Index: sys/amd64/amd64/vm_machdep.c
===================================================================
--- sys/amd64/amd64/vm_machdep.c
+++ sys/amd64/amd64/vm_machdep.c
@@ -245,9 +245,12 @@
 	td2->td_frame = (struct trapframe *)td2->td_md.md_stack_base - 1;
 	bcopy(td1->td_frame, td2->td_frame, sizeof(struct trapframe));
 
-	td2->td_frame->tf_rax = 0;		/* Child returns zero */
-	td2->td_frame->tf_rflags &= ~PSL_C;	/* success */
-	td2->td_frame->tf_rdx = 1;
+	if (__predict_true(p2->p_sysent->sv_cpu_fork == NULL)) {
+		td2->td_frame->tf_rax = 0;		/* Child returns zero */
+		td2->td_frame->tf_rflags &= ~PSL_C;	/* success */
+		td2->td_frame->tf_rdx = 1;		/* System V emulation */
+	} else
+		p2->p_sysent->sv_cpu_fork(td2);
 
 	/*
 	 * If the parent process has the trap bit set (i.e. a debugger
Index: sys/amd64/linux/linux_sysvec.c
===================================================================
--- sys/amd64/linux/linux_sysvec.c
+++ sys/amd64/linux/linux_sysvec.c
@@ -112,6 +112,7 @@
 
 static int	linux_copyout_strings(struct image_params *imgp,
 		    uintptr_t *stack_base);
+static void	linux_cpu_fork(struct thread *td);
 static int	linux_fixup_elf(uintptr_t *stack_base,
 		    struct image_params *iparams);
 static bool	linux_trans_osrel(const Elf_Note *note, int32_t *osrel);
@@ -269,6 +270,14 @@
 	set_pcb_flags(td->td_pcb, PCB_FULL_IRET);
 }
 
+static void
+linux_cpu_fork(struct thread *td)
+{
+	struct trapframe *frame = td->td_frame;
+
+	frame->tf_rax = 0;
+}
+
 static int
 linux_copyout_auxargs(struct image_params *imgp, uintptr_t base)
 {
@@ -790,6 +799,7 @@
 	.sv_onexit	= linux_on_exit,
 	.sv_ontdexit	= linux_thread_dtor,
 	.sv_setid_allowed = &linux_setid_allowed_query,
+	.sv_cpu_fork	= linux_cpu_fork,
 };
 
 static int
Index: sys/amd64/linux32/linux32_sysvec.c
===================================================================
--- sys/amd64/linux32/linux32_sysvec.c
+++ sys/amd64/linux32/linux32_sysvec.c
@@ -117,6 +117,7 @@
 		    struct image_params *iparams);
 static int	linux_copyout_strings(struct image_params *imgp,
 		    uintptr_t *stack_base);
+static void	linux32_cpu_fork(struct thread *td);
 static void     linux_sendsig(sig_t catcher, ksiginfo_t *ksi, sigset_t *mask);
 static void	linux_exec_setregs(struct thread *td,
 				   struct image_params *imgp, uintptr_t stack);
@@ -703,6 +704,14 @@
 	}
 }
 
+static void
+linux32_cpu_fork(struct thread *td)
+{
+	struct trapframe *frame = td->td_frame;
+
+	frame->tf_rax = 0;
+}
+
 /*
  * Clear registers on exec
  * XXX copied from ia32_signal.c.
@@ -956,6 +965,7 @@
 	.sv_onexit	= linux_on_exit,
 	.sv_ontdexit	= linux_thread_dtor,
 	.sv_setid_allowed = &linux_setid_allowed_query,
+	.sv_cpu_fork	= linux32_cpu_fork,
 };
 
 static int
Index: sys/i386/i386/vm_machdep.c
===================================================================
--- sys/i386/i386/vm_machdep.c
+++ sys/i386/i386/vm_machdep.c
@@ -258,9 +258,12 @@
 	    VM86_STACK_SPACE) - 1;
 	bcopy(td1->td_frame, td2->td_frame, sizeof(struct trapframe));
 
-	td2->td_frame->tf_eax = 0;		/* Child returns zero */
-	td2->td_frame->tf_eflags &= ~PSL_C;	/* success */
-	td2->td_frame->tf_edx = 1;
+	if (__predict_true(p2->p_sysent->sv_cpu_fork == NULL))
+		td2->td_frame->tf_eax = 0;		/* Child returns zero */
+		td2->td_frame->tf_eflags &= ~PSL_C;	/* success */
+		td2->td_frame->tf_edx = 1;		/* System V emulation */
+	} else
+		p2->p_sysent->sv_cpu_fork(td2);
 
 	/*
 	 * If the parent process has the trap bit set (i.e. a debugger
Index: sys/i386/linux/linux_sysvec.c
===================================================================
--- sys/i386/linux/linux_sysvec.c
+++ sys/i386/linux/linux_sysvec.c
@@ -96,6 +96,7 @@
 
 SET_DECLARE(linux_ioctl_handler_set, struct linux_ioctl_handler);
 
+static void	linux_cpu_fork(struct thread *td);
 static int	linux_fixup(uintptr_t *stack_base,
 		    struct image_params *iparams);
 static int	linux_fixup_elf(uintptr_t *stack_base,
@@ -803,6 +804,14 @@
 	}
 }
 
+static void
+linux_cpu_fork(struct thread *td)
+{
+	struct trapframe *frame = td->td_frame;
+
+	frame->tf_eax = 0;
+}
+
 /*
  * exec_setregs may initialize some registers differently than Linux
  * does, thus potentially confusing Linux binaries. If necessary, we
@@ -856,6 +865,7 @@
 	.sv_onexit	= linux_on_exit,
 	.sv_ontdexit	= linux_thread_dtor,
 	.sv_setid_allowed = &linux_setid_allowed_query,
+	.sv_cpu_fork	= linux_cpu_fork,
 };
 INIT_SYSENTVEC(aout_sysvec, &linux_sysvec);
 
@@ -898,6 +908,7 @@
 	.sv_onexit	= linux_on_exit,
 	.sv_ontdexit	= linux_thread_dtor,
 	.sv_setid_allowed = &linux_setid_allowed_query,
+	.sv_cpu_fork	= linux_cpu_fork,
 };
 
 static int
Index: sys/sys/sysent.h
===================================================================
--- sys/sys/sysent.h
+++ sys/sys/sysent.h
@@ -156,6 +156,7 @@
 	void		(*sv_ontdexit)(struct thread *td);
 	int		(*sv_setid_allowed)(struct thread *td,
 			    struct image_params *imgp);
+	void		(*sv_cpu_fork)(struct thread *);
 };
 
 #define	SV_ILP32	0x000100	/* 32-bit executable. */