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

From: Dmitry Osipenko
Subject: Re: [Qemu-devel] [PULL 17/28] hw/ptimer: Perform counter wrap around if timer already expired
Date: Fri, 24 Jun 2016 23:10:47 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.1.1

On 24.06.2016 21:37, Mark Cave-Ayland wrote:
> On 24/06/16 19:19, Dmitry Osipenko wrote:
>> On 24.06.2016 19:02, Peter Maydell wrote:
>>> On 24 June 2016 at 16:58, Mark Cave-Ayland
>>> <address@hidden> wrote:
>>>> On 06/06/16 15:47, Peter Maydell wrote:
>>>>> From: Dmitry Osipenko <address@hidden>
>>>>> 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.
> Hi Dmitry,
> Thanks for looking at this. The reproducer with NetBSD is fairly easy:
> ./qemu-system-sparc -cdrom NetBSD-6.1.5-sparc.iso -boot d
> Hit 1 followed by Enter for the first couple of questions, and by the time you
> get asked for the terminal settings you should find that the message has been
> emitted somewhere on the console.
> ATB,
> Mark.
> (repost due to mail client crash - apologies if this is a repeat)

Hi Mark,

Thanks for the guide. I successfully reproduced the issue (it's exactly what I
supposed in my previous reply) and will send fix for it shortly. Sorry for


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