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x86_64/hpet: guard against subtraction underflow #364

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Merged
dreamliner787-9 merged 1 commit into from
Jul 28, 2026
Merged

x86_64/hpet: guard against subtraction underflow #364

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31 changes: 18 additions & 13 deletions src/arch/x86_64/hpet.c
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Original file line number Diff line number Diff line change
Expand Up @@ -182,8 +182,15 @@ static bool timer_n_irq_edge_triggered(int n)

uint64_t timer_n_compute_timeout_delta_as_tsc(int n, uint32_t main_counter_val)
{
uint32_t delay_hpet_ticks = hpet_regs.comparators[n].current_comparator - main_counter_val;
return convert_ticks_by_frequency(delay_hpet_ticks, HPET_MAIN_COUNTER_HZ, guest_time_tsc_hz());
/* It is valid for main counter value to be greater than the current comparator because the VMM
* might not be scheduled for enough time. So we need to guard against subtraction underflow. */
int32_t delta = (int32_t)(hpet_regs.comparators[n].current_comparator - main_counter_val);
if (delta <= 0) {
/* Deadline in the past. hpet_maintenance will deal with the problem. */
return 0;
} else {
return convert_ticks_by_frequency(delta, HPET_MAIN_COUNTER_HZ, guest_time_tsc_hz());
}
}

void hpet_maintenance(uint8_t comparator);
Expand Down Expand Up @@ -229,19 +236,17 @@ void hpet_maintenance(uint8_t comparator)
uint32_t main_counter_val = main_counter_value();
uint64_t delay_tsc_ticks = timer_n_compute_timeout_delta_as_tsc(comparator, main_counter_val);

if (delay_tsc_ticks) {
if (hpet_regs.comparators[comparator].timeout_handle_valid) {
guest_time_cancel_timeout(hpet_regs.comparators[comparator].timeout_handle);
hpet_regs.comparators[comparator].timeout_handle_valid = false;
}
if (hpet_regs.comparators[comparator].timeout_handle_valid) {
guest_time_cancel_timeout(hpet_regs.comparators[comparator].timeout_handle);
hpet_regs.comparators[comparator].timeout_handle_valid = false;
}

hpet_regs.comparators[comparator].timeout_handle = guest_time_request_timeout(
delay_tsc_ticks, &hpet_handle_timer_ntfn, comparator);
assert(hpet_regs.comparators[comparator].timeout_handle != TIMEOUT_HANDLE_INVALID);
hpet_regs.comparators[comparator].timeout_handle_valid = true;
hpet_regs.comparators[comparator].timeout_handle = guest_time_request_timeout(delay_tsc_ticks,
&hpet_handle_timer_ntfn, comparator);
assert(hpet_regs.comparators[comparator].timeout_handle != TIMEOUT_HANDLE_INVALID);
hpet_regs.comparators[comparator].timeout_handle_valid = true;

hpet_regs.comparators[comparator].armed_comparator = hpet_regs.comparators[comparator].current_comparator;
}
hpet_regs.comparators[comparator].armed_comparator = hpet_regs.comparators[comparator].current_comparator;
}

static bool hpet_fault_on_config(uint64_t offset, uint8_t *comparator)
Expand Down
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