Re: [PATCH] util/qemu-timer.c: Don't warp timer from timerlist_rearm()

[Peter Maydell <peter.maydell@linaro.org>]

On Mon, 10 Feb 2025 at 13:58, Peter Maydell <peter.maydell@linaro.org> wrote:
>
> Currently we call icount_start_warp_timer() from timerlist_rearm().
> This produces incorrect behaviour, because timerlist_rearm() is
> called, for instance, when a timer callback modifies its timer.  We
> cannot decide here to warp the timer forwards to the next timer
> deadline merely because all_cpu_threads_idle() is true, because the
> timer callback we were called from (or some other callback later in
> the list of callbacks being invoked) may be about to raise a CPU
> interrupt and move a CPU from idle to ready.5A

(oops, stray editor damage "5A" at end of line)

>
> The only valid place to choose to warp the timer forward is from the
> main loop, when we know we have no outstanding IO or timer callbacks
> that might be about to wake up a CPU.

This raises actually another question: should the call to
icount_start_warp_timer() in main_loop_wait() maybe go after
qemu_clock_run_all_timers() rather than before? (Haven't tested
whether that breaks anything ;-))

-- PMM