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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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