ARM: Introduce ability to enable invalidate of BTB with ICIALLU on Cortex-A15 for CVE-2017-5715

As recommended by Arm in [1], ACTLR[0] (Enable invalidates of BTB)
needs to be set[2] for BTB to be invalidated on ICIALLU. This needs to
be done unconditionally for Cortex-A15 processors. Provide a config
option for platforms to enable this option based on impact analysis
for products.

NOTE: This patch in itself is NOT the final solution, this requires:
a) Implementation of v7_arch_cp15_set_acr on SoCs which may not
   provide direct access to ACR register.
b) Operating Systems such as Linux to provide adequate workaround in the
   right locations.
c) This workaround applies to only the boot processor. It is important
   to apply workaround as necessary (context-save-restore) around low
   power context loss OR additional processors as necessary in either
   firmware support OR elsewhere in OS.

[1] https://developer.arm.com/support/security-update
[2] http://infocenter.arm.com/help/topic/com.arm.doc.ddi0438c/BABGHIBG.html

Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: Tony Lindgren <tony@atomide.com>
Cc: Robin Murphy <robin.murphy@arm.com>
Cc: Florian Fainelli <f.fainelli@gmail.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Christoffer Dall <christoffer.dall@linaro.org>
Cc: Andre Przywara <Andre.Przywara@arm.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Tom Rini <trini@konsulko.com>
Cc: Michael Nazzareno Trimarchi <michael@amarulasolutions.com>

Signed-off-by: Nishanth Menon <nm@ti.com>
Tested-by: Fabio Estevam <fabio.estevam@nxp.com>

arch/arm/Kconfig

@@ -109,6 +109,7 @@ config SYS_ARM_MPU
 # CONFIG_ARM_ERRATA_798870
 # CONFIG_ARM_ERRATA_801819
 # CONFIG_ARM_CORTEX_A8_CVE_2017_5715
+# CONFIG_ARM_CORTEX_A15_CVE_2017_5715
 
 config ARM_ERRATA_430973
 	bool
@@ -182,6 +183,9 @@ config ARM_ERRATA_855873
 config ARM_CORTEX_A8_CVE_2017_5715
 	bool
 
+config ARM_CORTEX_A15_CVE_2017_5715
+	bool
+
 config CPU_ARM720T
 	bool
 	select SYS_CACHE_SHIFT_5

arch/arm/cpu/armv7/start.S

@@ -241,6 +241,14 @@ skip_errata_798870:
 skip_errata_801819:
 #endif
 
+#ifdef CONFIG_ARM_CORTEX_A15_CVE_2017_5715
+	mrc	p15, 0, r0, c1, c0, 1	@ read auxilary control register
+	orr	r0, r0, #1 << 0		@ Enable invalidates of BTB
+	push	{r1-r5}			@ Save the cpu info registers
+	bl	v7_arch_cp15_set_acr
+	pop	{r1-r5}			@ Restore the cpu info - fall through
+#endif
+
 #ifdef CONFIG_ARM_ERRATA_454179
 	mrc	p15, 0, r0, c1, c0, 1	@ Read ACR