x86, apic: Don't waste a vector to improve vector spread

We want to use a vector-assignment sequence that avoids stumbling onto
0x80 earlier in the sequence, in order to improve the spread of
vectors across priority levels on machines with a small number of
interrupt sources.  Right now, this is done by simply making the first
vector (0x31 or 0x41) completely unusable.  This is unnecessary; all
we need is to start assignment at a +1 offset, we don't actually need
to prohibit the usage of this vector once we have wrapped around.
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
LKML-Reference: <4B426550.6000209@kernel.org>

arch/x86/include/asm/irq_vectors.h

@@ -133,11 +133,12 @@
 #define MCE_SELF_VECTOR			0xeb
 
 /*
- * First APIC vector available to drivers: (vectors 0x30-0xee) we
- * start at 0x31 to spread out vectors evenly between priority
- * levels. (0x80 is the syscall vector)
+ * First APIC vector available to drivers: (vectors 0x30-0xee).  We
+ * start allocating at 0x31 to spread out vectors evenly between
+ * priority levels. (0x80 is the syscall vector)
  */
-#define FIRST_DEVICE_VECTOR		(IRQ15_VECTOR + 2)
+#define FIRST_DEVICE_VECTOR		(IRQ15_VECTOR + 1)
+#define VECTOR_OFFSET_START		1
 
 #define NR_VECTORS			 256
 

arch/x86/kernel/apic/io_apic.c

@@ -1162,7 +1162,8 @@ __assign_irq_vector(int irq, struct irq_cfg *cfg, const struct cpumask *mask)
 	 * Also, we've got to be careful not to trash gate
 	 * 0x80, because int 0x80 is hm, kind of importantish. ;)
 	 */
-	static int current_vector = FIRST_DEVICE_VECTOR, current_offset = 0;
+	static int current_vector = FIRST_DEVICE_VECTOR + VECTOR_OFFSET_START;
+	static int current_offset = VECTOR_OFFSET_START % 8;
 	unsigned int old_vector;
 	int cpu, err;
 	cpumask_var_t tmp_mask;