Re: [PATCH 2/6] kvm-s390: use hrtimer for clock wakeup from idle

[Avi Kivity <avi@redhat.com>]

Christian Borntraeger wrote:
>> Why can't this be done from the timer context (after adjusting the locks)?
>>     
>
> It can be done, in fact I did that in my first version. The thing is, we would need to change all local_int.locks to spinlock_irqs, since standard timers are softirqs and hrtimers and hardirqs. Enabling and disabling irqs is a relatively expensive operating on s390 (but locks via compare and swap are quite cheap). Since we take this specific lock in lots of places this lock is on some hot pathes. The idle wakeup on the other hand is not that critical.
>   

Makes sense.

Back when irqsave/irqrestore were expensive on x86 (P4 days, I think it 
was ~100 cycles) there was talk of using a software irq disable flag 
instead of using the hardware support.  So

- local_irq_disable() sets a flag

- interrupt handlers check the flag, if set they schedule the interrupt 
handler and return immediately

- local_irq_restore() clears the flag and fires and scheduled handlers

Now these operations are pretty cheap on x86, but maybe that can apply 
to s390.

-- 
error compiling committee.c: too many arguments to function