Use PFR0 to identify need for mitigation of CVE-2017-5915

If the CSV2 field reads as 1 then branch targets trained in one
context cannot affect speculative execution in a different context.
In that case skip the workaround on Cortex A75.

Change-Id: I4d5504cba516a67311fb5f0657b08f72909cbd38
Signed-off-by: Dimitris Papastamos <dimitris.papastamos@arm.com>

include/lib/aarch64/arch.h

@@ -117,6 +117,9 @@
 #define ID_AA64PFR0_SVE_SHIFT	U(32)
 #define ID_AA64PFR0_SVE_MASK	U(0xf)
 #define ID_AA64PFR0_SVE_LENGTH	U(4)
+#define ID_AA64PFR0_CSV2_SHIFT	U(56)
+#define ID_AA64PFR0_CSV2_MASK	U(0xf)
+#define ID_AA64PFR0_CSV2_LENGTH	U(4)
 
 /* ID_AA64DFR0_EL1.PMS definitions (for ARMv8.2+) */
 #define ID_AA64DFR0_PMS_SHIFT	U(32)

lib/cpus/aarch64/cortex_a75.S

@@ -13,8 +13,18 @@
 
 func cortex_a75_reset_func
 #if IMAGE_BL31 && WORKAROUND_CVE_2017_5715
+	mrs	x0, id_aa64pfr0_el1
+	ubfx	x0, x0, #ID_AA64PFR0_CSV2_SHIFT, #ID_AA64PFR0_CSV2_LENGTH
+	/*
+	 * If the field equals to 1 then branch targets trained in one
+	 * context cannot affect speculative execution in a different context.
+	 */
+	cmp	x0, #1
+	beq	1f
+
 	adr	x0, workaround_bpiall_vbar0_runtime_exceptions
 	msr	vbar_el3, x0
+1:
 #endif
 
 #if ENABLE_AMU