Merge branch kvm-arm64/configurable-id-regs into kvmarm/next

* kvm-arm64/configurable-id-regs:
  : Configurable ID register infrastructure, courtesy of Jing Zhang
  :
  : Create generalized infrastructure for allowing userspace to select the
  : supported feature set for a VM, so long as the feature set is a subset
  : of what hardware + KVM allows. This does not add any new features that
  : are user-configurable, and instead focuses on the necessary refactoring
  : to enable future work.
  :
  : As a consequence of the series, feature asymmetry is now deliberately
  : disallowed for KVM. It is unlikely that VMMs ever configured VMs with
  : asymmetry, nor does it align with the kernel's overall stance that
  : features must be uniform across all cores in the system.
  :
  : Furthermore, KVM incorrectly advertised an IMP_DEF PMU to guests for
  : some time. Migrations from affected kernels was supported by explicitly
  : allowing such an ID register value from userspace, and forwarding that
  : along to the guest. KVM now allows an IMP_DEF PMU version to be restored
  : through the ID register interface, but reinterprets the user value as
  : not implemented (0).
  KVM: arm64: Rip out the vestiges of the 'old' ID register scheme
  KVM: arm64: Handle ID register reads using the VM-wide values
  KVM: arm64: Use generic sanitisation for ID_AA64PFR0_EL1
  KVM: arm64: Use generic sanitisation for ID_(AA64)DFR0_EL1
  KVM: arm64: Use arm64_ftr_bits to sanitise ID register writes
  KVM: arm64: Save ID registers' sanitized value per guest
  KVM: arm64: Reuse fields of sys_reg_desc for idreg
  KVM: arm64: Rewrite IMPDEF PMU version as NI
  KVM: arm64: Make vCPU feature flags consistent VM-wide
  KVM: arm64: Relax invariance of KVM_ARM_VCPU_POWER_OFF
  KVM: arm64: Separate out feature sanitisation and initialisation

Signed-off-by: Oliver Upton <oliver.upton@linux.dev>

8 files changed, 482 insertions, 290 deletions

diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
index 3d4b547ae312..e753d989163f 100644
--- a/arch/arm64/include/asm/cpufeature.h
+++ b/arch/arm64/include/asm/cpufeature.h
@@ -918,6 +918,7 @@ static inline unsigned int get_vmid_bits(u64 mmfr1)
 	return 8;
 }
 
+s64 arm64_ftr_safe_value(const struct arm64_ftr_bits *ftrp, s64 new, s64 cur);
 struct arm64_ftr_reg *get_arm64_ftr_reg(u32 sys_id);
 
 extern struct arm64_ftr_override id_aa64mmfr1_override;
diff --git a/arch/arm64/include/asm/kvm_emulate.h b/arch/arm64/include/asm/kvm_emulate.h
index cf40d19a72f8..e14a3827a57e 100644
--- a/arch/arm64/include/asm/kvm_emulate.h
+++ b/arch/arm64/include/asm/kvm_emulate.h
@@ -62,12 +62,7 @@ static __always_inline bool vcpu_el1_is_32bit(struct kvm_vcpu *vcpu)
 #else
 static __always_inline bool vcpu_el1_is_32bit(struct kvm_vcpu *vcpu)
 {
-	struct kvm *kvm = vcpu->kvm;
-
-	WARN_ON_ONCE(!test_bit(KVM_ARCH_FLAG_REG_WIDTH_CONFIGURED,
-			       &kvm->arch.flags));
-
-	return test_bit(KVM_ARCH_FLAG_EL1_32BIT, &kvm->arch.flags);
+	return test_bit(KVM_ARM_VCPU_EL1_32BIT, vcpu->arch.features);
 }
 #endif
 
diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
index 6fcc552b40b5..bad7dfe9c16d 100644
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h
@@ -39,6 +39,7 @@
 #define KVM_MAX_VCPUS VGIC_V3_MAX_CPUS
 
 #define KVM_VCPU_MAX_FEATURES 7
+#define KVM_VCPU_VALID_FEATURES	(BIT(KVM_VCPU_MAX_FEATURES) - 1)
 
 #define KVM_REQ_SLEEP \
 	KVM_ARCH_REQ_FLAGS(0, KVM_REQUEST_WAIT | KVM_REQUEST_NO_WAKEUP)
@@ -229,25 +230,23 @@ struct kvm_arch {
 #define KVM_ARCH_FLAG_MTE_ENABLED			1
 	/* At least one vCPU has ran in the VM */
 #define KVM_ARCH_FLAG_HAS_RAN_ONCE			2
-	/*
-	 * The following two bits are used to indicate the guest's EL1
-	 * register width configuration. A value of KVM_ARCH_FLAG_EL1_32BIT
-	 * bit is valid only when KVM_ARCH_FLAG_REG_WIDTH_CONFIGURED is set.
-	 * Otherwise, the guest's EL1 register width has not yet been
-	 * determined yet.
-	 */
-#define KVM_ARCH_FLAG_REG_WIDTH_CONFIGURED		3
-#define KVM_ARCH_FLAG_EL1_32BIT				4
+	/* The vCPU feature set for the VM is configured */
+#define KVM_ARCH_FLAG_VCPU_FEATURES_CONFIGURED		3
 	/* PSCI SYSTEM_SUSPEND enabled for the guest */
-#define KVM_ARCH_FLAG_SYSTEM_SUSPEND_ENABLED		5
+#define KVM_ARCH_FLAG_SYSTEM_SUSPEND_ENABLED		4
 	/* VM counter offset */
-#define KVM_ARCH_FLAG_VM_COUNTER_OFFSET			6
+#define KVM_ARCH_FLAG_VM_COUNTER_OFFSET			5
 	/* Timer PPIs made immutable */
-#define KVM_ARCH_FLAG_TIMER_PPIS_IMMUTABLE		7
+#define KVM_ARCH_FLAG_TIMER_PPIS_IMMUTABLE		6
 	/* SMCCC filter initialized for the VM */
-#define KVM_ARCH_FLAG_SMCCC_FILTER_CONFIGURED		8
+#define KVM_ARCH_FLAG_SMCCC_FILTER_CONFIGURED		7
+	/* Initial ID reg values loaded */
+#define KVM_ARCH_FLAG_ID_REGS_INITIALIZED		8
 	unsigned long flags;
 
+	/* VM-wide vCPU feature set */
+	DECLARE_BITMAP(vcpu_features, KVM_VCPU_MAX_FEATURES);
+
 	/*
 	 * VM-wide PMU filter, implemented as a bitmap and big enough for
 	 * up to 2^10 events (ARMv8.0) or 2^16 events (ARMv8.1+).
@@ -257,18 +256,24 @@ struct kvm_arch {
 
 	cpumask_var_t supported_cpus;
 
-	u8 pfr0_csv2;
-	u8 pfr0_csv3;
-	struct {
-		u8 imp:4;
-		u8 unimp:4;
-	} dfr0_pmuver;
-
 	/* Hypercall features firmware registers' descriptor */
 	struct kvm_smccc_features smccc_feat;
 	struct maple_tree smccc_filter;
 
 	/*
+	 * Emulated CPU ID registers per VM
+	 * (Op0, Op1, CRn, CRm, Op2) of the ID registers to be saved in it
+	 * is (3, 0, 0, crm, op2), where 1<=crm<8, 0<=op2<8.
+	 *
+	 * These emulated idregs are VM-wide, but accessed from the context of a vCPU.
+	 * Atomic access to multiple idregs are guarded by kvm_arch.config_lock.
+	 */
+#define IDREG_IDX(id)		(((sys_reg_CRm(id) - 1) << 3) | sys_reg_Op2(id))
+#define IDREG(kvm, id)		((kvm)->arch.id_regs[IDREG_IDX(id)])
+#define KVM_ARM_ID_REG_NUM	(IDREG_IDX(sys_reg(3, 0, 0, 7, 7)) + 1)
+	u64 id_regs[KVM_ARM_ID_REG_NUM];
+
+	/*
 	 * For an untrusted host VM, 'pkvm.handle' is used to lookup
 	 * the associated pKVM instance in the hypervisor.
 	 */
diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index c743b1a5ae31..c01c52a3dd7e 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -800,7 +800,7 @@ static u64 arm64_ftr_set_value(const struct arm64_ftr_bits *ftrp, s64 reg,
 	return reg;
 }
 
