From: Dave Voutila <dv@sisu.io>
Subject: fix deadlock if pausing a vm with a busy-polling vcpu
To: tech@openbsd.org
Date: Thu, 12 Feb 2026 10:02:17 -0500

If a vcpu isn't halted because it's busy polling (like at the
bootloader prompt), issuing a pause via `vmctl pause <vm>` will
effectively deadlock. This removes the condition that the vcpu needs
to be in a halted state and aggressively pauses at the next vm exit.

If you want to see the current deadlock, boot an installer image and
at the boot prompt just type a single character to stop the auto-boot
from occurring. Now try to `vmctl pause`.

ok?


diff refs/heads/master refs/heads/vmd-pause-deadlock
commit - 012cf974ca6d25b889551a1e73afca0e5a6d0994
commit + 35699612346148cad906319a3a64d4862bcd0f15
blob - 6d571ce90fc27d700755eae58c2127ce874efd3f
blob + 4b43735e16d2910a4ca843f425e32fa643016df4
--- usr.sbin/vmd/vm.c
+++ usr.sbin/vmd/vm.c
@@ -826,2 +826,2 @@ vcpu_run_loop(void *arg)
 		halted = vcpu_hlt[n];
 		mutex_unlock(&vm_mtx);

-		/* If we are halted and need to pause, pause */
-		if (halted && paused) {
+		/* If we need to pause, wait on the barrier. */
+		if (paused) {
 			ret = pthread_barrier_wait(&vm_pause_barrier);
 			if (ret != 0 && ret != PTHREAD_BARRIER_SERIAL_THREAD) {
 				log_warnx("%s: could not wait on pause barrier (%d)",