Re: [Qemu-devel] [PULL 17/28] hw/ptimer: Perform counter wrap around if timer already expired

[Dmitry Osipenko <digetx@gmail.com>]

On 24.06.2016 19:02, Peter Maydell wrote:
> On 24 June 2016 at 16:58, Mark Cave-Ayland
> <mark.cave-ayland@ilande.co.uk> wrote:
>> On 06/06/16 15:47, Peter Maydell wrote:
>>
>>> From: Dmitry Osipenko <digetx@gmail.com>
>>>
>>> ptimer_get_count() might be called while QEMU timer already been expired.
>>> In that case ptimer would return counter = 0, which might be undesirable
>>> in case of polled timer. Do counter wrap around for periodic timer to keep
>>> it distributed. In order to achieve more accurate emulation behaviour of
>>> certain hardware, don't perform wrap around when in icount mode and return
>>> counter = 0 in that case (that doesn't affect polled counter
>>> distribution).
>>>
> 
>> Whilst testing Artyom's qemu-system-sparc patch today, I noticed another
>> regression which I've bisected down to the above commit.
>>
>> Booting my NetBSD/OpenBSD test images with current git master causes the
>> following warning to appear on the console: "WARNING: negative runtime;
>> monotonic clock has gone backwards".
>>
>> Could this be a regression or does qemu-system-sparc make an incorrect
>> assumption as to how the timer should work in this scenario?
> 
> I'm not sure -- Dmitry ?
> 
> thanks
> -- PMM
> 

The problem could be due to the IRQ being raised after the poll of the wrapped
around counter in non-icount mode, so CPU "sees the future". In that case patch
should be reverted and reworked. During the review of the patch we decided that
it shouldn't be an issue. I'll take a closer look at it and try to reproduce the
issue.

-- 
Dmitry