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Re: QEMU Clock record and replay

From: Philippe Mathieu-Daudé
Subject: Re: QEMU Clock record and replay
Date: Thu, 25 Feb 2021 16:43:36 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.7.0

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?>?
> - 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?
> - Let's say we record the clock values when the guest starts in KVM
> mode, and we replay the clock values when the guest starts in TCG mode.
> Does the record-replay functionality change a lot?
> - Also, I couldn't understand the specific purpose of replaying the
> 'virtual real time clock' ? Do I still need to record and replay the
> 'virtual real time clock' if we record the clock values in KVM mode, and
> replay the clock values in TCG mode?
> Thank you very much.
> Best Regards,
> Arnab

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