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QEMU Clock record and replay

From: Arnabjyoti Kalita
Subject: QEMU Clock record and replay
Date: Thu, 25 Feb 2021 20:39:20 +0530

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

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 -

- 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,

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