KVM: arm64: Handle ID register reads using the VM-wide values

Everything is in place now to use the generic ID register
infrastructure. Use the VM-wide values to service ID register reads.
The ID registers are invariant after the VM has started, so there is no
need for locking in that case. This is rather desirable for VM live
migration, as the needless lock contention could prolong the VM blackout
period.

Link: https://lore.kernel.org/r/20230609190054.1542113-11-oliver.upton@linux.dev
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>

Author: oupton

arch/arm64/kvm/sys_regs.c

@@ -1395,7 +1395,7 @@ static u64 kvm_read_sanitised_id_reg(struct kvm_vcpu *vcpu,
 
 static u64 read_id_reg(const struct kvm_vcpu *vcpu, const struct sys_reg_desc *r)
 {
-	return __kvm_read_sanitised_id_reg(vcpu, r);
+	return IDREG(vcpu->kvm, reg_to_encoding(r));
 }
 
 /*
@@ -1634,7 +1634,19 @@ 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;
 }