From mboxrd@z Thu Jan  1 00:00:00 1970
From: Klaas Zweck <khz@tzi.org>
Subject: question about local-apic timer
Date: Sat, 13 Apr 2002 17:42:40 +0200
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To: linux-smp <linux-smp@vger.kernel.org>

hi all,

i have a question about disabling the local-apic timer interrupt
on certain (but not all) cpus in linux MP-Systems to prevent the cpus
from entering kernel mode.
as i understood, the timer is a self generated irq by the local apic of 
a cpu
and is then received by this cpu itself.
i found it used in smp_local_timer_interrupt() function to
update process times.
i think it is initialized in arch/../apic.c in:hi all,

lvtt1_value = SET_APIC_TIMER_BASE(APIC_TIMER_BASE_DIV) |
                       APIC_LVT_TIMER_PERIODIC | LOCAL_TIMER_VECTOR;
apic_write_around(APIC_LVTT, lvtt1_value);


the APIC_LVT_TIMER_PERIODIC means ( as i understood intel mp spec )
that it is emitted at a certain frequency and is set to one as default 
value in linux.
the mp spec says that a zero value would mean to set the local-timer 
'one shot mode'
to be emitted only once (since reprogrammed).

so would it be sufficient for my problem ( to disable local timer irqs 
on some cpus)
to set this value to zero ??

and if so, is this possible at system runtime ??

since it is used to update process times this would produce problems in 
getting correct
values for this on cpus with local-timer disabled but if thats the only 
problem
coming up , this would be ok to me. (system is just for testing purposes.)

so thanks a lot for ideas and comments,
klaas