-static s64 arm64_ftr_safe_value(const struct arm64_ftr_bits *ftrp, s64 new,
+s64 arm64_ftr_safe_value(const struct arm64_ftr_bits *ftrp, s64 new,
 				s64 cur)
 {
 	s64 ret = 0;
diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
index 0f58fb69dacd..050f4ef57f10 100644
--- a/arch/arm64/kvm/arm.c
+++ b/arch/arm64/kvm/arm.c
@@ -121,22 +121,6 @@ static int kvm_arm_default_max_vcpus(void)
 	return vgic_present ? kvm_vgic_get_max_vcpus() : KVM_MAX_VCPUS;
 }
 
-static void set_default_spectre(struct kvm *kvm)
-{
-	/*
-	 * The default is to expose CSV2 == 1 if the HW isn't affected.
-	 * Although this is a per-CPU feature, we make it global because
-	 * asymmetric systems are just a nuisance.
-	 *
-	 * Userspace can override this as long as it doesn't promise
-	 * the impossible.
-	 */
-	if (arm64_get_spectre_v2_state() == SPECTRE_UNAFFECTED)
-		kvm->arch.pfr0_csv2 = 1;
-	if (arm64_get_meltdown_state() == SPECTRE_UNAFFECTED)
-		kvm->arch.pfr0_csv3 = 1;
-}
-
 /**
  * kvm_arch_init_vm - initializes a VM data structure
  * @kvm:	pointer to the KVM struct
@@ -180,14 +164,9 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
 	/* The maximum number of VCPUs is limited by the host's GIC model */
 	kvm->max_vcpus = kvm_arm_default_max_vcpus();
 
-	set_default_spectre(kvm);
 	kvm_arm_init_hypercalls(kvm);
 
-	/*
-	 * Initialise the default PMUver before there is a chance to
-	 * create an actual PMU.
-	 */
-	kvm->arch.dfr0_pmuver.imp = kvm_arm_pmu_get_pmuver_limit();
+	bitmap_zero(kvm->arch.vcpu_features, KVM_VCPU_MAX_FEATURES);
 
 	return 0;
 
@@ -1195,58 +1174,115 @@ int kvm_vm_ioctl_irq_line(struct kvm *kvm, struct kvm_irq_level *irq_level,
 	return -EINVAL;
 }
 
-static int kvm_vcpu_set_target(struct kvm_vcpu *vcpu,
-			       const struct kvm_vcpu_init *init)
+static int kvm_vcpu_init_check_features(struct kvm_vcpu *vcpu,
+					const struct kvm_vcpu_init *init)
 {
-	unsigned int i, ret;
-	u32 phys_target = kvm_target_cpu();
+	unsigned long features = init->features[0];
+	int i;
+
+	if (features & ~KVM_VCPU_VALID_FEATURES)
+		return -ENOENT;
+
+	for (i = 1; i < ARRAY_SIZE(init->features); i++) {
+		if (init->features[i])
+			return -ENOENT;
+	}
+
+	if (!test_bit(KVM_ARM_VCPU_EL1_32BIT, &features))
+		return 0;
 
-	if (init->target != phys_target)
+	if (!cpus_have_const_cap(ARM64_HAS_32BIT_EL1))
 		return -EINVAL;
 
-	/*
-	 * Secondary and subsequent calls to KVM_ARM_VCPU_INIT must
-	 * use the same target.
-	 */
-	if (vcpu->arch.target != -1 && vcpu->arch.target != init->target)
+	/* MTE is incompatible with AArch32 */
+	if (kvm_has_mte(vcpu->kvm))
 		return -EINVAL;
 
-	/* -ENOENT for unknown features, -EINVAL for invalid combinations. */
-	for (i = 0; i < sizeof(init->features) * 8; i++) {
-		bool set = (init->features[i / 32] & (1 << (i % 32)));
+	/* NV is incompatible with AArch32 */
+	if (test_bit(KVM_ARM_VCPU_HAS_EL2, &features))
+		return -EINVAL;
 
-		if (set && i >= KVM_VCPU_MAX_FEATURES)
-			return -ENOENT;
+	return 0;
+}
 
-		/*
-		 * Secondary and subsequent calls to KVM_ARM_VCPU_INIT must
-		 * use the same feature set.
-		 */
-		if (vcpu->arch.target != -1 && i < KVM_VCPU_MAX_FEATURES &&
-		    test_bit(i, vcpu->arch.features) != set)
-			return -EINVAL;
+static bool kvm_vcpu_init_changed(struct kvm_vcpu *vcpu,
+				  const struct kvm_vcpu_init *init)
+{
+	unsigned long features = init->features[0];
 
-		if (set)
-			set_bit(i, vcpu->arch.features);
-	}
+	return !bitmap_equal(vcpu->arch.features, &features, KVM_VCPU_MAX_FEATURES) ||
+			vcpu->arch.target != init->target;
+}
+
+static int __kvm_vcpu_set_target(struct kvm_vcpu *vcpu,
+				 const struct kvm_vcpu_init *init)
+{
+	unsigned long features = init->features[0];
+	struct kvm *kvm = vcpu->kvm;
+	int ret = -EINVAL;
+
+	mutex_lock(&kvm->arch.config_lock);
+
+	if (test_bit(KVM_ARCH_FLAG_VCPU_FEATURES_CONFIGURED, &kvm->arch.flags) &&
+	    !bitmap_equal(kvm->arch.vcpu_features, &features, KVM_VCPU_MAX_FEATURES))
+		goto out_unlock;
 
-	vcpu->arch.target = phys_target;
+	vcpu->arch.target = init->target;
+	bitmap_copy(vcpu->arch.features, &features, KVM_VCPU_MAX_FEATURES);
 
 	/* Now we know what it is, we can reset it. */
 	ret = kvm_reset_vcpu(vcpu);
 	if (ret) {
 		vcpu->arch.target = -1;
 		bitmap_zero(vcpu->arch.features, KVM_VCPU_MAX_FEATURES);
+		goto out_unlock;
 	}
 
+	bitmap_copy(kvm->arch.vcpu_features, &features, KVM_VCPU_MAX_FEATURES);
+	set_bit(KVM_ARCH_FLAG_VCPU_FEATURES_CONFIGURED, &kvm->arch.flags);
+
+out_unlock:
+	mutex_unlock(&kvm->arch.config_lock);
 	return ret;
 }
 
+static int kvm_vcpu_set_target(struct kvm_vcpu *vcpu,
+			       const struct kvm_vcpu_init *init)
+{
+	int ret;
+
+	if (init->target != kvm_target_cpu())
+		return -EINVAL;
+
+	ret = kvm_vcpu_init_check_features(vcpu, init);
+	if (ret)
+		return ret;
+
+	if (vcpu->arch.target == -1)
+		return __kvm_vcpu_set_target(vcpu, init);
+
+	if (kvm_vcpu_init_changed(vcpu, init))
+		return -EINVAL;
+
+	return kvm_reset_vcpu(vcpu);
+}
+
 static int kvm_arch_vcpu_ioctl_vcpu_init(struct kvm_vcpu *vcpu,
 					 struct kvm_vcpu_init *init)
 {
+	bool power_off = false;
 	int ret;
 
