diff --git a/arch/i386/kernel/acpi/boot.c b/arch/i386/kernel/acpi/boot.c
index b66c13c0cc0f..82754bb80e20 100644
--- a/arch/i386/kernel/acpi/boot.c
+++ b/arch/i386/kernel/acpi/boot.c
@@ -39,17 +39,14 @@
 
 #ifdef	CONFIG_X86_64
 
-static inline void acpi_madt_oem_check(char *oem_id, char *oem_table_id)
-{
-}
 extern void __init clustered_apic_check(void);
-static inline int ioapic_setup_disabled(void)
-{
-	return 0;
-}
 
+extern int gsi_irq_sharing(int gsi);
 #include <asm/proto.h>
 
+static inline int acpi_madt_oem_check(char *oem_id, char *oem_table_id) { return 0; }
+
+
 #else				/* X86 */
 
 #ifdef	CONFIG_X86_LOCAL_APIC
@@ -57,6 +54,8 @@ static inline int ioapic_setup_disabled(void)
 #include <mach_mpparse.h>
 #endif				/* CONFIG_X86_LOCAL_APIC */
 
+static inline int gsi_irq_sharing(int gsi) { return gsi; }
+
 #endif				/* X86 */
 
 #define BAD_MADT_ENTRY(entry, end) (					    \
@@ -459,7 +458,7 @@ int acpi_gsi_to_irq(u32 gsi, unsigned int *irq)
 		*irq = IO_APIC_VECTOR(gsi);
 	else
 #endif
-		*irq = gsi;
+		*irq = gsi_irq_sharing(gsi);
 	return 0;
 }
 
diff --git a/arch/x86_64/kernel/io_apic.c b/arch/x86_64/kernel/io_apic.c
index c8eee20cd519..97154ab058b4 100644
--- a/arch/x86_64/kernel/io_apic.c
+++ b/arch/x86_64/kernel/io_apic.c
@@ -57,7 +57,7 @@ int nr_ioapic_registers[MAX_IO_APICS];
  * Rough estimation of how many shared IRQs there are, can
  * be changed anytime.
  */
-#define MAX_PLUS_SHARED_IRQS NR_IRQS
+#define MAX_PLUS_SHARED_IRQS NR_IRQ_VECTORS
 #define PIN_MAP_SIZE (MAX_PLUS_SHARED_IRQS + NR_IRQS)
 
 /*
@@ -85,6 +85,7 @@ int vector_irq[NR_VECTORS] __read_mostly = { [0 ... NR_VECTORS - 1] = -1};
 	int pin;							\
 	struct irq_pin_list *entry = irq_2_pin + irq;			\
 									\
+	BUG_ON(irq >= NR_IRQS);						\
 	for (;;) {							\
 		unsigned int reg;					\
 		pin = entry->pin;					\
@@ -127,6 +128,8 @@ static void set_ioapic_affinity_irq(unsigned int irq, cpumask_t mask)
 }
 #endif
 
+static u8 gsi_2_irq[NR_IRQ_VECTORS] = { [0 ... NR_IRQ_VECTORS-1] = 0xFF };
+
 /*
  * The common case is 1:1 IRQ<->pin mappings. Sometimes there are
  * shared ISA-space IRQs, so we have to support them. We are super
@@ -137,6 +140,7 @@ static void add_pin_to_irq(unsigned int irq, int apic, int pin)
 	static int first_free_entry = NR_IRQS;
 	struct irq_pin_list *entry = irq_2_pin + irq;
 
+	BUG_ON(irq >= NR_IRQS);
 	while (entry->next)
 		entry = irq_2_pin + entry->next;
 
