[xen stable-4.18] x86/rtc: Avoid UIP flag being set for longer than expected

[patchbot@xxxxxxx]

commit af0e9ba44a58c87d6d135d8ffbf468b4ceac0a41
Author:     Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>
AuthorDate: Mon Apr 29 09:36:04 2024 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Mon Apr 29 09:36:04 2024 +0200

    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 <ross.lagerwall@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
    master commit: 43a07069863b419433dee12c9b58c1f7ce70aa97
    master date: 2024-04-23 14:09:18 +0200
---
 xen/arch/x86/hvm/rtc.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/xen/arch/x86/hvm/rtc.c b/xen/arch/x86/hvm/rtc.c
index 206b4296e9..4839374352 100644
--- a/xen/arch/x86/hvm/rtc.c
+++ b/xen/arch/x86/hvm/rtc.c
@@ -202,6 +202,7 @@ static void check_update_timer(RTCState *s)
         }
         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;
--
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