+	/*
+	 * Treat the power-off vCPU feature as ephemeral. Clear the bit to avoid
+	 * reflecting it in the finalized feature set, thus limiting its scope
+	 * to a single KVM_ARM_VCPU_INIT call.
+	 */
+	if (init->features[0] & KVM_ARM_VCPU_POWER_OFF) {
+		init->features[0] &= ~KVM_ARM_VCPU_POWER_OFF;
+		power_off = true;
+	}
+
 	ret = kvm_vcpu_set_target(vcpu, init);
 	if (ret)
 		return ret;
@@ -1275,7 +1311,7 @@ static int kvm_arch_vcpu_ioctl_vcpu_init(struct kvm_vcpu *vcpu,
 	 */
 	spin_lock(&vcpu->arch.mp_state_lock);
 
-	if (test_bit(KVM_ARM_VCPU_POWER_OFF, vcpu->arch.features))
+	if (power_off)
 		__kvm_arm_vcpu_power_off(vcpu);
 	else
 		WRITE_ONCE(vcpu->arch.mp_state.mp_state, KVM_MP_STATE_RUNNABLE);
diff --git a/arch/arm64/kvm/reset.c b/arch/arm64/kvm/reset.c
index b5dee8e57e77..bc8556b6f459 100644
--- a/arch/arm64/kvm/reset.c
+++ b/arch/arm64/kvm/reset.c
@@ -187,57 +187,6 @@ static int kvm_vcpu_enable_ptrauth(struct kvm_vcpu *vcpu)
 }
 
 /**
- * kvm_set_vm_width() - set the register width for the guest
- * @vcpu: Pointer to the vcpu being configured
- *
- * Set both KVM_ARCH_FLAG_EL1_32BIT and KVM_ARCH_FLAG_REG_WIDTH_CONFIGURED
- * in the VM flags based on the vcpu's requested register width, the HW
- * capabilities and other options (such as MTE).
- * When REG_WIDTH_CONFIGURED is already set, the vcpu settings must be
- * consistent with the value of the FLAG_EL1_32BIT bit in the flags.
- *
- * Return: 0 on success, negative error code on failure.
- */
-static int kvm_set_vm_width(struct kvm_vcpu *vcpu)
-{
-	struct kvm *kvm = vcpu->kvm;
-	bool is32bit;
-
-	is32bit = vcpu_has_feature(vcpu, KVM_ARM_VCPU_EL1_32BIT);
-
-	lockdep_assert_held(&kvm->arch.config_lock);
-
-	if (test_bit(KVM_ARCH_FLAG_REG_WIDTH_CONFIGURED, &kvm->arch.flags)) {
-		/*
-		 * The guest's register width is already configured.
-		 * Make sure that the vcpu is consistent with it.
-		 */
-		if (is32bit == test_bit(KVM_ARCH_FLAG_EL1_32BIT, &kvm->arch.flags))
-			return 0;
-
-		return -EINVAL;
-	}
-
-	if (!cpus_have_const_cap(ARM64_HAS_32BIT_EL1) && is32bit)
-		return -EINVAL;
-
-	/* MTE is incompatible with AArch32 */
-	if (kvm_has_mte(kvm) && is32bit)
-		return -EINVAL;
-
-	/* NV is incompatible with AArch32 */
-	if (vcpu_has_nv(vcpu) && is32bit)
-		return -EINVAL;
-
-	if (is32bit)
-		set_bit(KVM_ARCH_FLAG_EL1_32BIT, &kvm->arch.flags);
-
-	set_bit(KVM_ARCH_FLAG_REG_WIDTH_CONFIGURED, &kvm->arch.flags);
-
-	return 0;
-}
-
-/**
  * kvm_reset_vcpu - sets core registers and sys_regs to reset value
  * @vcpu: The VCPU pointer
  *
@@ -262,13 +211,6 @@ int kvm_reset_vcpu(struct kvm_vcpu *vcpu)
 	bool loaded;
 	u32 pstate;
 
-	mutex_lock(&vcpu->kvm->arch.config_lock);
-	ret = kvm_set_vm_width(vcpu);
-	mutex_unlock(&vcpu->kvm->arch.config_lock);
-
-	if (ret)
-		return ret;
-
 	spin_lock(&vcpu->arch.mp_state_lock);
 	reset_state = vcpu->arch.reset_state;
 	vcpu->arch.reset_state.reset = false;
diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
index 11ededa80737..1a13bab1a06c 100644
--- a/arch/arm64/kvm/sys_regs.c
+++ b/arch/arm64/kvm/sys_regs.c
@@ -42,6 +42,8 @@
  */
 
 static u64 sys_reg_to_index(const struct sys_reg_desc *reg);
+static int set_id_reg(struct kvm_vcpu *vcpu, const struct sys_reg_desc *rd,
+		      u64 val);
 
 static bool read_from_write_only(struct kvm_vcpu *vcpu,
 				 struct sys_reg_params *params,
@@ -540,10 +542,11 @@ static int get_bvr(struct kvm_vcpu *vcpu, const struct sys_reg_desc *rd,
 	return 0;
 }
 
-static void reset_bvr(struct kvm_vcpu *vcpu,
+static u64 reset_bvr(struct kvm_vcpu *vcpu,
 		      const struct sys_reg_desc *rd)
 {
 	vcpu->arch.vcpu_debug_state.dbg_bvr[rd->CRm] = rd->val;
+	return rd->val;
 }
 
 static bool trap_bcr(struct kvm_vcpu *vcpu,
@@ -576,10 +579,11 @@ static int get_bcr(struct kvm_vcpu *vcpu, const struct sys_reg_desc *rd,
 	return 0;
 }
 
-static void reset_bcr(struct kvm_vcpu *vcpu,
+static u64 reset_bcr(struct kvm_vcpu *vcpu,
 		      const struct sys_reg_desc *rd)
 {
 	vcpu->arch.vcpu_debug_state.dbg_bcr[rd->CRm] = rd->val;
+	return rd->val;
 }
 
 static bool trap_wvr(struct kvm_vcpu *vcpu,
@@ -613,10 +617,11 @@ static int get_wvr(struct kvm_vcpu *vcpu, const struct sys_reg_desc *rd,
 	return 0;
 }
 
-static void reset_wvr(struct kvm_vcpu *vcpu,
+static u64 reset_wvr(struct kvm_vcpu *vcpu,
 		      const struct sys_reg_desc *rd)
 {
 	vcpu->arch.vcpu_debug_state.dbg_wvr[rd->CRm] = rd->val;
+	return rd->val;
 }
 
 static bool trap_wcr(struct kvm_vcpu *vcpu,
@@ -649,25 +654,28 @@ static int get_wcr(struct kvm_vcpu *vcpu, const struct sys_reg_desc *rd,
 	return 0;
 }
 
-static void reset_wcr(struct kvm_vcpu *vcpu,
+static u64 reset_wcr(struct kvm_vcpu *vcpu,
 		      const struct sys_reg_desc *rd)
 {
 	vcpu->arch.vcpu_debug_state.dbg_wcr[rd->CRm] = rd->val;
+	return rd->val;
 }
 
-static void reset_amair_el1(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r)
+static u64 reset_amair_el1(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r)
 {
 	u64 amair = read_sysreg(amair_el1);
 	vcpu_write_sys_reg(vcpu, amair, AMAIR_EL1);
+	return amair;
 }
 
-static void reset_actlr(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r)
+static u64 reset_actlr(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r)
 {
 	u64 actlr = read_sysreg(actlr_el1);
 	vcpu_write_sys_reg(vcpu, actlr, ACTLR_EL1);
+	return actlr;
 }
 
-static void reset_mpidr(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r)
+static u64 reset_mpidr(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r)
 {
 	u64 mpidr;
 
