x86, x2apic: Fallback to xapic when BIOS doesn't setup interrupt-remapping

On some of the recent Intel SNB platforms, by default bios is pre-enabling
x2apic mode in the cpu with out setting up interrupt-remapping.
This case was resulting in the kernel to panic as the cpu is already in
x2apic mode but the OS was not able to enable interrupt-remapping (which
is a pre-req for using x2apic capability).

On these platforms all the apic-ids are < 255 and the kernel can fallback to
xapic mode if the bios has not enabled interrupt-remapping (which is
mostly the case if the bios has not exported interrupt-remapping tables to the
OS).

Reported-by: Berck E. Nash <flyboy@gmail.com>
Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Link: http://lkml.kernel.org/r/20111222014632.600418637@sbsiddha-desk.sc.intel.com
Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

2 files changed, 59 insertions, 18 deletions

diff --git a/arch/x86/kernel/apic/apic.c b/arch/x86/kernel/apic/apic.c
index 07832363b729..2c07aebbb6f2 100644
--- a/arch/x86/kernel/apic/apic.c
+++ b/arch/x86/kernel/apic/apic.c
@@ -146,7 +146,8 @@ __setup("apicpmtimer", setup_apicpmtimer);
 int x2apic_mode;
 #ifdef CONFIG_X86_X2APIC
 /* x2apic enabled before OS handover */
-static int x2apic_preenabled;
+int x2apic_preenabled;
+static int x2apic_disabled;
 static __init int setup_nox2apic(char *str)
 {
         if (x2apic_enabled()) {
@@ -1432,6 +1433,40 @@ void __init bsp_end_local_APIC_setup(void)
 }
 
 #ifdef CONFIG_X86_X2APIC
+/*
+ * Need to disable xapic and x2apic at the same time and then enable xapic mode
+ */
+static inline void __disable_x2apic(u64 msr)
+{
+        wrmsrl(MSR_IA32_APICBASE,
+               msr & ~(X2APIC_ENABLE | XAPIC_ENABLE));
+        wrmsrl(MSR_IA32_APICBASE, msr & ~X2APIC_ENABLE);
+}
+
+static void disable_x2apic(void)
+{
+        u64 msr;
+
+        if (!cpu_has_x2apic)
+                return;
+
+        rdmsrl(MSR_IA32_APICBASE, msr);
+        if (msr & X2APIC_ENABLE) {
+                u32 x2apic_id = read_apic_id();
+
+                if (x2apic_id >= 255)
+                        panic("Cannot disable x2apic, id: %08x\n", x2apic_id);
+
+                pr_info("Disabling x2apic\n");
+                __disable_x2apic(msr);
+
+                x2apic_disabled = 1;
+                x2apic_mode = 0;
+
+                register_lapic_address(mp_lapic_addr);
+        }
+}
+
 void check_x2apic(void)
 {
         if (x2apic_enabled()) {
@@ -1442,15 +1477,20 @@ void check_x2apic(void)
 
 void enable_x2apic(void)
 {
-        int msr, msr2;
+        u64 msr;
+
+        rdmsrl(MSR_IA32_APICBASE, msr);
+        if (x2apic_disabled) {
+                __disable_x2apic(msr);
+                return;
+        }
 
         if (!x2apic_mode)
                 return;
 
-        rdmsr(MSR_IA32_APICBASE, msr, msr2);
         if (!(msr & X2APIC_ENABLE)) {
                 printk_once(KERN_INFO "Enabling x2apic\n");
-                wrmsr(MSR_IA32_APICBASE, msr | X2APIC_ENABLE, msr2);
+                wrmsrl(MSR_IA32_APICBASE, msr | X2APIC_ENABLE);
         }
 }
 #endif /* CONFIG_X86_X2APIC */
@@ -1487,7 +1527,7 @@ void __init enable_IR_x2apic(void)
         ret = save_ioapic_entries();
         if (ret) {
                 pr_info("Saving IO-APIC state failed: %d\n", ret);
-                goto out;
+                return;
         }
 
         local_irq_save(flags);
@@ -1499,13 +1539,19 @@ void __init enable_IR_x2apic(void)
         else
                 ret = enable_IR();
 
+        if (!x2apic_supported())
+                goto nox2apic;
+
         if (ret < 0) {
                 /* IR is required if there is APIC ID > 255 even when running
                  * under KVM
                  */
                 if (max_physical_apicid > 255 ||
-                    !hypervisor_x2apic_available())
+                    !hypervisor_x2apic_available()) {
+                        if (x2apic_preenabled)
+                                disable_x2apic();
                         goto nox2apic;
+                }
                 /*
                  * without IR all CPUs can be addressed by IOAPIC/MSI
                  * only in physical mode
@@ -1513,8 +1559,10 @@ void __init enable_IR_x2apic(void)
                 x2apic_force_phys();
         }
 
-        if (ret == IRQ_REMAP_XAPIC_MODE)
+        if (ret == IRQ_REMAP_XAPIC_MODE) {
+                pr_info("x2apic not enabled, IRQ remapping is in xapic mode\n");
                 goto nox2apic;
+        }
 
         x2apic_enabled = 1;
 
@@ -1529,17 +1577,6 @@ nox2apic:
                 restore_ioapic_entries();
         legacy_pic->restore_mask();
         local_irq_restore(flags);
-
-out:
-        if (x2apic_enabled || !x2apic_supported())
-                return;
-
-        if (x2apic_preenabled)
-                panic("x2apic: enabled by BIOS but kernel init failed.");
-        else if (ret == IRQ_REMAP_XAPIC_MODE)
-                pr_info("x2apic not enabled, IRQ remapping is in xapic mode\n");
-        else if (ret < 0)
-                pr_info("x2apic not enabled, IRQ remapping init failed\n");
 }
 
 #ifdef CONFIG_X86_64
diff --git a/arch/x86/kernel/apic/io_apic.c b/arch/x86/kernel/apic/io_apic.c
index 6d939d7847e2..45b461fdb344 100644
--- a/arch/x86/kernel/apic/io_apic.c
+++ b/arch/x86/kernel/apic/io_apic.c
@@ -2948,6 +2948,10 @@ static inline void __init check_timer(void)
         }
         local_irq_disable();
         apic_printk(APIC_QUIET, KERN_INFO "..... failed :(.\n");
+        if (x2apic_preenabled)
+                apic_printk(APIC_QUIET, KERN_INFO
+                            "Perhaps problem with the pre-enabled x2apic mode\n"
+                            "Try booting with x2apic and interrupt-remapping disabled in the bios.\n");
         panic("IO-APIC + timer doesn't work!  Boot with apic=debug and send a "
                 "report.  Then try booting with the 'noapic' option.\n");
 out: