KVM: PPC: Book3S PR: Take SRCU read lock around RTAS kvm_read_guest() call

commit ef1af2e2 upstream.

This does for PR KVM what c9438092 ("KVM: PPC: Book3S HV: Take SRCU
read lock around kvm_read_guest() call") did for HV KVM, that is,
eliminate a "suspicious rcu_dereference_check() usage!" warning by
taking the SRCU lock around the call to kvmppc_rtas_hcall().

It also fixes a return of RESUME_HOST to return EMULATE_FAIL instead,
since kvmppc_h_pr() is supposed to return EMULATE_* values.
Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>

arch/powerpc/kvm/book3s_pr_papr.c

@@ -258,6 +258,8 @@ static int kvmppc_h_pr_xics_hcall(struct kvm_vcpu *vcpu, u32 cmd)
 
 int kvmppc_h_pr(struct kvm_vcpu *vcpu, unsigned long cmd)
 {
+	int rc, idx;
+
 	switch (cmd) {
 	case H_ENTER:
 		return kvmppc_h_pr_enter(vcpu);
@@ -286,8 +288,11 @@ int kvmppc_h_pr(struct kvm_vcpu *vcpu, unsigned long cmd)
 		break;
 	case H_RTAS:
 		if (list_empty(&vcpu->kvm->arch.rtas_tokens))
-			return RESUME_HOST;
-		if (kvmppc_rtas_hcall(vcpu))
+			break;
+		idx = srcu_read_lock(&vcpu->kvm->srcu);
+		rc = kvmppc_rtas_hcall(vcpu);
+		srcu_read_unlock(&vcpu->kvm->srcu, idx);
+		if (rc)
 			break;
 		kvmppc_set_gpr(vcpu, 3, 0);
 		return EMULATE_DONE;