[Xen-changelog] [xen-unstable] x86/apic: fix potential Protection Fault during shutdown

[Xen patchbot-unstable <patchbot@xxxxxxx>]

# HG changeset patch
# User Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
# Date 1308052065 -3600
# Node ID 0feb98534a87b9eab6c3ac94f078fa8efbfad9e0
# Parent  450f1d198e1e299b69489d513f591f0301cc5166
x86/apic: fix potential Protection Fault during shutdown

This is a rare case, but if the BIOS is set to uniprocessor, and Xen
is booted with 'lapic x2apic', Xen will switch into x2apic mode, which
will cause a protection fault when disabling the local APIC.  This
leads to a general protection fault as this code is also in the fault
handler.

When x2apic mode is enabled, the only tranlsation which does
not result in a protection fault is to clear both the EN and EXTD
bits, which is safe to do in all cases, even if you are in xapic
mode rather than x2apic mode.

The linux code from which this is derrived is protected by an
if ( ! x2apic_mode ...) clause which is how they get away with it.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Acked-by: Jan Beulich <jbeulich@xxxxxxxxxx>
---


diff -r 450f1d198e1e -r 0feb98534a87 xen/arch/x86/apic.c
--- a/xen/arch/x86/apic.c       Tue Jun 14 12:46:29 2011 +0100
+++ b/xen/arch/x86/apic.c       Tue Jun 14 12:47:45 2011 +0100
@@ -340,7 +340,8 @@
     if (enabled_via_apicbase) {
         uint64_t msr_content;
         rdmsrl(MSR_IA32_APICBASE, msr_content);
-        wrmsrl(MSR_IA32_APICBASE, msr_content & ~MSR_IA32_APICBASE_ENABLE);
+        wrmsrl(MSR_IA32_APICBASE, msr_content &
+               ~(MSR_IA32_APICBASE_ENABLE|MSR_IA32_APICBASE_EXTD));
     }
 }
 

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