target/arm: Handle overflow in calculation of next timer tick

In commit edac4d8a168 back in 2015 when we added support for
the virtual timer offset CNTVOFF_EL2, we didn't correctly update
the timer-recalculation code that figures out when the timer
interrupt is next going to change state. We got it wrong in
two ways:
 * for the 0->1 transition, we didn't notice that gt->cval + offset
   can overflow a uint64_t
 * for the 1->0 transition, we didn't notice that the transition
   might now happen before the count rolls over, if offset > count

In the former case, we end up trying to set the next interrupt
for a time in the past, which results in QEMU hanging as the
timer fires continuously.

In the latter case, we would fail to update the interrupt
status when we are supposed to.

Fix the calculations in both cases.

The test case is Alex Bennée's from the bug report, and tests
the 0->1 transition overflow case.

Fixes: edac4d8a168 ("target-arm: Add CNTVOFF_EL2")
Cc: qemu-stable@nongnu.org
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/60
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20231120173506.3729884-1-peter.maydell@linaro.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>

1 files changed, 21 insertions, 4 deletions

diff --git a/target/arm/helper.c b/target/arm/helper.c
index ff19709..2746d3f 100644
--- a/target/arm/helper.c
+++ b/target/arm/helper.c
@@ -2646,11 +2646,28 @@ static void gt_recalc_timer(ARMCPU *cpu, int timeridx)
         gt->ctl = deposit32(gt->ctl, 2, 1, istatus);
 
         if (istatus) {
-            /* Next transition is when count rolls back over to zero */
-            nexttick = UINT64_MAX;
+            /*
+             * Next transition is when (count - offset) rolls back over to 0.
+             * If offset > count then this is when count == offset;
+             * if offset <= count then this is when count == offset + 2^64
+             * For the latter case we set nexttick to an "as far in future
+             * as possible" value and let the code below handle it.
+             */
+            if (offset > count) {
+                nexttick = offset;
+            } else {
+                nexttick = UINT64_MAX;
+            }
         } else {
-            /* Next transition is when we hit cval */
-            nexttick = gt->cval + offset;
+            /*
+             * Next transition is when (count - offset) == cval, i.e.
+             * when count == (cval + offset).
+             * If that would overflow, then again we set up the next interrupt
+             * for "as far in the future as possible" for the code below.
+             */
+            if (uadd64_overflow(gt->cval, offset, &nexttick)) {
+                nexttick = UINT64_MAX;
+            }
         }
         /*
          * Note that the desired next expiry time might be beyond the