@@ -681,7 +689,10 @@ static void reset_mpidr(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r)
 	mpidr = (vcpu->vcpu_id & 0x0f) << MPIDR_LEVEL_SHIFT(0);
 	mpidr |= ((vcpu->vcpu_id >> 4) & 0xff) << MPIDR_LEVEL_SHIFT(1);
 	mpidr |= ((vcpu->vcpu_id >> 12) & 0xff) << MPIDR_LEVEL_SHIFT(2);
-	vcpu_write_sys_reg(vcpu, (1ULL << 31) | mpidr, MPIDR_EL1);
+	mpidr |= (1ULL << 31);
+	vcpu_write_sys_reg(vcpu, mpidr, MPIDR_EL1);
+
+	return mpidr;
 }
 
 static unsigned int pmu_visibility(const struct kvm_vcpu *vcpu,
@@ -693,13 +704,13 @@ static unsigned int pmu_visibility(const struct kvm_vcpu *vcpu,
 	return REG_HIDDEN;
 }
 
-static void reset_pmu_reg(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r)
+static u64 reset_pmu_reg(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r)
 {
 	u64 n, mask = BIT(ARMV8_PMU_CYCLE_IDX);
 
 	/* No PMU available, any PMU reg may UNDEF... */
 	if (!kvm_arm_support_pmu_v3())
-		return;
+		return 0;
 
 	n = read_sysreg(pmcr_el0) >> ARMV8_PMU_PMCR_N_SHIFT;
 	n &= ARMV8_PMU_PMCR_N_MASK;
@@ -708,33 +719,41 @@ static void reset_pmu_reg(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r)
 
 	reset_unknown(vcpu, r);
 	__vcpu_sys_reg(vcpu, r->reg) &= mask;
+
+	return __vcpu_sys_reg(vcpu, r->reg);
 }
 
-static void reset_pmevcntr(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r)
+static u64 reset_pmevcntr(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r)
 {
 	reset_unknown(vcpu, r);
 	__vcpu_sys_reg(vcpu, r->reg) &= GENMASK(31, 0);
+
+	return __vcpu_sys_reg(vcpu, r->reg);
 }
 
-static void reset_pmevtyper(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r)
+static u64 reset_pmevtyper(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r)
 {
 	reset_unknown(vcpu, r);
 	__vcpu_sys_reg(vcpu, r->reg) &= ARMV8_PMU_EVTYPE_MASK;
+
+	return __vcpu_sys_reg(vcpu, r->reg);
 }
 
-static void reset_pmselr(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r)
+static u64 reset_pmselr(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r)
 {
 	reset_unknown(vcpu, r);
 	__vcpu_sys_reg(vcpu, r->reg) &= ARMV8_PMU_COUNTER_MASK;
+
+	return __vcpu_sys_reg(vcpu, r->reg);
 }
 
-static void reset_pmcr(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r)
+static u64 reset_pmcr(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r)
 {
 	u64 pmcr;
 
 	/* No PMU available, PMCR_EL0 may UNDEF... */
 	if (!kvm_arm_support_pmu_v3())
-		return;
+		return 0;
 
 	/* Only preserve PMCR_EL0.N, and reset the rest to 0 */
 	pmcr = read_sysreg(pmcr_el0) & (ARMV8_PMU_PMCR_N_MASK << ARMV8_PMU_PMCR_N_SHIFT);
@@ -742,6 +761,8 @@ static void reset_pmcr(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r)
 		pmcr |= ARMV8_PMU_PMCR_LC;
 
 	__vcpu_sys_reg(vcpu, r->reg) = pmcr;
+
+	return __vcpu_sys_reg(vcpu, r->reg);
 }
 
 static bool check_pmu_access_disabled(struct kvm_vcpu *vcpu, u64 flags)
@@ -1174,25 +1195,89 @@ static bool access_arch_timer(struct kvm_vcpu *vcpu,
 	return true;
 }
 
-static u8 vcpu_pmuver(const struct kvm_vcpu *vcpu)
+static s64 kvm_arm64_ftr_safe_value(u32 id, const struct arm64_ftr_bits *ftrp,
+				    s64 new, s64 cur)
 {
-	if (kvm_vcpu_has_pmu(vcpu))
-		return vcpu->kvm->arch.dfr0_pmuver.imp;
+	struct arm64_ftr_bits kvm_ftr = *ftrp;
+
+	/* Some features have different safe value type in KVM than host features */
+	switch (id) {
+	case SYS_ID_AA64DFR0_EL1:
+		if (kvm_ftr.shift == ID_AA64DFR0_EL1_PMUVer_SHIFT)
+			kvm_ftr.type = FTR_LOWER_SAFE;
+		break;
+	case SYS_ID_DFR0_EL1:
+		if (kvm_ftr.shift == ID_DFR0_EL1_PerfMon_SHIFT)
+			kvm_ftr.type = FTR_LOWER_SAFE;
+		break;
+	}
 
-	return vcpu->kvm->arch.dfr0_pmuver.unimp;
+	return arm64_ftr_safe_value(&kvm_ftr, new, cur);
 }
 
-static u8 perfmon_to_pmuver(u8 perfmon)
+/**
+ * arm64_check_features() - Check if a feature register value constitutes
+ * a subset of features indicated by the idreg's KVM sanitised limit.
+ *
+ * This function will check if each feature field of @val is the "safe" value
+ * against idreg's KVM sanitised limit return from reset() callback.
+ * If a field value in @val is the same as the one in limit, it is always
+ * considered the safe value regardless For register fields that are not in
+ * writable, only the value in limit is considered the safe value.
+ *
+ * Return: 0 if all the fields are safe. Otherwise, return negative errno.
+ */
+static int arm64_check_features(struct kvm_vcpu *vcpu,
+				const struct sys_reg_desc *rd,
+				u64 val)
 {
-	switch (perfmon) {
-	case ID_DFR0_EL1_PerfMon_PMUv3:
-		return ID_AA64DFR0_EL1_PMUVer_IMP;
-	case ID_DFR0_EL1_PerfMon_IMPDEF:
-		return ID_AA64DFR0_EL1_PMUVer_IMP_DEF;
-	default:
-		/* Anything ARMv8.1+ and NI have the same value. For now. */
-		return perfmon;
+	const struct arm64_ftr_reg *ftr_reg;
+	const struct arm64_ftr_bits *ftrp = NULL;
+	u32 id = reg_to_encoding(rd);
+	u64 writable_mask = rd->val;
+	u64 limit = rd->reset(vcpu, rd);
+	u64 mask = 0;
+
+	/*
+	 * Hidden and unallocated ID registers may not have a corresponding
+	 * struct arm64_ftr_reg. Of course, if the register is RAZ we know the
+	 * only safe value is 0.
+	 */
+	if (sysreg_visible_as_raz(vcpu, rd))
+		return val ? -E2BIG : 0;
+
+	ftr_reg = get_arm64_ftr_reg(id);
+	if (!ftr_reg)
+		return -EINVAL;
+
+	ftrp = ftr_reg->ftr_bits;
+
+	for (; ftrp && ftrp->width; ftrp++) {
+		s64 f_val, f_lim, safe_val;
+		u64 ftr_mask;
+
+		ftr_mask = arm64_ftr_mask(ftrp);
+		if ((ftr_mask & writable_mask) != ftr_mask)
+			continue;
+
+		f_val = arm64_ftr_value(ftrp, val);
+		f_lim = arm64_ftr_value(ftrp, limit);
+		mask |= ftr_mask;
+
+		if (f_val == f_lim)
+			safe_val = f_val;
+		else
+			safe_val = kvm_arm64_ftr_safe_value(id, ftrp, f_val, f_lim);
+
+		if (safe_val != f_val)
+			return -E2BIG;
 	}
+
+	/* For fields that are not writable, values in limit are the safe values. */
+	if ((val & ~mask) != (limit & ~mask))
+		return -E2BIG;
+
+	return 0;
 }
 
 static u8 pmuver_to_perfmon(u8 pmuver)
@@ -1209,7 +1294,8 @@ static u8 pmuver_to_perfmon(u8 pmuver)
 }
 
