Thank you Pavel. You answered all of my questions. The clock record-replay functionality is much clearer now.
Best Regards, Arnab On Fri, Feb 26, 2021 at 11:54 AM Pavel Dovgalyuk <pavel.dovgal...@ispras.ru> wrote: > 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. > > >> - 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? > > Only if you want to replay just the clock. > But replaying whole execution includes instruction counting and thread > synchronization. > > >> - 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? > > This clock is used for icount-related timers. For now icount is required > for record replay. If you want to replay only host clock, you don't need > this one. > > Pavel Dovgalyuk >
