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    Subject[rfc 3/4] x86: apic - introduce dummy apic operations
    In case if apic was disabled by kernel option
    or hardware limits we could use dummy operations
    such as apic->write at moment.

    Same time the patch fixes the missed EOI in
    do_IRQ function (which has place if kernel
    is compiled as X86-32 and interrupt without
    handler happens where apic was not asked to
    be disabled via kernel option).

    Note that native_apic_write_dummy consists of
    WARN_ON_ONCE to catch any unpredictable
    writes on enabled APICs. Could be removed
    after long time of testing.

    Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
    ---
    arch/x86/include/asm/apic.h | 3 ++-
    arch/x86/kernel/apic/apic.c | 24 ++++++++++++++++++++++++
    arch/x86/kernel/irq.c | 10 ++--------
    3 files changed, 28 insertions(+), 9 deletions(-)

    Index: linux-2.6.git/arch/x86/include/asm/apic.h
    =====================================================================
    --- linux-2.6.git.orig/arch/x86/include/asm/apic.h
    +++ linux-2.6.git/arch/x86/include/asm/apic.h
    @@ -212,6 +212,7 @@ static inline void ack_x2APIC_irq(void)
    }
    #endif

    +extern void apic_disable(void);
    extern int lapic_get_maxlvt(void);
    extern void clear_local_APIC(void);
    extern void connect_bsp_APIC(void);
    @@ -252,7 +253,7 @@ static inline void lapic_shutdown(void)
    #define local_apic_timer_c2_ok 1
    static inline void init_apic_mappings(void) { }
    static inline void disable_local_APIC(void) { }
    -
    +static inline void apic_disable(void) { }
    #endif /* !CONFIG_X86_LOCAL_APIC */

    #ifdef CONFIG_X86_64
    Index: linux-2.6.git/arch/x86/kernel/apic/apic.c
    =====================================================================
    --- linux-2.6.git.orig/arch/x86/kernel/apic/apic.c
    +++ linux-2.6.git/arch/x86/kernel/apic/apic.c
    @@ -233,6 +233,24 @@ static int modern_apic(void)
    return lapic_get_version() >= 0x14;
    }

    +/*
    + * bare function to substitute write operation
    + * and it's _that_ fast :)
    + */
    +void native_apic_write_dummy(u32 reg, u32 v)
    +{
    + WARN_ON_ONCE((cpu_has_apic || !disable_apic));
    +}
    +
    +/*
    + * right after this call apic->write doesn't do anything
    + * note that there is no restore operation it works one way
    + */
    +void apic_disable(void)
    +{
    + apic->write = native_apic_write_dummy;
    +}
    +
    void native_apic_wait_icr_idle(void)
    {
    while (apic_read(APIC_ICR) & APIC_ICR_BUSY)
    @@ -1592,6 +1610,12 @@ void __init init_apic_mappings(void)
    */
    if (boot_cpu_physical_apicid == -1U)
    boot_cpu_physical_apicid = read_apic_id();
    +
    + /* lets check if we may to NOP'ify apic operations */
    + if (!cpu_has_apic) {
    + pr_info("APIC: disable apic facility\n");
    + apic_disable();
    + }
    }

    /*
    Index: linux-2.6.git/arch/x86/kernel/irq.c
    =====================================================================
    --- linux-2.6.git.orig/arch/x86/kernel/irq.c
    +++ linux-2.6.git/arch/x86/kernel/irq.c
    @@ -27,7 +27,6 @@ void ack_bad_irq(unsigned int irq)
    if (printk_ratelimit())
    pr_err("unexpected IRQ trap at vector %02x\n", irq);

    -#ifdef CONFIG_X86_LOCAL_APIC
    /*
    * Currently unexpected vectors happen only on SMP and APIC.
    * We _must_ ack these because every local APIC has only N
    @@ -37,9 +36,7 @@ void ack_bad_irq(unsigned int irq)
    * completely.
    * But only ack when the APIC is enabled -AK
    */
    - if (cpu_has_apic)
    - ack_APIC_irq();
    -#endif
    + ack_APIC_irq();
    }

    #define irq_stats(x) (&per_cpu(irq_stat, x))
    @@ -224,10 +221,7 @@ unsigned int __irq_entry do_IRQ(struct p
    irq = __get_cpu_var(vector_irq)[vector];

    if (!handle_irq(irq, regs)) {
    -#ifdef CONFIG_X86_64
    - if (!disable_apic)
    - ack_APIC_irq();
    -#endif
    + ack_APIC_irq();

    if (printk_ratelimit())
    pr_emerg("%s: %d.%d No irq handler for vector (irq %d)\n",


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