 /* Read a sanitised cpufeature ID register by sys_reg_desc */
-static u64 read_id_reg(const struct kvm_vcpu *vcpu, struct sys_reg_desc const *r)
+static u64 __kvm_read_sanitised_id_reg(const struct kvm_vcpu *vcpu,
+				       const struct sys_reg_desc *r)
 {
 	u32 id = reg_to_encoding(r);
 	u64 val;
@@ -1220,19 +1306,6 @@ static u64 read_id_reg(const struct kvm_vcpu *vcpu, struct sys_reg_desc const *r
 	val = read_sanitised_ftr_reg(id);
 
 	switch (id) {
-	case SYS_ID_AA64PFR0_EL1:
-		if (!vcpu_has_sve(vcpu))
-			val &= ~ARM64_FEATURE_MASK(ID_AA64PFR0_EL1_SVE);
-		val &= ~ARM64_FEATURE_MASK(ID_AA64PFR0_EL1_AMU);
-		val &= ~ARM64_FEATURE_MASK(ID_AA64PFR0_EL1_CSV2);
-		val |= FIELD_PREP(ARM64_FEATURE_MASK(ID_AA64PFR0_EL1_CSV2), (u64)vcpu->kvm->arch.pfr0_csv2);
-		val &= ~ARM64_FEATURE_MASK(ID_AA64PFR0_EL1_CSV3);
-		val |= FIELD_PREP(ARM64_FEATURE_MASK(ID_AA64PFR0_EL1_CSV3), (u64)vcpu->kvm->arch.pfr0_csv3);
-		if (kvm_vgic_global_state.type == VGIC_V3) {
-			val &= ~ARM64_FEATURE_MASK(ID_AA64PFR0_EL1_GIC);
-			val |= FIELD_PREP(ARM64_FEATURE_MASK(ID_AA64PFR0_EL1_GIC), 1);
-		}
-		break;
 	case SYS_ID_AA64PFR1_EL1:
 		if (!kvm_has_mte(vcpu->kvm))
 			val &= ~ARM64_FEATURE_MASK(ID_AA64PFR1_EL1_MTE);
@@ -1253,22 +1326,6 @@ static u64 read_id_reg(const struct kvm_vcpu *vcpu, struct sys_reg_desc const *r
 		if (!cpus_have_final_cap(ARM64_HAS_WFXT))
 			val &= ~ARM64_FEATURE_MASK(ID_AA64ISAR2_EL1_WFxT);
 		break;
-	case SYS_ID_AA64DFR0_EL1:
-		/* Limit debug to ARMv8.0 */
-		val &= ~ARM64_FEATURE_MASK(ID_AA64DFR0_EL1_DebugVer);
-		val |= FIELD_PREP(ARM64_FEATURE_MASK(ID_AA64DFR0_EL1_DebugVer), 6);
-		/* Set PMUver to the required version */
-		val &= ~ARM64_FEATURE_MASK(ID_AA64DFR0_EL1_PMUVer);
-		val |= FIELD_PREP(ARM64_FEATURE_MASK(ID_AA64DFR0_EL1_PMUVer),
-				  vcpu_pmuver(vcpu));
-		/* Hide SPE from guests */
-		val &= ~ARM64_FEATURE_MASK(ID_AA64DFR0_EL1_PMSVer);
-		break;
-	case SYS_ID_DFR0_EL1:
-		val &= ~ARM64_FEATURE_MASK(ID_DFR0_EL1_PerfMon);
-		val |= FIELD_PREP(ARM64_FEATURE_MASK(ID_DFR0_EL1_PerfMon),
-				  pmuver_to_perfmon(vcpu_pmuver(vcpu)));
-		break;
 	case SYS_ID_AA64MMFR2_EL1:
 		val &= ~ID_AA64MMFR2_EL1_CCIDX_MASK;
 		break;
@@ -1280,6 +1337,28 @@ static u64 read_id_reg(const struct kvm_vcpu *vcpu, struct sys_reg_desc const *r
 	return val;
 }
 
+static u64 kvm_read_sanitised_id_reg(struct kvm_vcpu *vcpu,
+				     const struct sys_reg_desc *r)
+{
+	return __kvm_read_sanitised_id_reg(vcpu, r);
+}
+
+static u64 read_id_reg(const struct kvm_vcpu *vcpu, const struct sys_reg_desc *r)
+{
+	return IDREG(vcpu->kvm, reg_to_encoding(r));
+}
+
+/*
+ * Return true if the register's (Op0, Op1, CRn, CRm, Op2) is
+ * (3, 0, 0, crm, op2), where 1<=crm<8, 0<=op2<8.
+ */
+static inline bool is_id_reg(u32 id)
+{
+	return (sys_reg_Op0(id) == 3 && sys_reg_Op1(id) == 0 &&
+		sys_reg_CRn(id) == 0 && sys_reg_CRm(id) >= 1 &&
+		sys_reg_CRm(id) < 8);
+}
+
 static unsigned int id_visibility(const struct kvm_vcpu *vcpu,
 				  const struct sys_reg_desc *r)
 {
@@ -1341,88 +1420,113 @@ static unsigned int sve_visibility(const struct kvm_vcpu *vcpu,
 	return REG_HIDDEN;
 }
 
-static int set_id_aa64pfr0_el1(struct kvm_vcpu *vcpu,
-			       const struct sys_reg_desc *rd,
-			       u64 val)
+static u64 read_sanitised_id_aa64pfr0_el1(struct kvm_vcpu *vcpu,
+					  const struct sys_reg_desc *rd)
 {
-	u8 csv2, csv3;
+	u64 val = read_sanitised_ftr_reg(SYS_ID_AA64PFR0_EL1);
+
+	if (!vcpu_has_sve(vcpu))
+		val &= ~ID_AA64PFR0_EL1_SVE_MASK;
 
 	/*
-	 * Allow AA64PFR0_EL1.CSV2 to be set from userspace as long as
-	 * it doesn't promise more than what is actually provided (the
-	 * guest could otherwise be covered in ectoplasmic residue).
+	 * The default is to expose CSV2 == 1 if the HW isn't affected.
+	 * Although this is a per-CPU feature, we make it global because
+	 * asymmetric systems are just a nuisance.
+	 *
+	 * Userspace can override this as long as it doesn't promise
+	 * the impossible.
 	 */
-	csv2 = cpuid_feature_extract_unsigned_field(val, ID_AA64PFR0_EL1_CSV2_SHIFT);
-	if (csv2 > 1 ||
-	    (csv2 && arm64_get_spectre_v2_state() != SPECTRE_UNAFFECTED))
-		return -EINVAL;
+	if (arm64_get_spectre_v2_state() == SPECTRE_UNAFFECTED) {
+		val &= ~ID_AA64PFR0_EL1_CSV2_MASK;
+		val |= SYS_FIELD_PREP_ENUM(ID_AA64PFR0_EL1, CSV2, IMP);
+	}
+	if (arm64_get_meltdown_state() == SPECTRE_UNAFFECTED) {
+		val &= ~ID_AA64PFR0_EL1_CSV3_MASK;
+		val |= SYS_FIELD_PREP_ENUM(ID_AA64PFR0_EL1, CSV3, IMP);
+	}
 
-	/* Same thing for CSV3 */
-	csv3 = cpuid_feature_extract_unsigned_field(val, ID_AA64PFR0_EL1_CSV3_SHIFT);
-	if (csv3 > 1 ||
-	    (csv3 && arm64_get_meltdown_state() != SPECTRE_UNAFFECTED))
-		return -EINVAL;
+	if (kvm_vgic_global_state.type == VGIC_V3) {
+		val &= ~ID_AA64PFR0_EL1_GIC_MASK;
+		val |= SYS_FIELD_PREP_ENUM(ID_AA64PFR0_EL1, GIC, IMP);
+	}
 
