_STATUS
and return:

do_machine_check()
{
	/* offline cpus may show up for the party - but don't need to do anything here - send them back home */
	if (!(cpu_online(smp_processor_id())) {
		mce_wrmsrl(MSR_IA32_MCG_STATUS, 0);
		return;
	}

If we are crashing because of a machine check - I wonder how useful it is to run kdump().  There are a very
small set of ways that you can induce a machine check from program action - normally the problem is that
something bad happened in the h/w ... a kdump will just fill your disk and waste your time looking at what
the s/w was dong when the machine check happened.

-Tony