Re: [Xen-devel] [PATCH ARM v8 1/4] mini-os: arm: time

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Sun, 2014-10-26 at 09:51 +0000, Thomas Leonard wrote:
> On 21 October 2014 11:50, Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:
> > On Fri, 2014-10-03 at 10:20 +0100, Thomas Leonard wrote:
> >> Based on an initial patch by Karim Raslan.
> >>
> >> Signed-off-by: Karim Allah Ahmed <karim.allah.ahmed@xxxxxxxxx>
> >> Signed-off-by: Thomas Leonard <talex5@xxxxxxxxx>
> >
> > Acked-by: Ian Campbell <ian.campbell@xxxxxxxxxx>
> >
> >> +/* Wall-clock time is not currently available on ARM, so this is always 
> >> zero for now:
> >> + * 
> >> http://wiki.xenproject.org/wiki/Xen_ARM_TODO#Expose_Wallclock_time_to_guests
> >
> > I have some slightly hacky patches for this, I really should dust them
> > off and submit them...
> >
> >> +void block_domain(s_time_t until)
> >> +{
> >> +    uint64_t until_count = ns_to_ticks(until) + cntvct_at_init;
> >> +    ASSERT(irqs_disabled());
> >> +    if (read_virtual_count() < until_count)
> >> +    {
> >> +        set_vtimer_compare(until_count);
> >> +        __asm__ __volatile__("wfi");
> >> +        unset_vtimer_compare();
> >> +
> >> +        /* Give the IRQ handler a chance to handle whatever woke us up. */
> >> +        local_irq_enable();
> >> +        local_irq_disable();
> >> +    }
> >
> > Just wondering, is this not roughly equivalent to a wfi loop with
> > interrupts enabled?
> 
> I'm not quite sure what you mean.
> 
> If we enable interrupts before the wfi then I think the following could occur:
> 
> 1. Application checks for work, finds none and calls block_domain.
> 2. block_domain enables interrupts.
> 3. An interrupt occurs.
> 4. The interrupt handler sets a flag indicating work to do.
> 5. wfi is called, putting the domain to sleep, even though there is work to 
> do.
> 
> Enabling IRQs after block_domain ensures we can't sleep while we have
> work to do.

Ah, yes.

Ian.



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