-	/* We can only differ with CSV[23], and anything else is an error */
-	val ^= read_id_reg(vcpu, rd);
-	val &= ~(ARM64_FEATURE_MASK(ID_AA64PFR0_EL1_CSV2) |
-		 ARM64_FEATURE_MASK(ID_AA64PFR0_EL1_CSV3));
-	if (val)
-		return -EINVAL;
+	val &= ~ID_AA64PFR0_EL1_AMU_MASK;
 
-	vcpu->kvm->arch.pfr0_csv2 = csv2;
-	vcpu->kvm->arch.pfr0_csv3 = csv3;
+	return val;
+}
 
-	return 0;
+static u64 read_sanitised_id_aa64dfr0_el1(struct kvm_vcpu *vcpu,
+					  const struct sys_reg_desc *rd)
+{
+	u64 val = read_sanitised_ftr_reg(SYS_ID_AA64DFR0_EL1);
+
+	/* Limit debug to ARMv8.0 */
+	val &= ~ID_AA64DFR0_EL1_DebugVer_MASK;
+	val |= SYS_FIELD_PREP_ENUM(ID_AA64DFR0_EL1, DebugVer, IMP);
+
+	/*
+	 * Only initialize the PMU version if the vCPU was configured with one.
+	 */
+	val &= ~ID_AA64DFR0_EL1_PMUVer_MASK;
+	if (kvm_vcpu_has_pmu(vcpu))
+		val |= SYS_FIELD_PREP(ID_AA64DFR0_EL1, PMUVer,
+				      kvm_arm_pmu_get_pmuver_limit());
+
+	/* Hide SPE from guests */
+	val &= ~ID_AA64DFR0_EL1_PMSVer_MASK;
+
+	return val;
 }
 
 static int set_id_aa64dfr0_el1(struct kvm_vcpu *vcpu,
 			       const struct sys_reg_desc *rd,
 			       u64 val)
 {
-	u8 pmuver, host_pmuver;
-	bool valid_pmu;
-
-	host_pmuver = kvm_arm_pmu_get_pmuver_limit();
+	u8 pmuver = SYS_FIELD_GET(ID_AA64DFR0_EL1, PMUVer, val);
 
 	/*
-	 * Allow AA64DFR0_EL1.PMUver to be set from userspace as long
-	 * as it doesn't promise more than what the HW gives us. We
-	 * allow an IMPDEF PMU though, only if no PMU is supported
-	 * (KVM backward compatibility handling).
+	 * Prior to commit 3d0dba5764b9 ("KVM: arm64: PMU: Move the
+	 * ID_AA64DFR0_EL1.PMUver limit to VM creation"), KVM erroneously
+	 * exposed an IMP_DEF PMU to userspace and the guest on systems w/
+	 * non-architectural PMUs. Of course, PMUv3 is the only game in town for
+	 * PMU virtualization, so the IMP_DEF value was rather user-hostile.
+	 *
+	 * At minimum, we're on the hook to allow values that were given to
+	 * userspace by KVM. Cover our tracks here and replace the IMP_DEF value
+	 * with a more sensible NI. The value of an ID register changing under
+	 * the nose of the guest is unfortunate, but is certainly no more
+	 * surprising than an ill-guided PMU driver poking at impdef system
+	 * registers that end in an UNDEF...
 	 */
-	pmuver = FIELD_GET(ARM64_FEATURE_MASK(ID_AA64DFR0_EL1_PMUVer), val);
-	if ((pmuver != ID_AA64DFR0_EL1_PMUVer_IMP_DEF && pmuver > host_pmuver))
-		return -EINVAL;
+	if (pmuver == ID_AA64DFR0_EL1_PMUVer_IMP_DEF)
+		val &= ~ID_AA64DFR0_EL1_PMUVer_MASK;
 
-	valid_pmu = (pmuver != 0 && pmuver != ID_AA64DFR0_EL1_PMUVer_IMP_DEF);
-
-	/* Make sure view register and PMU support do match */
-	if (kvm_vcpu_has_pmu(vcpu) != valid_pmu)
-		return -EINVAL;
+	return set_id_reg(vcpu, rd, val);
+}
 
-	/* We can only differ with PMUver, and anything else is an error */
-	val ^= read_id_reg(vcpu, rd);
-	val &= ~ARM64_FEATURE_MASK(ID_AA64DFR0_EL1_PMUVer);
-	if (val)
-		return -EINVAL;
+static u64 read_sanitised_id_dfr0_el1(struct kvm_vcpu *vcpu,
+				      const struct sys_reg_desc *rd)
+{
+	u8 perfmon = pmuver_to_perfmon(kvm_arm_pmu_get_pmuver_limit());
+	u64 val = read_sanitised_ftr_reg(SYS_ID_DFR0_EL1);
 
-	if (valid_pmu)
-		vcpu->kvm->arch.dfr0_pmuver.imp = pmuver;
-	else
-		vcpu->kvm->arch.dfr0_pmuver.unimp = pmuver;
+	val &= ~ID_DFR0_EL1_PerfMon_MASK;
+	if (kvm_vcpu_has_pmu(vcpu))
+		val |= SYS_FIELD_PREP(ID_DFR0_EL1, PerfMon, perfmon);
 
-	return 0;
+	return val;
 }
 
 static int set_id_dfr0_el1(struct kvm_vcpu *vcpu,
 			   const struct sys_reg_desc *rd,
 			   u64 val)
 {
-	u8 perfmon, host_perfmon;
-	bool valid_pmu;
+	u8 perfmon = SYS_FIELD_GET(ID_DFR0_EL1, PerfMon, val);
 
-	host_perfmon = pmuver_to_perfmon(kvm_arm_pmu_get_pmuver_limit());
+	if (perfmon == ID_DFR0_EL1_PerfMon_IMPDEF) {
+		val &= ~ID_DFR0_EL1_PerfMon_MASK;
+		perfmon = 0;
+	}
 
 	/*
 	 * Allow DFR0_EL1.PerfMon to be set from userspace as long as
@@ -1430,29 +1534,10 @@ static int set_id_dfr0_el1(struct kvm_vcpu *vcpu,
 	 * AArch64 side (as everything is emulated with that), and
 	 * that this is a PMUv3.
 	 */
-	perfmon = FIELD_GET(ARM64_FEATURE_MASK(ID_DFR0_EL1_PerfMon), val);
-	if ((perfmon != ID_DFR0_EL1_PerfMon_IMPDEF && perfmon > host_perfmon) ||
-	    (perfmon != 0 && perfmon < ID_DFR0_EL1_PerfMon_PMUv3))
+	if (perfmon != 0 && perfmon < ID_DFR0_EL1_PerfMon_PMUv3)
 		return -EINVAL;
 
-	valid_pmu = (perfmon != 0 && perfmon != ID_DFR0_EL1_PerfMon_IMPDEF);
-
-	/* Make sure view register and PMU support do match */
-	if (kvm_vcpu_has_pmu(vcpu) != valid_pmu)
-		return -EINVAL;
-
-	/* We can only differ with PerfMon, and anything else is an error */
-	val ^= read_id_reg(vcpu, rd);
-	val &= ~ARM64_FEATURE_MASK(ID_DFR0_EL1_PerfMon);
-	if (val)
-		return -EINVAL;
-
-	if (valid_pmu)
-		vcpu->kvm->arch.dfr0_pmuver.imp = perfmon_to_pmuver(perfmon);
-	else
-		vcpu->kvm->arch.dfr0_pmuver.unimp = perfmon_to_pmuver(perfmon);
-
-	return 0;
+	return set_id_reg(vcpu, rd, val);
 }
 
