xen/irq: Cleanup the find_unbound_irq

The "find_unbound_irq" is a bit unusual - it allocates
virtual IRQ (event channels) in reverse order. This means
starting at the "top" of the available IRQs (nr_irqs) down
to the GSI/MSI IRQs (nr_irqs_gsi). Lets document this and
also make the variables easier to understand.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

diff --git a/drivers/xen/events.c b/drivers/xen/events.c
index 31af0ac..4d4a23d 100644 (file)
--- a/drivers/xen/events.c
+++ b/drivers/xen/events.c
@@ -405,15 +405,21 @@ static int find_unbound_irq(void)
 {
        struct irq_data *data;
        int irq, res;
-       int start = get_nr_hw_irqs();
+       int bottom = get_nr_hw_irqs();
+       int top = nr_irqs-1;
 
-       if (start == nr_irqs)
+       if (bottom == nr_irqs)
                goto no_irqs;
 
-       /* nr_irqs is a magic value. Must not use it.*/
-       for (irq = nr_irqs-1; irq > start; irq--) {
+       /* This loop starts from the top of IRQ space and goes down.
+        * We need this b/c if we have a PCI device in a Xen PV guest
+        * we do not have an IO-APIC (though the backend might have them)
+        * mapped in. To not have a collision of physical IRQs with the Xen
+        * event channels start at the top of the IRQ space for virtual IRQs.
+        */
+       for (irq = top; irq > bottom; irq--) {
                data = irq_get_irq_data(irq);
-               /* only 0->15 have init'd desc; handle irq > 16 */
+               /* only 15->0 have init'd desc; handle irq > 16 */
                if (!data)
                        break;
                if (data->chip == &no_irq_chip)
@@ -424,7 +430,7 @@ static int find_unbound_irq(void)
                        return irq;
        }
 
-       if (irq == start)
+       if (irq == bottom)
                goto no_irqs;
 
        res = irq_alloc_desc_at(irq, -1);