ARM: spectre-v2: add Cortex A8 and A15 validation of the IBE bit

When the branch predictor hardening is enabled, firmware must have set
the IBE bit in the auxiliary control register.  If this bit has not
been set, the Spectre workarounds will not be functional.

Add validation that this bit is set, and print a warning at alert level
if this is not the case.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Boot-tested-by: Tony Lindgren <tony@atomide.com>
Reviewed-by: Tony Lindgren <tony@atomide.com>

arch/arm/mm/proc-v7-bugs.c

@@ -0,0 +1,36 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <linux/kernel.h>
+#include <linux/smp.h>
+
+static __maybe_unused void cpu_v7_check_auxcr_set(bool *warned,
+						  u32 mask, const char *msg)
+{
+	u32 aux_cr;
+
+	asm("mrc p15, 0, %0, c1, c0, 1" : "=r" (aux_cr));
+
+	if ((aux_cr & mask) != mask) {
+		if (!*warned)
+			pr_err("CPU%u: %s", smp_processor_id(), msg);
+		*warned = true;
+	}
+}
+
+static DEFINE_PER_CPU(bool, spectre_warned);
+
+static void check_spectre_auxcr(bool *warned, u32 bit)
+{
+	if (IS_ENABLED(CONFIG_HARDEN_BRANCH_PREDICTOR) &&
+		cpu_v7_check_auxcr_set(warned, bit,
+				       "Spectre v2: firmware did not set auxiliary control register IBE bit, system vulnerable\n");
+}
+
+void cpu_v7_ca8_ibe(void)
+{
+	check_spectre_auxcr(this_cpu_ptr(&spectre_warned), BIT(6));
+}
+
+void cpu_v7_ca15_ibe(void)
+{
+	check_spectre_auxcr(this_cpu_ptr(&spectre_warned), BIT(0));
+}