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Subject[PATCH 09/23] 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."

[v2: Added an explanation in the code]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
---
drivers/xen/events.c | 24 +++++++++++++++++++-----
1 files changed, 19 insertions(+), 5 deletions(-)

diff --git a/drivers/xen/events.c b/drivers/xen/events.c
index e1bd0be..8ae696f 100644
--- a/drivers/xen/events.c
+++ b/drivers/xen/events.c
@@ -370,7 +370,11 @@ 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--) {
desc = irq_to_desc(irq);
/* only 0->15 have init'd desc; handle irq > 16 */
if (desc == NULL)
@@ -383,8 +387,8 @@ static int find_unbound_irq(void)
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))
@@ -393,6 +397,9 @@ static int find_unbound_irq(void)
dynamic_irq_init_keep_chip_data(irq);

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

static bool identity_mapped_irq(unsigned irq)
@@ -546,8 +553,15 @@ static int find_irq_by_gsi(unsigned gsi)
return -1;
}

-/*
- * Allocate a physical irq, along with a vector. We don't assign an
+/* xen_allocate_irq might allocate irqs from the top down, as a
+ * consequence don't assume that the irq number returned has a low value
+ * or can be used as a pirq number unless you know otherwise.
+ *
+ * One notable exception is when xen_allocate_irq is called passing an
+ * hardware gsi as argument, in that case the irq number returned
+ * matches the gsi number passed as first argument.
+
+ * Note: We don't assign an
* event channel until the irq actually started up. Return an
* existing irq if we've already got one for the gsi.
*/
--
1.7.0.4


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