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Re: QEMU Clock record and replay

From: Arnabjyoti Kalita
Subject: Re: QEMU Clock record and replay
Date: Sat, 27 Feb 2021 22:01:07 +0530

The host clock refers to one of the clock types used by QEMU -

@QEMU_CLOCK_HOST: host clock
* The host clock should be used for device models that emulate accurate
* real time sources. It will continue to run when the virtual machine
* is suspended, and it will reflect system time changes the host may
* undergo (e.g. due to NTP).


Best Regards,

On Fri, Feb 26, 2021 at 3:25 PM Alex Bennée <alex.bennee@linaro.org> wrote:

Pavel Dovgalyuk <pavel.dovgalyuk@ispras.ru> writes:

> On 25.02.2021 18:43, Philippe Mathieu-Daudé wrote:
>> Cc'ing Pavel/Alex.
>> On 2/25/21 4:09 PM, Arnabjyoti Kalita wrote:
>>> Hello all,
>>> I am trying to understand how the clock values are recorded and replayed
>>> in QEMU (when it runs in TCG mode). I have been specifically following
>>> the document that has been provided here -
>>> https://github.com/qemu/qemu/blob/master/docs/replay.txt
>>> <https://github.com/qemu/qemu/blob/master/docs/replay.txt>
>>> I have the following 4 questions to ask-
>>> - Why are clock values being replayed off a cache and not directly off
>>> the file as seen here -
>>> https://github.com/qemu/qemu/blob/stable-5.0/replay/replay-time.c#L45?
>>> <https://github.com/qemu/qemu/blob/stable-5.0/replay/replay-time.c#L45?>?
> The cache was needed when replay does not follow the record behavior.
> E.g., included additional deadline calculations or timer checks.
> I'm not sure, that it is still needed, and I'll check it during future
> refactoring session.
>>> - I have a requirement to record and replay host clock values. Can it so
>>> happen that if incorrect values of the host clock are replayed, then the
>>> timers in the guest start expiring incorrectly, during replay?
> This is a bug, when the host clock is replayed incorrectly.

When you say host clock what do you mean? Surely all the cycle counters
are VIRTUAL? Or do you mean things like the RTC?

Alex Bennée

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