linux-kernel - [tip:x86/urgent] x86/apic, doc: Justification for disabling IO APIC before Local APIC

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Date:	Wed, 4 Dec 2013 19:36:30 -0800
From:	tip-bot for Fenghua Yu <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, hpa@...or.com, mingo@...nel.org,
	fenghua.yu@...el.com, tglx@...utronix.de, hpa@...ux.intel.com
Subject: [tip:x86/urgent] x86/apic, doc:
  Justification for disabling IO APIC before Local APIC

Commit-ID:  2885432aaf15c1b7e65c787bfe7c5fec428296f0
Gitweb:     http://git.kernel.org/tip/2885432aaf15c1b7e65c787bfe7c5fec428296f0
Author:     Fenghua Yu <fenghua.yu@...el.com>
AuthorDate: Wed, 4 Dec 2013 16:07:49 -0800
Committer:  H. Peter Anvin <hpa@...ux.intel.com>
CommitDate: Wed, 4 Dec 2013 19:33:21 -0800

x86/apic, doc: Justification for disabling IO APIC before Local APIC

Since erratum AVR31 in "Intel Atom Processor C2000 Product Family
Specification Update" is now published, I added a justification
comment for disabling IO APIC before Local APIC, as changed in commit:

522e66464467 x86/apic: Disable I/O APIC before shutdown of the local APIC

Signed-off-by: Fenghua Yu <fenghua.yu@...el.com>
Link: http://lkml.kernel.org/r/1386202069-51515-1-git-send-email-fenghua.yu@intel.com
Signed-off-by: H. Peter Anvin <hpa@...ux.intel.com>
---
 arch/x86/kernel/reboot.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/arch/x86/kernel/reboot.c b/arch/x86/kernel/reboot.c
index da3c599..c752cb4 100644
--- a/arch/x86/kernel/reboot.c
+++ b/arch/x86/kernel/reboot.c
@@ -558,6 +558,17 @@ void native_machine_shutdown(void)
 {
 	/* Stop the cpus and apics */
 #ifdef CONFIG_X86_IO_APIC
+	/*
+	 * Disabling IO APIC before local APIC is a workaround for
+	 * erratum AVR31 in "Intel Atom Processor C2000 Product Family
+	 * Specification Update". In this situation, interrupts that target
+	 * a Logical Processor whose Local APIC is either in the process of
+	 * being hardware disabled or software disabled are neither delivered
+	 * nor discarded. When this erratum occurs, the processor may hang.
+	 *
+	 * Even without the erratum, it still makes sense to quiet IO APIC
+	 * before disabling Local APIC.
+	 */
 	disable_IO_APIC();
 #endif
 
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