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>

diff --git a/arch/x86/include/asm/irq_vectors.h b/arch/x86/include/asm/irq_vectors.h
index dbc81ac..585a428 100644 (file)
--- a/arch/x86/include/asm/irq_vectors.h
+++ b/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
 

diff --git a/arch/x86/kernel/apic/io_apic.c b/arch/x86/kernel/apic/io_apic.c
index d9cd1f1..e9ba090 100644 (file)
--- a/arch/x86/kernel/apic/io_apic.c
+++ b/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;