Re: Controlling time in QEMU

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

On 6/6/25 12:03 PM, Pierrick Bouvier wrote:
> Hi,
> 
> I recently needed to slow down time within a virtual machine, due to a
> timeout being hit because my QEMU binary which was not fast enough (gcov
> debug build if you're curious about the use case).
> 
> Currently, people tend to use -icount shift=X with low values for that,
> as it roughly maps time to how many instructions were ran.
> As well, I've been working on ips (Instructions per second) plugin [1],
> which allows to slow down QEMU execution by sleeping, as suggested by
> Alex. It's using a new clock update API [2], that is limited to move
> time faster (jump to future time), but not slower.
> 
> After looking at how all this implemented, I am wondering why we don't
> simply fake real time QEMU is using as a reference, which is based on
> gettimeofday [3].
> 
> Using faketime, which fakes gettimeofday I've been able to manipulate
> QEMU time:
> - faketime -f '+0 x10' qemu-system-aarch64: boot timestamp is 14s
> - faketime -f '+0 x0.1' qemu-system-aarch64: boot timestamp is 0.13s
> In both cases, the real boot time (QEMU execution) is around 3s.
> 
> It works as expected, speeding or slowing execution without impacting
> QEMU speed itself, which can run without needing to do any artificial
> sleep or clock manipulation. If a program in guest vm sleeps with an
> absolute time, it will be multiplied by the factor, which results in
> significant slowdown. But it can't be avoided, by design.
> By using a time factor big enough (x200), it can even trigger
> rcu_preempt stall detection in Linux guest kernel.
> 
> As it seems a bit too good to be true, time for questions:
> - Has it already been considered?
> - Any obvious downside I might have skipped?
> - Would that be interesting to offer such a feature in QEMU itself
>     (-time-dilation 0.10)?
> 
> [1]
> https://gitlab.com/qemu-project/qemu/-/commit/72db6d54a4a9203736261fc63f32f6cd3486b7e4
> [2]
> https://gitlab.com/qemu-project/qemu/-/commit/847a65dd76bf1868c7966a2b2608dcd00cb68dc7
> [3]
> https://gitlab.com/qemu-project/qemu/-/blob/master/include/qemu/timer.h?ref_type=heads#L809
> [4] https://github.com/wolfcw/libfaketime
> 
> Regards,
> Pierrick

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