# Commit 43a07069863b419433dee12c9b58c1f7ce70aa97 # Date 2024-04-23 14:09:18 +0200 # Author Ross Lagerwall # Committer Jan Beulich x86/rtc: Avoid UIP flag being set for longer than expected In a test, OVMF reported an error initializing the RTC without indicating the precise nature of the error. The only plausible explanation I can find is as follows: As part of the initialization, OVMF reads register C and then reads register A repatedly until the UIP flag is not set. If this takes longer than 100 ms, OVMF fails and reports an error. This may happen with the following sequence of events: At guest time=0s, rtc_init() calls check_update_timer() which schedules update_timer for t=(1 - 244us). At t=1s, the update_timer function happens to have been called >= 244us late. In the timer callback, it sets the UIP flag and schedules update_timer2 for t=1s. Before update_timer2 runs, the guest reads register C which calls check_update_timer(). check_update_timer() stops the scheduled update_timer2 and since the guest time is now outside of the update cycle, it schedules update_timer for t=(2 - 244us). The UIP flag will therefore be set for a whole second from t=1 to t=2 while the guest repeatedly reads register A waiting for the UIP flag to clear. Fix it by clearing the UIP flag when scheduling update_timer. I was able to reproduce this issue with a synthetic test and this resolves the issue. Signed-off-by: Ross Lagerwall Reviewed-by: Jan Beulich --- a/xen/arch/x86/hvm/rtc.c +++ b/xen/arch/x86/hvm/rtc.c @@ -202,6 +202,7 @@ static void check_update_timer(RTCState } else { + s->hw.cmos_data[RTC_REG_A] &= ~RTC_UIP; next_update_time = (USEC_PER_SEC - guest_usec - 244) * NS_PER_USEC; expire_time = NOW() + next_update_time; s->next_update_time = expire_time;