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Re: [Xen-devel] ARM Generic Timer interrupt

On Wed, 2014-05-28 at 12:32 +0100, Julien Grall wrote:
> On 05/28/2014 11:10 AM, Ian Campbell wrote:
> > On Tue, 2014-05-27 at 13:11 +0100, Stefano Stabellini wrote:
> >>> But, I have question:
> >>> Should the Hypervisor masks virtual timer IRQ on his own?
> >>> It is a guest's resource and the guest itself should decide what to do.
> >>> For example, I see that Linux Kernel (3.8) sets and clears timer 
> >>> interrupt mask by itself.
> >>
> >> In principle I agree with you that the vtimer is a guest resource.
> >> However in practice if we don't mask the irq we can easily get into an
> >> interrupt storm situation: if the guest doesn't handle the interrupt
> >> immediately we could keep receiving the vtimer irq in the hypervisor and
> >> busy loop around it.
> > 
> > Do we not do a priority drop on the interrupt when we receive it, so we
> > won't get any more interrupts from the timer until it acks the
> > interrupt?
> The timer interrupt is acked directly by Xen. We can't wait the guest
> VCPU as EOI the interrupt because the guest may have move to another
> pCPU by this time.

Surely we can arrange to handle that though. The way we currently handle
the timer stuff always seemed suboptimal to me.

> If so, you will lose the interrupt timer on this pCPU forever.
> Regards,

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