@@ -144,7 +148,7 @@ static void add_pin_to_irq(unsigned int irq, int apic, int pin)
 		entry->next = first_free_entry;
 		entry = irq_2_pin + entry->next;
 		if (++first_free_entry >= PIN_MAP_SIZE)
-			panic("io_apic.c: whoops");
+			panic("io_apic.c: ran out of irq_2_pin entries!");
 	}
 	entry->apic = apic;
 	entry->pin = pin;
@@ -420,6 +424,7 @@ int IO_APIC_get_PCI_irq_vector(int bus, int slot, int pin)
 				best_guess = irq;
 		}
 	}
+	BUG_ON(best_guess >= NR_IRQS);
 	return best_guess;
 }
 
@@ -610,6 +615,64 @@ static inline int irq_trigger(int idx)
 	return MPBIOS_trigger(idx);
 }
 
+static int next_irq = 16;
+
+/*
+ * gsi_irq_sharing -- Name overload!  "irq" can be either a legacy IRQ
+ * in the range 0-15, a linux IRQ in the range 0-223, or a GSI number
+ * from ACPI, which can reach 800 in large boxen.
+ *
+ * Compact the sparse GSI space into a sequential IRQ series and reuse
+ * vectors if possible.
+ */
+int gsi_irq_sharing(int gsi)
+{
+	int i, tries, vector;
+
+	BUG_ON(gsi >= NR_IRQ_VECTORS);
+
+	if (platform_legacy_irq(gsi))
+		return gsi;
+
+	if (gsi_2_irq[gsi] != 0xFF)
+		return (int)gsi_2_irq[gsi];
+
+	tries = NR_IRQS;
+  try_again:
+	vector = assign_irq_vector(gsi);
+
+	/*
+	 * Sharing vectors means sharing IRQs, so scan irq_vectors for previous
+	 * use of vector and if found, return that IRQ.  However, we never want
+	 * to share legacy IRQs, which usually have a different trigger mode
+	 * than PCI.
+	 */
+	for (i = 0; i < NR_IRQS; i++)
+		if (IO_APIC_VECTOR(i) == vector)
+			break;
+	if (platform_legacy_irq(i)) {
+		if (--tries >= 0) {
+			IO_APIC_VECTOR(i) = 0;
+			goto try_again;
+		}
+		panic("gsi_irq_sharing: didn't find an IRQ using vector 0x%02X for GSI %d", vector, gsi);
+	}
+	if (i < NR_IRQS) {
+		gsi_2_irq[gsi] = i;
+		printk(KERN_INFO "GSI %d sharing vector 0x%02X and IRQ %d\n",
+				gsi, vector, i);
+		return i;
+	}
+
+	i = next_irq++;
+	BUG_ON(i >= NR_IRQS);
+	gsi_2_irq[gsi] = i;
+	IO_APIC_VECTOR(i) = vector;
+	printk(KERN_INFO "GSI %d assigned vector 0x%02X and IRQ %d\n",
+			gsi, vector, i);
+	return i;
+}
+
 static int pin_2_irq(int idx, int apic, int pin)
 {
 	int irq, i;
@@ -639,6 +702,7 @@ static int pin_2_irq(int idx, int apic, int pin)
 			while (i < apic)
 				irq += nr_ioapic_registers[i++];
 			irq += pin;
+			irq = gsi_irq_sharing(irq);
 			break;
 		}
 		default:
@@ -648,6 +712,7 @@ static int pin_2_irq(int idx, int apic, int pin)
 			break;
 		}
 	}
+	BUG_ON(irq >= NR_IRQS);
 
 	/*
 	 * PCI IRQ command line redirection. Yes, limits are hardcoded.
@@ -663,6 +728,7 @@ static int pin_2_irq(int idx, int apic, int pin)
 			}
 		}
 	}
+	BUG_ON(irq >= NR_IRQS);
 	return irq;
 }
 
@@ -690,8 +756,8 @@ int assign_irq_vector(int irq)
 {
 	static int current_vector = FIRST_DEVICE_VECTOR, offset = 0;
 
