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From: "Pavel Dovgalyuk" <dovgaluk@ispras.ru>
References: <CAMo8BfLRP8qUgJAkU3fSwE_H6EmqYmPKO_MhMn7B6WpwGWUudw@mail.gmail.com>
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Date: Tue, 10 Jan 2017 11:31:10 +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>, 'qemu-devel' <qemu-devel@nongnu.org>
Cc: 'Richard Henderson' <rth@twiddle.net>, 'Alex Bligh' <alex@alex.org.uk>, 'Pavel Dovgaluk' <Pavel.Dovgaluk@ispras.ru>

> 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.

Maybe you need the accuracy that is higher than the precision of the virtual clock?
Every executed instruction in icount mode takes fixed period of virtual time.

Pavel Dovgalyuk