This should solve the problem.
Index: sys/arch/mips64/mips64/vm_machdep.c
===================================================================
RCS file: /OpenBSD/src/sys/arch/mips64/mips64/vm_machdep.c,v
retrieving revision 1.37
diff -u -p -r1.37 vm_machdep.c
--- sys/arch/mips64/mips64/vm_machdep.c 2 Sep 2017 15:56:29 -0000 1.37
+++ sys/arch/mips64/mips64/vm_machdep.c 20 Nov 2019 11:03:22 -0000
@@ -135,7 +135,7 @@ cpu_fork(struct proc *p1, struct proc *p
}
pcb->pcb_context.val[10] = (register_t)proc_trampoline;
pcb->pcb_context.val[8] = (register_t)pcb +
- USPACE - sizeof(struct trapframe);
+ ((USPACE - sizeof(struct trapframe)) & ~_STACKALIGNBYTES);
pcb->pcb_context.val[1] = (register_t)arg;
pcb->pcb_context.val[0] = (register_t)func;
}