c/s 22475 removed the early checking without replacement, neglecting
the fact that x2apic_enabled must be set early for APIC register
accesses done during second stage ACPI table parsing (rooted at
acpi_boot_init()) to work correctly. Without this, particularly
determination of the boot CPU won't work, resulting in an attempt to
bring up that CPU again as a secondary one (which fails).

Restore the functionality, now calling it from generic_apic_probe().

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxxxx>

--- a/xen/arch/x86/apic.c
+++ b/xen/arch/x86/apic.c
@@ -957,19 +957,10 @@ void x2apic_ap_setup(void)
 void __init x2apic_bsp_setup(void)
 {
     struct IO_APIC_route_entry **ioapic_entries = NULL;
-    uint64_t msr_content;
 
     if ( !cpu_has_x2apic )
         return;
 
-    /* Check whether x2apic mode was already enabled by the BIOS. */
-    rdmsrl(MSR_IA32_APICBASE, msr_content);
-    if ( msr_content & MSR_IA32_APICBASE_EXTD )
-    {
-        printk("x2APIC mode is already enabled by BIOS.\n");
-        x2apic_enabled = 1;
-    }
-
     if ( !opt_x2apic )
     {
         if ( !x2apic_enabled )
--- a/xen/arch/x86/genapic/probe.c
+++ b/xen/arch/x86/genapic/probe.c
@@ -59,8 +59,10 @@ custom_param("apic", genapic_apic_force)
 
 void __init generic_apic_probe(void) 
 { 
-	int i;
-	int changed = cmdline_apic = (genapic != NULL);
+	int i, changed;
+
+	check_x2apic_preenabled();
+	cmdline_apic = changed = (genapic != NULL);
 
 	for (i = 0; !changed && apic_probe[i]; i++) { 
 		if (apic_probe[i]->probe()) {
--- a/xen/arch/x86/genapic/x2apic.c
+++ b/xen/arch/x86/genapic/x2apic.c
@@ -24,6 +24,8 @@
 #include <asm/genapic.h>
 #include <asm/apic.h>
 #include <asm/io_apic.h>
+#include <asm/msr.h>
+#include <asm/processor.h>
 #include <xen/smp.h>
 #include <asm/mach-default/mach_mpparse.h>
 
@@ -123,3 +125,20 @@ const struct genapic *__init apic_x2apic
 {
     return x2apic_phys ? &apic_x2apic_phys : &apic_x2apic_cluster;
 }
+
+void __init check_x2apic_preenabled(void)
+{
+    u32 lo, hi;
+
+    if ( !cpu_has_x2apic )
+        return;
+
+    /* Check whether x2apic mode was already enabled by the BIOS. */
+    rdmsr(MSR_IA32_APICBASE, lo, hi);
+    if ( lo & MSR_IA32_APICBASE_EXTD )
+    {
+        printk("x2APIC mode is already enabled by BIOS.\n");
+        x2apic_enabled = 1;
+        genapic = apic_x2apic_probe();
+    }
+}
--- a/xen/include/asm-x86/apic.h
+++ b/xen/include/asm-x86/apic.h
@@ -25,6 +25,7 @@ extern int apic_verbosity;
 extern bool_t x2apic_enabled;
 extern bool_t directed_eoi_enabled;
 
+void check_x2apic_preenabled(void);
 void x2apic_bsp_setup(void);
 void x2apic_ap_setup(void);
 const struct genapic *apic_x2apic_probe(void);