Re: [PATCH V9 1/3] irq: Allow to pass the IRQF_TIMER flag with percpu irq request

[Marc Zyngier]

On 25/04/17 10:49, Daniel Lezcano wrote:
> On Tue, Apr 25, 2017 at 10:10:12AM +0100, Marc Zyngier wrote:

[...]

>>> +static inline void setup_timings(struct irq_desc *desc, struct irqaction *act)
>>> +{
>>> +	/*
>>> +	 * We don't need the measurement because the idle code already
>>> +	 * knows the next expiry event.
>>> +	 */
>>> +	if (act->flags & __IRQF_TIMER)
>>> +		return;
>>
>> And that's where this is really wrong for the KVM guest timer. As I
>> said, this timer is under complete control of the guest, and the rest of
>> the system doesn't know about it. KVM itself will only find out when the
>> vcpu does a VM exit for a reason or another, and will just save/restore
>> the state in order to be able to give the timer to another guest.
>>
>> The idle code is very much *not* aware of anything concerning that guest
>> timer.
> 
> Just for my own curiosity, if there are two VM (VM1 and VM2). VM1 sets a timer1
> at <time> and exits, VM2 runs and sets a timer2 at <time+delta>.
> 
> The timer1 for VM1 is supposed to expire while VM2 is running. IIUC the virtual
> timer is under control of VM2 and will expire at <time+delta>.
> 
> Is the host wake up with the SW timer and switch in VM1 which in turn restores
> the timer and jump in the virtual timer irq handler?

Indeed. The SW timer causes VM1 to wake-up, either on the same CPU
(preempting VM2) or on another. The timer is then restored with the
pending virtual interrupt injected, and the guest does what it has to
with it.

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...