-	BUG_ON(irq >= NR_IRQ_VECTORS);
-	if (IO_APIC_VECTOR(irq) > 0)
+	BUG_ON(irq != AUTO_ASSIGN && (unsigned)irq >= NR_IRQ_VECTORS);
+	if (irq != AUTO_ASSIGN && IO_APIC_VECTOR(irq) > 0)
 		return IO_APIC_VECTOR(irq);
 next:
 	current_vector += 8;
@@ -699,9 +765,8 @@ next:
 		goto next;
 
 	if (current_vector >= FIRST_SYSTEM_VECTOR) {
-		offset++;
-		if (!(offset%8))
-			return -ENOSPC;
+		/* If we run out of vectors on large boxen, must share them. */
+		offset = (offset + 1) % 8;
 		current_vector = FIRST_DEVICE_VECTOR + offset;
 	}
 
@@ -1917,6 +1982,7 @@ int io_apic_set_pci_routing (int ioapic, int pin, int irq, int edge_level, int a
 	entry.polarity = active_high_low;
 	entry.mask = 1;					 /* Disabled (masked) */
 
+	irq = gsi_irq_sharing(irq);
 	/*
 	 * IRQs < 16 are already in the irq_2_pin[] map
 	 */
diff --git a/arch/x86_64/kernel/mpparse.c b/arch/x86_64/kernel/mpparse.c
index f16d38d09daf..8f6958e79455 100644
--- a/arch/x86_64/kernel/mpparse.c
+++ b/arch/x86_64/kernel/mpparse.c
@@ -218,7 +218,7 @@ static void __init MP_intsrc_info (struct mpc_config_intsrc *m)
 			m->mpc_irqtype, m->mpc_irqflag & 3,
 			(m->mpc_irqflag >> 2) & 3, m->mpc_srcbus,
 			m->mpc_srcbusirq, m->mpc_dstapic, m->mpc_dstirq);
-	if (++mp_irq_entries == MAX_IRQ_SOURCES)
+	if (++mp_irq_entries >= MAX_IRQ_SOURCES)
 		panic("Max # of irq sources exceeded!!\n");
 }
 
diff --git a/include/asm-x86_64/desc.h b/include/asm-x86_64/desc.h
index 68ac3c62fe3d..1a3d380f9d5d 100644
--- a/include/asm-x86_64/desc.h
+++ b/include/asm-x86_64/desc.h
@@ -98,16 +98,19 @@ static inline void _set_gate(void *adr, unsigned type, unsigned long func, unsig
 
 static inline void set_intr_gate(int nr, void *func) 
 { 
+	BUG_ON((unsigned)nr > 0xFF);
 	_set_gate(&idt_table[nr], GATE_INTERRUPT, (unsigned long) func, 0, 0); 
 } 
 
 static inline void set_intr_gate_ist(int nr, void *func, unsigned ist) 
 { 
+	BUG_ON((unsigned)nr > 0xFF);
 	_set_gate(&idt_table[nr], GATE_INTERRUPT, (unsigned long) func, 0, ist); 
 } 
 
 static inline void set_system_gate(int nr, void *func) 
 { 
+	BUG_ON((unsigned)nr > 0xFF);
 	_set_gate(&idt_table[nr], GATE_INTERRUPT, (unsigned long) func, 3, 0); 
 } 
 
diff --git a/include/asm-x86_64/mpspec.h b/include/asm-x86_64/mpspec.h
index f267e10c023d..6b375384f5c2 100644
--- a/include/asm-x86_64/mpspec.h
+++ b/include/asm-x86_64/mpspec.h
@@ -157,7 +157,8 @@ struct mpc_config_lintsrc
  */
 
 #define MAX_MP_BUSSES 256
-#define MAX_IRQ_SOURCES 256
+/* Each PCI slot may be a combo card with its own bus.  4 IRQ pins per slot. */
+#define MAX_IRQ_SOURCES (MAX_MP_BUSSES * 4)
 enum mp_bustype {
 	MP_BUS_ISA = 1,
 	MP_BUS_EISA,