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

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

On Fri, Nov 14, 2014 at 10:29:26AM +0000, Ian Campbell wrote:
> On Thu, 2014-11-13 at 16:29 +0000, Thomas Leonard wrote:
> > On 27 October 2014 10:34, Ian Campbell <Ian.Campbell@citrix.com> wrote:
> > > On Sun, 2014-10-26 at 09:51 +0000, Thomas Leonard wrote:
> > >> On 21 October 2014 11:50, Ian Campbell <Ian.Campbell@citrix.com> 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@gmail.com>
> > >> >> Signed-off-by: Thomas Leonard <talex5@gmail.com>
> > >> >
> > >> > Acked-by: Ian Campbell <ian.campbell@citrix.com>
> > >> >
> > >> >> +/* 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.
> > 
> > So, can this patch be applied as-is now?
> 
> We are now post-rc2 in the 4.5.0 release process, so the answer would be
> "needs a release exception, but it's a feature so probably not" (and it
> would have been a bit dubious towards the end of October too, which was
> post rc1, and feature freeze was the end of September in any case).
> 
> However this is part of a new mini-os port which isn't even hooked into
> the main build system yet (AFAICT), so in that sense it is utterly
> harmless to apply. On the other hand there is a bunch more patches to
> come which are needed to make the mini-os port actually useful, and I'm
> not sure those are all utterly harmless e.g. to common or x86 code (as
> in I've not gone looked at the diffstat for the remaining patches), so
> in that sense there's no harm waiting for 4.6 development to open.
> 
> I defer to the release manager (Konrad, CCd) on this...

I would prefer to defer this to Xen 4.6 to keep the amount of patches
going in staging to be bug-fixes.

Thank you.
> 
>