[Xen-devel] [PATCH] x86/kexec: Fix kexec on systems which boot in x2apic mode

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

Moving straight from fully disabled to x2apic mode is an illegal state
transition, and causes an unconditional #GP fault.  Bounce through xapic mode
to avoid the fault.

In addition, avoid bouncing through the various apic modes if the mode is
already correct.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CC: Jan Beulich <JBeulich@xxxxxxxx>
---
 xen/arch/x86/apic.c |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/xen/arch/x86/apic.c b/xen/arch/x86/apic.c
index 0e5e302..bbcc0a1 100644
--- a/xen/arch/x86/apic.c
+++ b/xen/arch/x86/apic.c
@@ -314,7 +314,7 @@ void disable_local_APIC(void)
                ~(MSR_IA32_APICBASE_ENABLE|MSR_IA32_APICBASE_EXTD));
     }
 
-    if ( kexecing )
+    if ( kexecing && (current_local_apic_mode() != apic_boot_mode) )
     {
         uint64_t msr_content;
         rdmsrl(MSR_IA32_APICBASE, msr_content);
@@ -330,7 +330,9 @@ void disable_local_APIC(void)
             wrmsrl(MSR_IA32_APICBASE, msr_content);
             break;
         case APIC_MODE_X2APIC:
-            msr_content |= (MSR_IA32_APICBASE_ENABLE|MSR_IA32_APICBASE_EXTD);
+            msr_content |= MSR_IA32_APICBASE_ENABLE;
+            wrmsrl(MSR_IA32_APICBASE, msr_content);
+            msr_content |= MSR_IA32_APICBASE_EXTD;
             wrmsrl(MSR_IA32_APICBASE, msr_content);
             break;
         default:
-- 
1.7.10.4


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