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Re: [Qemu-devel] implementing architectural timers using QEMU timers

From: Max Filippov
Subject: Re: [Qemu-devel] implementing architectural timers using QEMU timers
Date: Tue, 10 Jan 2017 10:39:08 -0800

On Tue, Jan 10, 2017 at 12:31 AM, Pavel Dovgalyuk <address@hidden> wrote:
>> From: Max Filippov [mailto:address@hidden
>> 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?

> 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.

-- Max

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