Re: [Xen-devel] [PATCH 1/2] xen/arm: support the ARM generic virtual timer

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Thu, 24 Jan 2013, Tim Deegan wrote:
> At 16:17 +0000 on 09 Jan (1357748277), Stefano Stabellini wrote:
> > +static void vtimer_interrupt(int irq, void *dev_id, struct cpu_user_regs 
> > *regs)
> > +{
> > +    current->arch.virt_timer.ctl = READ_CP32(CNTV_CTL);
> > +    WRITE_CP32(current->arch.virt_timer.ctl | CNTx_CTL_MASK, CNTV_CTL);
> 
> This is masking the vtimer interrupt in a way that's visible to the
> currently running guest.  Is that going to confuse the guest?
>
> When we talked about this before I had imagined that the masking would
> happen in the GIC, where the guest doesn't see it. 

I know it is not ideal but it is safe, it is not creating any problems to
Linux and it is even the same thing that KVM does).
More importantly I just couldn't get the GIC strategy to work correctly
(actually it work decently well on the physical Versatile Express but it
hangs the emulator, this tells me that it is not the way it was intended
to be used by the ARM people).


> > +    vgic_vcpu_inject_irq(current, irq, 1);
> > +}
> > +
> >  /* Set up the timer interrupt on this CPU */
> >  void __cpuinit init_timer_interrupt(void)
> >  {
> > @@ -172,6 +183,7 @@ void __cpuinit init_timer_interrupt(void)
> >  
> >      /* XXX Need to find this IRQ number from devicetree? */
> >      request_irq(26, timer_interrupt, 0, "hyptimer", NULL);
> > +    request_irq(27, vtimer_interrupt, 0, "virtimer", NULL);
> >      request_irq(30, timer_interrupt, 0, "phytimer", NULL);
> >  }
> >  
> > +static void virt_timer_expired(void *data)
> > +{
> > +    struct vtimer *t = data;
> > +    vcpu_wake(t->v);
> 
> Shouldn't this also inject the irq?  Otherwise when an unscheduled
> guest's timer goes off, we take two interrupts - one for the hyp timer
> to call this function and then another immediately after
> virt_timer_restore() which causes us to inject the vtimer irq. 
> If we injected the irq here (and arranged to mask the vtimer irq) we'd
> avoid a second interrupt.

This might be a good idea, I'll give it a try.

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