 /*
@@ -1465,18 +1550,60 @@ static int set_id_dfr0_el1(struct kvm_vcpu *vcpu,
 static int get_id_reg(struct kvm_vcpu *vcpu, const struct sys_reg_desc *rd,
 		      u64 *val)
 {
+	/*
+	 * Avoid locking if the VM has already started, as the ID registers are
+	 * guaranteed to be invariant at that point.
+	 */
+	if (kvm_vm_has_ran_once(vcpu->kvm)) {
+		*val = read_id_reg(vcpu, rd);
+		return 0;
+	}
+
+	mutex_lock(&vcpu->kvm->arch.config_lock);
 	*val = read_id_reg(vcpu, rd);
+	mutex_unlock(&vcpu->kvm->arch.config_lock);
+
 	return 0;
 }
 
 static int set_id_reg(struct kvm_vcpu *vcpu, const struct sys_reg_desc *rd,
 		      u64 val)
 {
-	/* This is what we mean by invariant: you can't change it. */
-	if (val != read_id_reg(vcpu, rd))
-		return -EINVAL;
+	u32 id = reg_to_encoding(rd);
+	int ret;
 
-	return 0;
+	mutex_lock(&vcpu->kvm->arch.config_lock);
+
+	/*
+	 * Once the VM has started the ID registers are immutable. Reject any
+	 * write that does not match the final register value.
+	 */
+	if (kvm_vm_has_ran_once(vcpu->kvm)) {
+		if (val != read_id_reg(vcpu, rd))
+			ret = -EBUSY;
+		else
+			ret = 0;
+
+		mutex_unlock(&vcpu->kvm->arch.config_lock);
+		return ret;
+	}
+
+	ret = arm64_check_features(vcpu, rd, val);
+	if (!ret)
+		IDREG(vcpu->kvm, id) = val;
+
+	mutex_unlock(&vcpu->kvm->arch.config_lock);
+
+	/*
+	 * arm64_check_features() returns -E2BIG to indicate the register's
+	 * feature set is a superset of the maximally-allowed register value.
+	 * While it would be nice to precisely describe this to userspace, the
+	 * existing UAPI for KVM_SET_ONE_REG has it that invalid register
+	 * writes return -EINVAL.
+	 */
+	if (ret == -E2BIG)
+		ret = -EINVAL;
+	return ret;
 }
 
 static int get_raz_reg(struct kvm_vcpu *vcpu, const struct sys_reg_desc *rd,
@@ -1516,7 +1643,7 @@ static bool access_clidr(struct kvm_vcpu *vcpu, struct sys_reg_params *p,
  * Fabricate a CLIDR_EL1 value instead of using the real value, which can vary
  * by the physical CPU which the vcpu currently resides in.
  */
-static void reset_clidr(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r)
+static u64 reset_clidr(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r)
 {
 	u64 ctr_el0 = read_sanitised_ftr_reg(SYS_CTR_EL0);
 	u64 clidr;
@@ -1564,6 +1691,8 @@ static void reset_clidr(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r)
 		clidr |= 2 << CLIDR_TTYPE_SHIFT(loc);
 
 	__vcpu_sys_reg(vcpu, r->reg) = clidr;
+
+	return __vcpu_sys_reg(vcpu, r->reg);
 }
 
 static int set_clidr(struct kvm_vcpu *vcpu, const struct sys_reg_desc *rd,
@@ -1663,6 +1792,17 @@ static unsigned int elx2_visibility(const struct kvm_vcpu *vcpu,
 	.visibility = elx2_visibility,		\
 }
 
+/*
+ * Since reset() callback and field val are not used for idregs, they will be
+ * used for specific purposes for idregs.
+ * The reset() would return KVM sanitised register value. The value would be the
+ * same as the host kernel sanitised value if there is no KVM sanitisation.
+ * The val would be used as a mask indicating writable fields for the idreg.
+ * Only bits with 1 are writable from userspace. This mask might not be
+ * necessary in the future whenever all ID registers are enabled as writable
+ * from userspace.
+ */
+
 /* sys_reg_desc initialiser for known cpufeature ID registers */
 #define ID_SANITISED(name) {			\
 	SYS_DESC(SYS_##name),			\
@@ -1670,6 +1810,8 @@ static unsigned int elx2_visibility(const struct kvm_vcpu *vcpu,
 	.get_user = get_id_reg,			\
 	.set_user = set_id_reg,			\
 	.visibility = id_visibility,		\
+	.reset = kvm_read_sanitised_id_reg,	\
+	.val = 0,				\
 }
 
 /* sys_reg_desc initialiser for known cpufeature ID registers */
@@ -1679,6 +1821,8 @@ static unsigned int elx2_visibility(const struct kvm_vcpu *vcpu,
 	.get_user = get_id_reg,			\
 	.set_user = set_id_reg,			\
 	.visibility = aa32_id_visibility,	\
+	.reset = kvm_read_sanitised_id_reg,	\
+	.val = 0,				\
 }
 
 /*
@@ -1691,7 +1835,9 @@ static unsigned int elx2_visibility(const struct kvm_vcpu *vcpu,
 	.access = access_id_reg,			\
 	.get_user = get_id_reg,				\
 	.set_user = set_id_reg,				\
-	.visibility = raz_visibility			\
+	.visibility = raz_visibility,			\
+	.reset = kvm_read_sanitised_id_reg,		\
+	.val = 0,					\
 }
 
 /*
@@ -1705,6 +1851,8 @@ static unsigned int elx2_visibility(const struct kvm_vcpu *vcpu,
 	.get_user = get_id_reg,			\
 	.set_user = set_id_reg,			\
 	.visibility = raz_visibility,		\
+	.reset = kvm_read_sanitised_id_reg,	\
+	.val = 0,				\
 }
 
 static bool access_sp_el1(struct kvm_vcpu *vcpu,
@@ -1806,9 +1954,13 @@ static const struct sys_reg_desc sys_reg_descs[] = {
 	/* CRm=1 */
 	AA32_ID_SANITISED(ID_PFR0_EL1),
 	AA32_ID_SANITISED(ID_PFR1_EL1),
-	{ SYS_DESC(SYS_ID_DFR0_EL1), .access = access_id_reg,
-	  .get_user = get_id_reg, .set_user = set_id_dfr0_el1,
-	  .visibility = aa32_id_visibility, },
+	{ SYS_DESC(SYS_ID_DFR0_EL1),
+	  .access = access_id_reg,
+	  .get_user = get_id_reg,
+	  .set_user = set_id_dfr0_el1,
+	  .visibility = aa32_id_visibility,
+	  .reset = read_sanitised_id_dfr0_el1,
+	  .val = ID_DFR0_EL1_PerfMon_MASK, },
 	ID_HIDDEN(ID_AFR0_EL1),
 	AA32_ID_SANITISED(ID_MMFR0_EL1),
 	AA32_ID_SANITISED(ID_MMFR1_EL1),
@@ -1837,8 +1989,12 @@ static const struct sys_reg_desc sys_reg_descs[] = {
 
 	/* AArch64 ID registers */
 	/* CRm=4 */
-	{ SYS_DESC(SYS_ID_AA64PFR0_EL1), .access = access_id_reg,
-	  .get_user = get_id_reg, .set_user = set_id_aa64pfr0_el1, },
+	{ SYS_DESC(SYS_ID_AA64PFR0_EL1),
+	  .access = access_id_reg,
+	  .get_user = get_id_reg,
+	  .set_user = set_id_reg,
+	  .reset = read_sanitised_id_aa64pfr0_el1,
+	  .val = ID_AA64PFR0_EL1_CSV2_MASK | ID_AA64PFR0_EL1_CSV3_MASK, },
 	ID_SANITISED(ID_AA64PFR1_EL1),
 	ID_UNALLOCATED(4,2),
 	ID_UNALLOCATED(4,3),
@@ -1848,8 +2004,12 @@ static const struct sys_reg_desc sys_reg_descs[] = {
 	ID_UNALLOCATED(4,7),
 
