Re: [PATCH v2 0/2] control guest time using a dilation factor

[Pierrick Bouvier <pierrick.bouvier@linaro.org>]

On 6/27/25 5:24 PM, Pierrick Bouvier wrote:
> Depending on host cpu speed, and QEMU optimization level, it may sometimes be
> needed to slow or accelerate time guest is perceiving. A common scenario is
> hitting a timeout during a boot process, because some operations were not
> finished on time.
> 
> An existing solution for that is -icount shift=X, with low values, which will
> roughly map virtual time to how many instructions were executed.
> 
> This series introduces another approach, based on faking host time returned to
> the guest, by applying a time-dilation factor. Time will go slower/faster for
> the guest, without impacting QEMU emulation speed.
> 
> It may eventually be used to fix some of the timeouts we hit in CI, by slowing
> down time in VM, to be less sensitive to varying cpu performance.
> 
> v2
> --
> 
> In review, Paolo mentioned timers deadline should redilated in the other
> direction. After going through this part, it seems that arrival is always based
> on one of the clocks we have (which is dilated already), so I don't think we
> should redilate that, as this would create a discordance between time set, and
> real time when this happen. Feel free to correct me if this is wrong.
> 
> - keep start time per clock and apply accordingly
> - apply time dilation for cpu_get_host_ticks as well
> - use a default factor of 1.0
> - rename cli option to -rtc speed-factor
> - forbid to use option with kvm, as time is not provided by QEMU for guest
> 
> Pierrick Bouvier (2):
>    qemu/timer: introduce time dilation factor
>    system/rtc: introduce -rtc speed-factor option
> 
>   include/qemu/timer.h     | 60 ++++++++++++++++++++++++++++------------
>   system/rtc.c             | 11 ++++++++
>   system/vl.c              |  9 ++++++
>   util/qemu-timer-common.c |  7 +++++
>   qemu-options.hx          |  7 ++++-
>   5 files changed, 75 insertions(+), 19 deletions(-)
> 

Trying another ping on this series.
It has been useful to reduce number of interruptions in a guest when 
running on a slow machine.

Regards,
Pierrick