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>

1 files changed, 12 insertions, 6 deletions

diff --git a/drivers/xen/events.c b/drivers/xen/events.c
index 31af0ac31a98..4d4a23d4c9f6 100644
--- 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);