asm/head: balance branches to avoid link stack predictor mispredicts

The Linux wrapper for OPAL call and return is arranged like this:

  __opal_call:
      mflr   r0
      std    r0,PPC_STK_LROFF(r1)
      LOAD_REG_ADDR(r11, opal_return)
      mtlr   r11
      hrfid  -> OPAL

  opal_return:
      ld     r0,PPC_STK_LROFF(r1)
      mtlr   r0
      blr

When skiboot returns to Linux, it branches to LR (i.e., opal_return)
with a blr. This unbalances the link stack predictor and will cause
mispredicts back up the return stack.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Stewart Smith <stewart@linux.ibm.com>

asm/head.S

@@ -1090,7 +1090,12 @@ opal_entry:
 	lwz	%r11,CPUTHREAD_IN_OPAL_CALL(%r12)
 	subi	%r11,%r11,1
 	stw	%r11,CPUTHREAD_IN_OPAL_CALL(%r12)
-	blr
+	/*
+	 * blr with BH=01b means it's not a function return, OPAL was entered
+	 * via (h)rfid not bl, so we don't have a corresponding link stack
+	 * prediction to return to here.
+	 */
+	bclr	20,0,1
 
 .global start_kernel
 start_kernel: