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From: "Pavel Dovgalyuk" <dovgaluk@ispras.ru>
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Date: Thu, 12 Jan 2017 14:28:03 +0300
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Subject: Re: [Qemu-devel] implementing architectural timers using QEMU timers
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To: 'Max Filippov' <jcmvbkbc@gmail.com>
Cc: 'qemu-devel' <qemu-devel@nongnu.org>, 'Richard Henderson' <rth@twiddle.net>, 'Alex Bligh' <alex@alex.org.uk>, 'Pavel Dovgaluk' <Pavel.Dovgaluk@ispras.ru>

> From: Max Filippov [mailto:jcmvbkbc@gmail.com]
> On Tue, Jan 10, 2017 at 12:31 AM, Pavel Dovgalyuk <dovgaluk@ispras.ru> wrote:
> >> From: Max Filippov [mailto:jcmvbkbc@gmail.com]
> >
> >> I'm trying to reimplement xtensa CCOUNT (cycle counter) and
> >> CCOMPARE (CCOUNT-based timer interrupts) using QEMU
> >> timers. That is CCOUNT value is derived from the
> >> QEMU_CLOCK_VIRTUAL clock and CCOMPARE interrupts are
> >> generated from the QEMU_CLOCK_VIRTUAL timer callbacks.
> >> The code is here:
> >>   https://github.com/OSLL/qemu-xtensa/commits/xtensa-ccount
> >>
> >> I've got the following issues doing that:
> >>
> >> - I thought that could be improved in -icount mode, so I tried that.
> >>   It is better with -icount, but it's still not 100% accurate. That is
> >>   I was able to observe guest reading QEMU clock value that is
> >>   past QEMU timer deadline before that timer callback was
> >>   invoked.
> >
> > icount is meant to be 100% accurate.
> > tcg_get_icount_limit function calculates the deadline before the soonest
> > virtual timer and executes number of instructions that will fit this
> > timeout.
> 
> Ok, looks like what happens in my case is that instruction that
> sets CCOMPARE and thus changes remaining icount does not
> cause exit from the cpu_exec. So merely ending TB on
> QEMU_CLOCK_VIRTUAL timer update is not enough, I need to
> throw an exception of some kind? Or does the timer code need
> to take care of that?

Yes, it seems that you should end the block with an exception,
to allow icount loop recalculate the timeouts.

Pavel Dovgalyuk