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Fix SMP=4 boot hang regression #111

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Merged
merged 1 commit into from
Nov 2, 2025
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Fix SMP=4 boot hang regression #111

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28 changes: 13 additions & 15 deletions main.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1191,22 +1191,20 @@ static int semu_run(emu_state_t *emu)
size_t pfd_count = 0;
int timer_index = -1;

/* Add periodic timer fd (1ms interval for guest timer emulation).
* Only add timer when ALL harts are active (none idle) to allow
* poll() to sleep when any harts are in WFI. When harts are idle,
* timer updates can be deferred until they wake up.
*
* During SMP boot (started_harts < vm->n_hart), always include the
* timer to ensure secondary harts can complete initialization. Only
* apply conditional exclusion after all harts have started.
*
* For single-hart configurations (n_hart == 1), disable
* optimization entirely to avoid boot issues, as the first hart
* starts immediately.
/* Add periodic timer fd (1ms interval). Excluded when harts are
* idle to allow poll() sleep, but always included during:
* 1. Single-hart mode (n_hart == 1)
* 2. Boot phase (!boot_complete) - prevents deadlock when kernel
* briefly puts all harts in WFI while waiting for timer IRQ
* 3. Active execution (idle_harts == 0)
*/
bool all_harts_started = (started_harts >= vm->n_hart);
const uint64_t BOOT_SETTLE_ITERATIONS = 5000;
bool boot_complete =
all_harts_started &&
(emu->peripheral_update_ctr > BOOT_SETTLE_ITERATIONS);
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@cubic-dev-ai cubic-dev-ai bot Nov 2, 2025 •
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peripheral_update_ctr never exceeds 64 because it is a countdown reset to 64 each tick, so this > BOOT_SETTLE_ITERATIONS check never succeeds and boot_complete stays false. That keeps the timer and UART fds permanently active, undoing the post-boot idle optimization this block is supposed to preserve.

Prompt for AI agents 
Address the following comment on main.c at line 1217:

<comment>`peripheral_update_ctr` never exceeds 64 because it is a countdown reset to 64 each tick, so this `&gt; BOOT_SETTLE_ITERATIONS` check never succeeds and `boot_complete` stays false. That keeps the timer and UART fds permanently active, undoing the post-boot idle optimization this block is supposed to preserve.</comment>

<file context>
@@ -1203,10 +1203,20 @@ static int semu_run(emu_state_t *emu)
+            const uint64_t BOOT_SETTLE_ITERATIONS = 5000;
+            bool boot_complete =
+                all_harts_started &amp;&amp;
+                (emu-&gt;peripheral_update_ctr &gt; BOOT_SETTLE_ITERATIONS);
+            bool harts_active = (vm-&gt;n_hart == 1) || !boot_complete ||
+                                (idle_harts == 0);
</file context>
Fix with Cubic

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@jserv jserv Nov 2, 2025

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peripheral_update_ctr is a countdown timer (64→0→64) and will never exceed 5000, meaning boot_complete stays false permanently. This breaks the intended idle optimization.

bool harts_active =
(vm->n_hart == 1) || !all_harts_started || (idle_harts == 0);
(vm->n_hart == 1) || !boot_complete || (idle_harts == 0);
#ifdef __APPLE__
/* macOS: use kqueue with EVFILT_TIMER */
if (kq >= 0 && pfd_count < poll_capacity && harts_active) {
Expand All @@ -1227,7 +1225,7 @@ static int semu_run(emu_state_t *emu)
/* Add UART input fd (stdin for keyboard input).
* Only add UART when:
* 1. Single-hart configuration (n_hart == 1), OR
* 2. Not all harts started (!all_harts_started), OR
* 2. Boot not complete (!boot_complete), OR
* 3. All harts are active (idle_harts == 0), OR
* 4. A hart is actively waiting for UART input
*
Expand All @@ -1236,7 +1234,7 @@ static int semu_run(emu_state_t *emu)
* input (Ctrl+A x) may be delayed by up to poll_timeout (10ms)
* when harts are idle, which is acceptable for an emulator.
*/
bool need_uart = (vm->n_hart == 1) || !all_harts_started ||
bool need_uart = (vm->n_hart == 1) || !boot_complete ||
(idle_harts == 0) || emu->uart.has_waiting_hart;
if (emu->uart.in_fd >= 0 && pfd_count < poll_capacity &&
need_uart) {
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