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

From: Mark Cave-Ayland
Subject: Re: [Qemu-devel] [PULL 17/28] hw/ptimer: Perform counter wrap around if timer already expired
Date: Fri, 24 Jun 2016 19:37:35 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Icedove/45.1.0

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

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 ?

-- 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

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.



(repost due to mail client crash - apologies if this is a repeat)

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