[PATCH v3 03/13] arm64: Take into account ID_AA64PFR0_EL1.CSV3

[Will Deacon]

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: Suzuki K Poulose <suzuki.poul...@arm.com>
Signed-off-by: Will Deacon <will.dea...@arm.com>
---
 arch/arm64/include/asm/sysreg.h | 1 +
 arch/arm64/kernel/cpufeature.c  | 8 +++++++-
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/include/asm/sysreg.h b/arch/arm64/include/asm/sysreg.h
index 08cc88574659..ae519bbd3f9e 100644
--- a/arch/arm64/include/asm/sysreg.h
+++ b/arch/arm64/include/asm/sysreg.h
@@ -437,6 +437,7 @@
 #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
diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index 9f0545dfe497..d723fc071f39 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -145,6 +145,7 @@ 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),
@@ -851,6 +852,8 @@ 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",
@@ -862,7 +865,9 @@ 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)
@@ -967,6 +972,7 @@ 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,
-- 
2.1.4