diff --git a/sys/amd64/amd64/vm_machdep.c b/sys/amd64/amd64/vm_machdep.c
--- a/sys/amd64/amd64/vm_machdep.c
+++ b/sys/amd64/amd64/vm_machdep.c
@@ -198,7 +198,6 @@
 	bcopy(&p1->p_md, mdp2, sizeof(*mdp2));
 
 	/*
-	 * Create a new fresh stack for the new process.
 	 * Copy the trap frame for the return to user mode as if from a
 	 * syscall.  This copies most of the user mode register values.
 	 */
@@ -606,7 +605,10 @@
 
 
 	/*
-	 * Create a new fresh stack for the new thread.
+	 * Copy user general-purpose registers.
+	 *
+	 * Some of these registers are rewritten by cpu_set_upcall()
+	 * and linux_set_upcall_kse().
 	 */
 	bcopy(td0->td_frame, td->td_frame, sizeof(struct trapframe));
 
diff --git a/sys/i386/i386/vm_machdep.c b/sys/i386/i386/vm_machdep.c
--- a/sys/i386/i386/vm_machdep.c
+++ b/sys/i386/i386/vm_machdep.c
@@ -205,7 +205,6 @@
 	bcopy(&p1->p_md, mdp2, sizeof(*mdp2));
 
 	/*
-	 * Create a new fresh stack for the new process.
 	 * Copy the trap frame for the return to user mode as if from a
 	 * syscall.  This copies most of the user mode register values.
 	 * The -VM86_STACK_SPACE (-16) is so we can expand the trapframe
@@ -473,7 +472,10 @@
 	}
 
 	/*
-	 * Create a new fresh stack for the new thread.
+	 * Copy user general-purpose registers.
+	 *
+	 * Some of these registers are rewritten by cpu_set_upcall()
+	 * and linux_set_upcall_kse().
 	 */
 	bcopy(td0->td_frame, td->td_frame, sizeof(struct trapframe));