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Subject[PATCH 1/5] Skip timer works.patch
Add a way to disable the timer IRQ routing check via a boot option.  The
VMI timer code uses this to avoid triggering the pester Mingo code, which
probes for some very unusual and broken motherboard routings. It fires
100% of the time when using a paravirtual delay mechanism instead of
using a realtime delay, since there is no elapsed real time, and the 4 timer
IRQs have not yet been delivered.

In addition, it is entirely possible, though improbable, that this bug
could surface on real hardware which picks a particularly bad time to enter
SMM mode, causing a long latency during one of the timer IRQs.

While here, make check_timer be __init.

Signed-off-by: Zachary Amsden <>

--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -603,8 +603,6 @@ and is between 256 and 4096 characters.

hugepages= [HW,IA-32,IA-64] Maximal number of HugeTLB pages.

- noirqbalance [IA-32,SMP,KNL] Disable kernel irq balancing
- [HW] Put keyboard port into non-translated mode
i8042.dumbkbd [HW] Pretend that controller can only read data from
keyboard and cannot control its state
@@ -1060,8 +1058,13 @@ and is between 256 and 4096 characters.
in certain environments such as networked servers or
real-time systems.

+ noirqbalance [IA-32,SMP,KNL] Disable kernel irq balancing
noirqdebug [IA-32] Disables the code which attempts to detect and
disable unhandled interrupt sources.
+ noirqtest [IA-32,APIC] Disables the code which tests for broken
+ timer IRQ sources.

noisapnp [ISAPNP] Disables ISA PnP code.

--- a/arch/i386/kernel/io_apic.c
+++ b/arch/i386/kernel/io_apic.c
@@ -1864,6 +1864,15 @@ static void __init setup_ioapic_ids_from
static void __init setup_ioapic_ids_from_mpc(void) { }

+int timer_irq_really_works __initdata;
+int __init irqtest_disable(char *str)
+ timer_irq_really_works = 1;
+ return 1;
+__setup("noirqtest", irqtest_disable);
* There is a nasty bug in some older SMP boards, their mptable lies
* about the timer IRQ. We do the following to work around the situation:
@@ -1872,9 +1881,12 @@ static void __init setup_ioapic_ids_from
* - if this function detects that timer IRQs are defunct, then we fall
* back to ISA timer IRQs
-static int __init timer_irq_works(void)
+int __init timer_irq_works(void)
unsigned long t1 = jiffies;
+ if (timer_irq_really_works)
+ return 1;

/* Let ten ticks pass... */
@@ -2146,7 +2158,7 @@ int timer_uses_ioapic_pin_0;
* is so screwy. Thanks to Brian Perkins for testing/hacking this beast
* fanatically on his truly buggy board.
-static inline void check_timer(void)
+static inline void __init check_timer(void)
int apic1, pin1, apic2, pin2;
int vector;
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