x86: restore x2apic pre-enabled check logic

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@novell.com>

2 files changed, 23 insertions, 2 deletions

diff --git a/xen/arch/x86/genapic/probe.c b/xen/arch/x86/genapic/probe.c
index 7768d36d71..20e72922db 100644
--- 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()) {
diff --git a/xen/arch/x86/genapic/x2apic.c b/xen/arch/x86/genapic/x2apic.c
index 98afc66b1b..7cf8989947 100644
--- 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_probe(void)
 {
     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();
+    }
+}