In case if apic was disabled by kernel option
or hardware limits we could use dummy operations
such as apic->write at moment.

Same time the patch fixes the missed EOI in
do_IRQ function (which has place if kernel
is compiled as X86-32 and interrupt without
handler happens where apic was not asked to
be disabled via kernel option).

Note that native_apic_write_dummy consists of
WARN_ON_ONCE to catch any unpredictable
writes on enabled APICs. Could be removed
after long time of testing.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
---
 arch/x86/include/asm/apic.h |    3 ++-
 arch/x86/kernel/apic/apic.c |   24 ++++++++++++++++++++++++
 arch/x86/kernel/irq.c       |   10 ++--------
 3 files changed, 28 insertions(+), 9 deletions(-)

Index: linux-2.6.git/arch/x86/include/asm/apic.h
=====================================================================
--- linux-2.6.git.orig/arch/x86/include/asm/apic.h
+++ linux-2.6.git/arch/x86/include/asm/apic.h
@@ -212,6 +212,7 @@ static inline void ack_x2APIC_irq(void)
 }
 #endif
 
+extern void apic_disable(void);
 extern int lapic_get_maxlvt(void);
 extern void clear_local_APIC(void);
 extern void connect_bsp_APIC(void);
@@ -252,7 +253,7 @@ static inline void lapic_shutdown(void) 
 #define local_apic_timer_c2_ok		1
 static inline void init_apic_mappings(void) { }
 static inline void disable_local_APIC(void) { }
-
+static inline void apic_disable(void) { }
 #endif /* !CONFIG_X86_LOCAL_APIC */
 
 #ifdef CONFIG_X86_64
Index: linux-2.6.git/arch/x86/kernel/apic/apic.c
=====================================================================
--- linux-2.6.git.orig/arch/x86/kernel/apic/apic.c
+++ linux-2.6.git/arch/x86/kernel/apic/apic.c
@@ -233,6 +233,24 @@ static int modern_apic(void)
 	return lapic_get_version() >= 0x14;
 }
 
+/*
+ * bare function to substitute write operation
+ * and it's _that_ fast :)
+ */
+void native_apic_write_dummy(u32 reg, u32 v)
+{
+	WARN_ON_ONCE((cpu_has_apic || !disable_apic));
+}
+
+/*
+ * right after this call apic->write doesn't do anything
+ * note that there is no restore operation it works one way
+ */
+void apic_disable(void)
+{
+	apic->write = native_apic_write_dummy;
+}
+
 void native_apic_wait_icr_idle(void)
 {
 	while (apic_read(APIC_ICR) & APIC_ICR_BUSY)
@@ -1592,6 +1610,12 @@ void __init init_apic_mappings(void)
 	 */
 	if (boot_cpu_physical_apicid == -1U)
 		boot_cpu_physical_apicid = read_apic_id();
+
+	/* lets check if we may to NOP'ify apic operations */
+	if (!cpu_has_apic) {
+		pr_info("APIC: disable apic facility\n");
+		apic_disable();
+	}
 }
 
 /*
Index: linux-2.6.git/arch/x86/kernel/irq.c
=====================================================================
--- linux-2.6.git.orig/arch/x86/kernel/irq.c
+++ linux-2.6.git/arch/x86/kernel/irq.c
@@ -27,7 +27,6 @@ void ack_bad_irq(unsigned int irq)
 	if (printk_ratelimit())
 		pr_err("unexpected IRQ trap at vector %02x\n", irq);
 
-#ifdef CONFIG_X86_LOCAL_APIC
 	/*
 	 * Currently unexpected vectors happen only on SMP and APIC.
 	 * We _must_ ack these because every local APIC has only N
@@ -37,9 +36,7 @@ void ack_bad_irq(unsigned int irq)
 	 * completely.
 	 * But only ack when the APIC is enabled -AK
 	 */
-	if (cpu_has_apic)
-		ack_APIC_irq();
-#endif
+	ack_APIC_irq();
 }
 
 #define irq_stats(x)		(&per_cpu(irq_stat, x))
@@ -224,10 +221,7 @@ unsigned int __irq_entry do_IRQ(struct p
 	irq = __get_cpu_var(vector_irq)[vector];
 
 	if (!handle_irq(irq, regs)) {
-#ifdef CONFIG_X86_64
-		if (!disable_apic)
-			ack_APIC_irq();
-#endif
+		ack_APIC_irq();
 
 		if (printk_ratelimit())
 			pr_emerg("%s: %d.%d No irq handler for vector (irq %d)\n",

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