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Subject[PATCH 09/20] xen: Find an unbound irq number in reverse order (high to low).
Date
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.

This patch also includes the fix, found by Ian Campbell, titled
"xen: fix off-by-one error in find_unbound_irq."

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
---
drivers/xen/events.c | 13 ++++++++++---
1 files changed, 10 insertions(+), 3 deletions(-)
diff --git a/drivers/xen/events.c b/drivers/xen/events.c
index 35718db..cd633fa 100644
--- a/drivers/xen/events.c
+++ b/drivers/xen/events.c
@@ -364,12 +364,16 @@ static int find_unbound_irq(void)
struct irq_desc *desc;
int start = get_nr_hw_irqs();

- for (irq = start; irq < nr_irqs; irq++)
+ if (start == nr_irqs)
+ goto no_irqs;
+
+ /* 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)
- panic("No available IRQ to bind to: increase nr_irqs!\n");
+ if (irq == start)
+ goto no_irqs;

desc = irq_to_desc_alloc_node(irq, 0);
if (WARN_ON(desc == NULL))
@@ -378,6 +382,9 @@ static int find_unbound_irq(void)
dynamic_irq_init(irq);

return irq;
+
+no_irqs:
+ panic("No available IRQ to bind to: increase nr_irqs!\n");
}

static bool identity_mapped_irq(unsigned irq)
--
1.7.0.1


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