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

[Daniel Lezcano]

On Mon, Apr 24, 2017 at 08:14:54PM +0100, Marc Zyngier wrote:
> On 24/04/17 19:59, Daniel Lezcano wrote:
> > On Mon, Apr 24, 2017 at 07:46:43PM +0100, Marc Zyngier wrote:
> >> On 24/04/17 15:01, Daniel Lezcano wrote:
> >>> In the next changes, we track when the interrupts occur in order to
> >>> statistically compute when is supposed to happen the next interrupt.
> >>>
> >>> In all the interruptions, it does not make sense to store the timer interrupt
> >>> occurences and try to predict the next interrupt as when know the expiration
> >>> time.
> >>>
> >>> The request_irq() has a irq flags parameter and the timer drivers use it to
> >>> pass the IRQF_TIMER flag, letting us know the interrupt is coming from a timer.
> >>> Based on this flag, we can discard these interrupts when tracking them.
> >>>
> >>> But, the API request_percpu_irq does not allow to pass a flag, hence specifying
> >>> if the interrupt type is a timer.
> >>>
> >>> Add a function request_percpu_irq_flags() where we can specify the flags. The
> >>> request_percpu_irq() function is changed to be a wrapper to
> >>> request_percpu_irq_flags() passing a zero flag parameter.
> >>>
> >>> Change the timers using request_percpu_irq() to use request_percpu_irq_flags()
> >>> instead with the IRQF_TIMER flag set.
> >>>
> >>> For now, in order to prevent a misusage of this parameter, only the IRQF_TIMER
> >>> flag (or zero) is a valid parameter to be passed to the
> >>> request_percpu_irq_flags() function.
> >>
> >> [...]
> >>
> >>> diff --git a/virt/kvm/arm/arch_timer.c b/virt/kvm/arm/arch_timer.c
> >>> index 35d7100..602e0a8 100644
> >>> --- a/virt/kvm/arm/arch_timer.c
> >>> +++ b/virt/kvm/arm/arch_timer.c
> >>> @@ -523,8 +523,9 @@ int kvm_timer_hyp_init(void)
> >>>  		host_vtimer_irq_flags = IRQF_TRIGGER_LOW;
> >>>  	}
> >>>  
> >>> -	err = request_percpu_irq(host_vtimer_irq, kvm_arch_timer_handler,
> >>> -				 "kvm guest timer", kvm_get_running_vcpus());
> >>> +	err = request_percpu_irq_flags(host_vtimer_irq, kvm_arch_timer_handler,
> >>> +				       IRQF_TIMER, "kvm guest timer",
> >>> +				       kvm_get_running_vcpus());
> >>>  	if (err) {
> >>>  		kvm_err("kvm_arch_timer: can't request interrupt %d (%d)\n",
> >>>  			host_vtimer_irq, err);
> >>>
> >>
> >> How is that useful? This timer is controlled by the guest OS, and not
> >> the host kernel. Can you explain how you intend to make use of that
> >> information in this case?
> > 
> > Isn't it a source of interruption on the host kernel?
> 
> Only to cause an exit of the VM, and not under the control of the host.
> This isn't triggering any timer related action on the host code either.
> 
> Your patch series seems to assume some kind of predictability of the
> timer interrupt, which can make sense on the host. Here, this interrupt
> is shared among *all* guests running on this system.
> 
> Maybe you could explain why you think this interrupt is relevant to what
> you're trying to achieve?

If this interrupt does not happen on the host, we don't care.

The flag IRQF_TIMER is used by the spurious irq handler in the try_one_irq()
function. However the per cpu timer interrupt will be discarded in the function
before because it is per cpu.

IMO, for consistency reason, adding the IRQF_TIMER makes sense. Other than
that, as the interrupt is not happening on the host, this flag won't be used.

Do you want to drop this change?

  -- Daniel



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