KVM: arm64: Validate FGT register descriptions against RES0 masks

In order to point out to the unsuspecting KVM hacker that they
are missing something somewhere, validate that the known FGT bits
do not intersect with the corresponding RES0 mask, as computed at
boot time.

THis check is also performed at boot time, ensuring that there is
no runtime overhead.

Signed-off-by: Marc Zyngier <maz@kernel.org>

arch/arm64/include/asm/kvm_host.h

@@ -1611,5 +1611,6 @@ void kvm_set_vm_id_reg(struct kvm *kvm, u32 reg, u64 val);
 	(kvm_has_feat((k), ID_AA64MMFR3_EL1, S1POE, IMP))
 
 void compute_fgu(struct kvm *kvm, enum fgt_group_id fgt);
+void check_feature_map(void);
 
 #endif /* __ARM64_KVM_HOST_H__ */

arch/arm64/kvm/config.c

@@ -494,6 +494,35 @@ static const struct reg_bits_to_feat_map hafgrtr_feat_map[] = {
 		   FEAT_AMUv1),
 };
 
+static void __init check_feat_map(const struct reg_bits_to_feat_map *map,
+				  int map_size, u64 res0, const char *str)
+{
+	u64 mask = 0;
+
+	for (int i = 0; i < map_size; i++)
+		mask |= map[i].bits;
+
+	if (mask != ~res0)
+		kvm_err("Undefined %s behaviour, bits %016llx\n",
+			str, mask ^ ~res0);
+}
+
+void __init check_feature_map(void)
+{
+	check_feat_map(hfgrtr_feat_map, ARRAY_SIZE(hfgrtr_feat_map),
+		       hfgrtr_masks.res0, hfgrtr_masks.str);
+	check_feat_map(hfgwtr_feat_map, ARRAY_SIZE(hfgwtr_feat_map),
+		       hfgwtr_masks.res0, hfgwtr_masks.str);
+	check_feat_map(hfgitr_feat_map, ARRAY_SIZE(hfgitr_feat_map),
+		       hfgitr_masks.res0, hfgitr_masks.str);
+	check_feat_map(hdfgrtr_feat_map, ARRAY_SIZE(hdfgrtr_feat_map),
+		       hdfgrtr_masks.res0, hdfgrtr_masks.str);
+	check_feat_map(hdfgwtr_feat_map, ARRAY_SIZE(hdfgwtr_feat_map),
+		       hdfgwtr_masks.res0, hdfgwtr_masks.str);
+	check_feat_map(hafgrtr_feat_map, ARRAY_SIZE(hafgrtr_feat_map),
+		       hafgrtr_masks.res0, hafgrtr_masks.str);
+}
+
 static bool idreg_feat_match(struct kvm *kvm, const struct reg_bits_to_feat_map *map)
 {
 	u64 regval = kvm->arch.id_regs[map->regidx];

arch/arm64/kvm/sys_regs.c

@@ -5208,6 +5208,8 @@ int __init kvm_sys_reg_table_init(void)
 
 	ret = populate_nv_trap_config();
 
+	check_feature_map();
+
 	for (i = 0; !ret && i < ARRAY_SIZE(sys_reg_descs); i++)
 		ret = populate_sysreg_config(sys_reg_descs + i, i);