 	/* CRm=5 */
-	{ SYS_DESC(SYS_ID_AA64DFR0_EL1), .access = access_id_reg,
-	  .get_user = get_id_reg, .set_user = set_id_aa64dfr0_el1, },
+	{ SYS_DESC(SYS_ID_AA64DFR0_EL1),
+	  .access = access_id_reg,
+	  .get_user = get_id_reg,
+	  .set_user = set_id_aa64dfr0_el1,
+	  .reset = read_sanitised_id_aa64dfr0_el1,
+	  .val = ID_AA64DFR0_EL1_PMUVer_MASK, },
 	ID_SANITISED(ID_AA64DFR1_EL1),
 	ID_UNALLOCATED(5,2),
 	ID_UNALLOCATED(5,3),
@@ -2237,6 +2397,8 @@ static const struct sys_reg_desc sys_reg_descs[] = {
 	EL2_REG(SP_EL2, NULL, reset_unknown, 0),
 };
 
+static const struct sys_reg_desc *first_idreg;
+
 static bool trap_dbgdidr(struct kvm_vcpu *vcpu,
 			struct sys_reg_params *p,
 			const struct sys_reg_desc *r)
@@ -2927,6 +3089,28 @@ static bool emulate_sys_reg(struct kvm_vcpu *vcpu,
 	return false;
 }
 
+static void kvm_reset_id_regs(struct kvm_vcpu *vcpu)
+{
+	const struct sys_reg_desc *idreg = first_idreg;
+	u32 id = reg_to_encoding(idreg);
+	struct kvm *kvm = vcpu->kvm;
+
+	if (test_bit(KVM_ARCH_FLAG_ID_REGS_INITIALIZED, &kvm->arch.flags))
+		return;
+
+	lockdep_assert_held(&kvm->arch.config_lock);
+
+	/* Initialize all idregs */
+	while (is_id_reg(id)) {
+		IDREG(kvm, id) = idreg->reset(vcpu, idreg);
+
+		idreg++;
+		id = reg_to_encoding(idreg);
+	}
+
+	set_bit(KVM_ARCH_FLAG_ID_REGS_INITIALIZED, &kvm->arch.flags);
+}
+
 /**
  * kvm_reset_sys_regs - sets system registers to reset value
  * @vcpu: The VCPU pointer
@@ -2938,9 +3122,17 @@ void kvm_reset_sys_regs(struct kvm_vcpu *vcpu)
 {
 	unsigned long i;
 
-	for (i = 0; i < ARRAY_SIZE(sys_reg_descs); i++)
-		if (sys_reg_descs[i].reset)
-			sys_reg_descs[i].reset(vcpu, &sys_reg_descs[i]);
+	kvm_reset_id_regs(vcpu);
+
+	for (i = 0; i < ARRAY_SIZE(sys_reg_descs); i++) {
+		const struct sys_reg_desc *r = &sys_reg_descs[i];
+
+		if (is_id_reg(reg_to_encoding(r)))
+			continue;
+
+		if (r->reset)
+			r->reset(vcpu, r);
+	}
 }
 
 /**
@@ -3041,19 +3233,21 @@ id_to_sys_reg_desc(struct kvm_vcpu *vcpu, u64 id,
  */
 
 #define FUNCTION_INVARIANT(reg)						\
-	static void get_##reg(struct kvm_vcpu *v,			\
+	static u64 get_##reg(struct kvm_vcpu *v,			\
 			      const struct sys_reg_desc *r)		\
 	{								\
 		((struct sys_reg_desc *)r)->val = read_sysreg(reg);	\
+		return ((struct sys_reg_desc *)r)->val;			\
 	}
 
 FUNCTION_INVARIANT(midr_el1)
 FUNCTION_INVARIANT(revidr_el1)
 FUNCTION_INVARIANT(aidr_el1)
 
-static void get_ctr_el0(struct kvm_vcpu *v, const struct sys_reg_desc *r)
+static u64 get_ctr_el0(struct kvm_vcpu *v, const struct sys_reg_desc *r)
 {
 	((struct sys_reg_desc *)r)->val = read_sanitised_ftr_reg(SYS_CTR_EL0);
+	return ((struct sys_reg_desc *)r)->val;
 }
 
 /* ->val is filled in by kvm_sys_reg_table_init() */
@@ -3345,6 +3539,7 @@ int kvm_arm_copy_sys_reg_indices(struct kvm_vcpu *vcpu, u64 __user *uindices)
 
 int __init kvm_sys_reg_table_init(void)
 {
+	struct sys_reg_params params;
 	bool valid = true;
 	unsigned int i;
 
@@ -3363,5 +3558,11 @@ int __init kvm_sys_reg_table_init(void)
 	for (i = 0; i < ARRAY_SIZE(invariant_sys_regs); i++)
 		invariant_sys_regs[i].reset(NULL, &invariant_sys_regs[i]);
 
+	/* Find the first idreg (SYS_ID_PFR0_EL1) in sys_reg_descs. */
+	params = encoding_to_params(SYS_ID_PFR0_EL1);
+	first_idreg = find_reg(&params, sys_reg_descs, ARRAY_SIZE(sys_reg_descs));
+	if (!first_idreg)
+		return -EINVAL;
+
 	return 0;
 }
diff --git a/arch/arm64/kvm/sys_regs.h b/arch/arm64/kvm/sys_regs.h
index 6b11f2cc7146..c65c129b3500 100644
--- a/arch/arm64/kvm/sys_regs.h
+++ b/arch/arm64/kvm/sys_regs.h
@@ -27,6 +27,13 @@ struct sys_reg_params {
 	bool	is_write;
 };
 
+#define encoding_to_params(reg)						\
+	((struct sys_reg_params){ .Op0 = sys_reg_Op0(reg),		\
+				  .Op1 = sys_reg_Op1(reg),		\
+				  .CRn = sys_reg_CRn(reg),		\
+				  .CRm = sys_reg_CRm(reg),		\
+				  .Op2 = sys_reg_Op2(reg) })
+
 #define esr_sys64_to_params(esr)                                               \
 	((struct sys_reg_params){ .Op0 = ((esr) >> 20) & 3,                    \
 				  .Op1 = ((esr) >> 14) & 0x7,                  \
@@ -64,13 +71,16 @@ struct sys_reg_desc {
 		       struct sys_reg_params *,
 		       const struct sys_reg_desc *);
 
-	/* Initialization for vcpu. */
-	void (*reset)(struct kvm_vcpu *, const struct sys_reg_desc *);
+	/*
+	 * Initialization for vcpu. Return initialized value, or KVM
+	 * sanitized value for ID registers.
+	 */
+	u64 (*reset)(struct kvm_vcpu *, const struct sys_reg_desc *);
 
 	/* Index into sys_reg[], or 0 if we don't need to save it. */
 	int reg;
 
-	/* Value (usually reset value) */
+	/* Value (usually reset value), or write mask for idregs */
 	u64 val;
 
 	/* Custom get/set_user functions, fallback to generic if NULL */
@@ -123,19 +133,21 @@ static inline bool read_zero(struct kvm_vcpu *vcpu,
 }
 
 /* Reset functions */
-static inline void reset_unknown(struct kvm_vcpu *vcpu,
+static inline u64 reset_unknown(struct kvm_vcpu *vcpu,
 				 const struct sys_reg_desc *r)
 {
 	BUG_ON(!r->reg);
 	BUG_ON(r->reg >= NR_SYS_REGS);
 	__vcpu_sys_reg(vcpu, r->reg) = 0x1de7ec7edbadc0deULL;
+	return __vcpu_sys_reg(vcpu, r->reg);
 }
 
-static inline void reset_val(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r)
+static inline u64 reset_val(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r)
 {
 	BUG_ON(!r->reg);
 	BUG_ON(r->reg >= NR_SYS_REGS);
 	__vcpu_sys_reg(vcpu, r->reg) = r->val;
+	return __vcpu_sys_reg(vcpu, r->reg);
 }
 
 static inline unsigned int sysreg_visibility(const struct kvm_vcpu *vcpu,