arm64: Take into account ID_AA64PFR0_EL1.CSV3

For non-KASLR kernels where the KPTI behaviour has not been overridden
on the command line we can use ID_AA64PFR0_EL1.CSV3 to determine whether
or not we should unmap the kernel whilst running at EL0.
Reviewed-by: NSuzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: NWill Deacon <will.deacon@arm.com>
Signed-off-by: NCatalin Marinas <catalin.marinas@arm.com>

arch/arm64/include/asm/sysreg.h

@@ -438,6 +438,7 @@
 #define ID_AA64ISAR1_DPB_SHIFT		0
 
 /* id_aa64pfr0 */
+#define ID_AA64PFR0_CSV3_SHIFT		60
 #define ID_AA64PFR0_SVE_SHIFT		32
 #define ID_AA64PFR0_GIC_SHIFT		24
 #define ID_AA64PFR0_ASIMD_SHIFT		20

arch/arm64/kernel/cpufeature.c

@@ -146,6 +146,7 @@ static const struct arm64_ftr_bits ftr_id_aa64isar1[] = {
 };
 
 static const struct arm64_ftr_bits ftr_id_aa64pfr0[] = {
+	ARM64_FTR_BITS(FTR_HIDDEN, FTR_NONSTRICT, FTR_LOWER_SAFE, ID_AA64PFR0_CSV3_SHIFT, 4, 0),
 	ARM64_FTR_BITS(FTR_VISIBLE, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64PFR0_SVE_SHIFT, 4, 0),
 	ARM64_FTR_BITS(FTR_HIDDEN, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64PFR0_GIC_SHIFT, 4, 0),
 	S_ARM64_FTR_BITS(FTR_VISIBLE, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64PFR0_ASIMD_SHIFT, 4, ID_AA64PFR0_ASIMD_NI),
@@ -852,6 +853,8 @@ static int __kpti_forced; /* 0: not forced, >0: forced on, <0: forced off */
 static bool unmap_kernel_at_el0(const struct arm64_cpu_capabilities *entry,
 				int __unused)
 {
+	u64 pfr0 = read_sanitised_ftr_reg(SYS_ID_AA64PFR0_EL1);
+
 	/* Forced on command line? */
 	if (__kpti_forced) {
 		pr_info_once("kernel page table isolation forced %s by command line option\n",
@@ -863,7 +866,9 @@ static bool unmap_kernel_at_el0(const struct arm64_cpu_capabilities *entry,
 	if (IS_ENABLED(CONFIG_RANDOMIZE_BASE))
 		return true;
 
-	return false;
+	/* Defer to CPU feature registers */
+	return !cpuid_feature_extract_unsigned_field(pfr0,
+						     ID_AA64PFR0_CSV3_SHIFT);
 }
 
 static int __init parse_kpti(char *str)
@@ -968,6 +973,7 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
 	},
 #ifdef CONFIG_UNMAP_KERNEL_AT_EL0
 	{
+		.desc = "Kernel page table isolation (KPTI)",
 		.capability = ARM64_UNMAP_KERNEL_AT_EL0,
 		.def_scope = SCOPE_SYSTEM,
 		.matches = unmap_kernel_at_el0,