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

[Daniel Lezcano <daniel.lezcano@linaro.org>]

On Tue, Apr 25, 2017 at 11:21:21AM +0100, Marc Zyngier wrote:
> 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 for clarification.

So there is a virtual timer with real registers / interruption (waking up the
host) for the running VMs and SW timers for non-running VMs.

What is the benefit of having such mechanism instead of real timers injecting
interrupts in the VM without the virtual timer + save/restore? Efficiency in
the running VMs when setting up timers (saving privileges change overhead)?

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