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Re: [PATCH 06/10] target/arm: Detect overflow when calculating next PMU

From: Richard Henderson
Subject: Re: [PATCH 06/10] target/arm: Detect overflow when calculating next PMU interrupt
Date: Thu, 11 Aug 2022 10:51:25 -0700
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.11.0

On 8/11/22 10:16, Peter Maydell wrote:
In pmccntr_op_finish() and pmevcntr_op_finish() we calculate the next
point at which we will get an overflow and need to fire the PMU
interrupt or set the overflow flag.  We do this by calculating the
number of nanoseconds to the overflow event and then adding it to
qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL).  However, we don't check
whether that signed addition overflows, which can happen if the next
PMU interrupt would happen massively far in the future (250 years or

Since QEMU assumes that "when the QEMU_CLOCK_VIRTUAL rolls over" is
"never", the sensible behaviour in this situation is simply to not
try to set the timer if it would be beyond that point.  Detect the
overflow, and skip setting the timer in that case.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
  target/arm/helper.c | 22 ++++++++++++++--------
  1 file changed, 14 insertions(+), 8 deletions(-)

diff --git a/target/arm/helper.c b/target/arm/helper.c
index 434885d024a..b7a420981f8 100644
--- a/target/arm/helper.c
+++ b/target/arm/helper.c
@@ -1227,10 +1227,13 @@ static void pmccntr_op_finish(CPUARMState *env)
          int64_t overflow_in = cycles_ns_per(remaining_cycles);
if (overflow_in > 0) {
-            int64_t overflow_at = qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL) +
-                overflow_in;
-            ARMCPU *cpu = env_archcpu(env);
-            timer_mod_anticipate_ns(cpu->pmu_timer, overflow_at);
+            int64_t overflow_at;
+            if (!sadd64_overflow(qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL),
+                                overflow_in, &overflow_at)) {

Maybe my mailer, but it looks like alignment is off on the continuation line 

+            int64_t overflow_at;
+            if (!sadd64_overflow(qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL),
+                                 overflow_in, &overflow_at)) {

but not here.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>


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