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    Subject[PATCH] x86_64: Make NR_IRQS configurable in Kconfig

    Currrently on a SMP system we can theoretically support
    NR_CPUS*224 irqs. Unfortunately our data structures
    don't cope will with that many irqs, nor does hardware
    typically provide that many irq sources.

    With the number of cores starting to follow Moores
    law, and the apicid limits being raised beyond an 8bit
    number trying to track our current maximum with our
    current data structures would be fatal and wasteful.

    So this patch decouples the number of irqs we support
    from the number of cpus. We can revisit this decision
    once someone reworks the current data structures.

    Signed-off-by: Eric W. Biederman <>
    arch/x86_64/Kconfig | 13 +++++++++++++
    include/asm-x86_64/irq.h | 3 ++-
    2 files changed, 15 insertions(+), 1 deletions(-)

    diff --git a/arch/x86_64/Kconfig b/arch/x86_64/Kconfig
    index 7598d99..d744e5b 100644
    --- a/arch/x86_64/Kconfig
    +++ b/arch/x86_64/Kconfig
    @@ -384,6 +384,19 @@ config NR_CPUS
    This is purely to save memory - each supported CPU requires
    memory in the static kernel configuration.

    +config NR_IRQS
    + int "Maximum number of IRQs (224-4096)"
    + range 256 4096
    + depends on SMP
    + default "4096"
    + help
    + This allows you to specify the maximum number of IRQs which this
    + kernel will support. Current maximum is 4096 IRQs as that
    + is slightly larger than has observed in the field.
    + This is purely to save memory - each supported IRQ requires
    + memory in the static kernel configuration.
    config HOTPLUG_CPU
    bool "Support for hot-pluggable CPUs (EXPERIMENTAL)"
    depends on SMP && HOTPLUG && EXPERIMENTAL
    diff --git a/include/asm-x86_64/irq.h b/include/asm-x86_64/irq.h
    index 5006c6e..34b264a 100644
    --- a/include/asm-x86_64/irq.h
    +++ b/include/asm-x86_64/irq.h
    @@ -31,7 +31,8 @@ #define NR_VECTORS 256

    #define FIRST_SYSTEM_VECTOR 0xef /* duplicated in hw_irq.h */

    -#define NR_IRQS (NR_VECTORS + (32 *NR_CPUS))
    +/* We can use at most NR_CPUS*224 irqs at one time */
    +#define NR_IRQS (CONFIG_NR_IRQS)

    static __inline__ int irq_canonicalize(int irq)
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      Last update: 2006-08-07 17:31    [W:0.020 / U:4.000 seconds]
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