ARM: 9199/1: spectre-bhb: use local DSB and elide ISB in loop8 sequence

The loop8 mitigation for Spectre-BHB only requires a CPU local DSB
rather than a systemwide one, which is much more costly. And by the same
reasoning as why it is justified to omit the ISB after BPIALL, we can
also elide the ISB and rely on the exception return for the context
synchronization.
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>

arch/arm/kernel/entry-armv.S

@@ -1131,8 +1131,9 @@ vector_bhb_loop8_\name:
 3:	W(b)	. + 4
 	subs	r0, r0, #1
 	bne	3b
-	dsb
-	isb
+	dsb	nsh
+	@ isb not needed due to "movs pc, lr" in the vector stub
+	@ which gives a "context synchronisation".
 	b	2b
 ENDPROC(vector_bhb_loop8_\name)
 	.previous

arch/arm/kernel/entry-common.S

@@ -164,7 +164,7 @@ ENTRY(vector_bhb_loop8_swi)
 1:	b	2f
 2:	subs	r8, r8, #1
 	bne	1b
-	dsb
+	dsb	nsh
 	isb
 	b	3f
 ENDPROC(vector_bhb_loop8_swi)