Re: QEMU Clock record and replay

Re: QEMU Clock record and replay

[Philippe Mathieu-Daudé]

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
> 
> 

Re: QEMU Clock record and replay

[Pavel Dovgalyuk]

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

Re: QEMU Clock record and replay

[Alex Bennée]

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

Re: QEMU Clock record and replay

[Arnabjyoti Kalita]

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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).
```

Thanks.

Best Regards,
Arnab

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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Re: QEMU Clock record and replay

[Arnabjyoti Kalita]

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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.dovgalyuk@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
>

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