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

[Thomas Leonard <talex5@xxxxxxxxx>]

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.


-- 
Dr Thomas Leonard        http://0install.net/
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