On 2/10/25 05:58, Peter Maydell 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.5AThe 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. For Arm guests, this bug was mostly latent until the refactoring commit f6fc36deef6abc ("target/arm/helper: Implement CNTHCTL_EL2.CNT[VP]MASK"), which exposed it because it refactored a timer callback so that it happened to call timer_mod() first and raise the interrupt second, when it had previously raised the interrupt first and called timer_mod() afterwards. This call seems to have originally derived from the pre-record-and-replay icount code, which (as of e.g. commit db1a49726c3c in 2010) in this location did a call to qemu_notify_event(), necessary to get the icount code in the vCPU round-robin thread to stop and recalculate the icount deadline when a timer was reprogrammed from the IO thread. In current QEMU, everything is done on the vCPU thread when we are in icount mode, so there's no need to try to notify another thread here. I suspect that the other reason why this call was doing icount timer warping is that it pre-dates commit efab87cf79077a from 2015, which added a call to icount_start_warp_timer() to main_loop_wait(). Once the call in timerlist_rearm() has been removed, if the timer callbacks don't cause any CPU to be woken up then we will end up calling icount_start_warp_timer() from main_loop_wait() when the rr main loop code calls rr_wait_io_event(). Remove the incorrect call from timerlist_rearm(). Cc: qemu-sta...@nongnu.org Signed-off-by: Peter Maydell <peter.mayd...@linaro.org> --- As far as I can tell, this is the right thing, and it fixes the "icount warps the timer when it should not" bug I'm trying to address, but I'm not super familiar with the icount subsystem or its evolution, so it's possible I've accidentally broken some other setup here. This does pass the tcg, functional and avocado tests, including the record-and-replay ones. I've cc'd it to stable as a bugfix, but it definitely merits careful review first. ---
I don't completely grok icount either, but this patch at least conforms to my expectations. The fact that it passes record/replay is a really good smoke test.
So fwiw, Reviewed-by: Richard Henderson <richard.hender...@linaro.org> r~
