linux-kernel - Re: [PATCH V9 1/3] irq: Allow to pass the IRQF

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Message-ID: <20170424185909.GD2137@mai>
Date:   Mon, 24 Apr 2017 20:59:09 +0200
From:   Daniel Lezcano <daniel.lezcano@...aro.org>
To:     Marc Zyngier <marc.zyngier@....com>
Cc:     tglx@...utronix.de, Mark Rutland <mark.rutland@....com>,
        Vineet Gupta <vgupta@...opsys.com>,
        Patrice Chotard <patrice.chotard@...com>,
        Kukjin Kim <kgene@...nel.org>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        Javier Martinez Canillas <javier@....samsung.com>,
        Christoffer Dall <christoffer.dall@...aro.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-snps-arc@...ts.infradead.org, kernel@...inux.com,
        linux-samsung-soc@...r.kernel.org, kvmarm@...ts.cs.columbia.edu,
        kvm@...r.kernel.org
Subject: Re: [PATCH V9 1/3] irq: Allow to pass the IRQF_TIMER flag with
 percpu irq request

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?

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