s390: do not bypass BPENTER for interrupt system calls

BugLink: http://bugs.launchpad.net/bugs/1768474

[ Upstream commit d5feec04 ]

The system call path can be interrupted before the switch back to the
standard branch prediction with BPENTER has been done. The critical
section cleanup code skips forward to .Lsysc_do_svc and bypasses the
BPENTER. In this case the kernel and all subsequent code will run with
the limited branch prediction.

Fixes: eacf67eb9b32 ("s390: run user space and KVM guests with modified branch prediction")
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

arch/s390/kernel/entry.S

@@ -1299,7 +1299,8 @@ cleanup_critical:
 	srag	%r9,%r9,23
 	jz	0f
 	mvc	__TI_last_break(8,%r12),16(%r11)
-0:	# set up saved register r11
+0:	BPENTER __TI_flags(%r12),_TIF_ISOLATE_BP
+	# set up saved register r11
 	lg	%r15,__LC_KERNEL_STACK
 	la	%r9,STACK_FRAME_OVERHEAD(%r15)
 	stg	%r9,24(%r11)		# r11 pt_regs pointer