[Xen-devel] [PATCH 27/31] Find an unbound irq number in reverse order (high to low).

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

From: root <root@xxxxxxxxxxxxxxxxxxxxx>

In earlier Xen Linux kernels, the IRQ mapping was a straight 1:1 and the
find_unbound_irq started looking around 256 for open IRQs and up. IRQs
from 0 to 255 were reserved for PCI devices.  Previous to this patch,
the 'find_unbound_irq'  started looking at get_nr_hw_irqs() number.
For privileged  domain where the ACPI information is available that
returns the upper-bound of what the GSIs. For non-privileged PV domains,
where ACPI is no-existent the get_nr_hw_irqs() reports the IRQ_LEGACY (16).
With PCI passthrough enabled, and with PCI cards that have IRQs pinned
to a higher number than 16 we collide with previously allocated IRQs.
Specifically the PCI IRQs collide with the IPI's for Xen functions
(as they are allocated earlier).
For example:

00:00.11 USB Controller: ATI Technologies Inc SB700 USB OHCI1 Controller 
(prog-if 10 [OHCI])
        ...
        Interrupt: pin A routed to IRQ 18

[root@localhost ~]# cat /proc/interrupts | head
           CPU0       CPU1       CPU2
 16:      38186          0          0   xen-dyn-virq      timer0
 17:        149          0          0   xen-dyn-ipi       spinlock0
 18:        962          0          0   xen-dyn-ipi       resched0

and when the USB controller is loaded, the kernel reports:
IRQ handler type mismatch for IRQ 18
current handler: resched0

One way to fix this is to reverse the logic when looking for un-used
IRQ numbers and start with the highest available number. With that,
we would get:

           CPU0       CPU1       CPU2
... snip ..
292:         35          0          0   xen-dyn-ipi       callfunc0
293:       3992          0          0   xen-dyn-ipi       resched0
294:        224          0          0   xen-dyn-ipi       spinlock0
295:      57183          0          0   xen-dyn-virq      timer0
NMI:          0          0          0   Non-maskable interrupts
.. snip ..

And interrupts for PCI cards are now accessible.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
---
 drivers/xen/events.c |    5 +++--
 1 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/xen/events.c b/drivers/xen/events.c
index e9f7551..b2c1b09 100644
--- a/drivers/xen/events.c
+++ b/drivers/xen/events.c
@@ -371,11 +371,12 @@ static int find_unbound_irq(void)
        struct irq_desc *desc;
        int start = get_nr_hw_irqs();
 
-       for (irq = start; irq < nr_irqs; irq++)
+       /* nr_irqs is a magic value. Must not use it.*/
+       for (irq = nr_irqs-1; irq > start; irq--)
                if (irq_info[irq].type == IRQT_UNBOUND)
                        break;
 
-       if (irq == nr_irqs)
+       if (irq == start || irq == nr_irqs)
                panic("No available IRQ to bind to: increase nr_irqs!\n");
 
        desc = irq_to_desc_alloc_node(irq, 0);
-- 
1.6.2.5


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