KVM: PPC: Book3S PR: Manage single-step mode

Until now, when we connect gdb to the QEMU gdb-server, the
single-step mode is not managed.

This patch adds this, only for kvm-pr:

If KVM_GUESTDBG_SINGLESTEP is set, we enable single-step trace bit in the
MSR (MSR_SE) just before the __kvmppc_vcpu_run(), and disable it just after.
In kvmppc_handle_exit_pr, instead of routing the interrupt to
the guest, we return to host, with KVM_EXIT_DEBUG reason.
Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/kvm/book3s_pr.c

@@ -882,6 +882,24 @@ void kvmppc_set_fscr(struct kvm_vcpu *vcpu, u64 fscr)
 }
 #endif
 
+static void kvmppc_setup_debug(struct kvm_vcpu *vcpu)
+{
+	if (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP) {
+		u64 msr = kvmppc_get_msr(vcpu);
+
+		kvmppc_set_msr(vcpu, msr | MSR_SE);
+	}
+}
+
+static void kvmppc_clear_debug(struct kvm_vcpu *vcpu)
+{
+	if (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP) {
+		u64 msr = kvmppc_get_msr(vcpu);
+
+		kvmppc_set_msr(vcpu, msr & ~MSR_SE);
+	}
+}
+
 int kvmppc_handle_exit_pr(struct kvm_run *run, struct kvm_vcpu *vcpu,
 			  unsigned int exit_nr)
 {
@@ -1207,10 +1225,18 @@ int kvmppc_handle_exit_pr(struct kvm_run *run, struct kvm_vcpu *vcpu,
 		break;
 #endif
 	case BOOK3S_INTERRUPT_MACHINE_CHECK:
-	case BOOK3S_INTERRUPT_TRACE:
 		kvmppc_book3s_queue_irqprio(vcpu, exit_nr);
 		r = RESUME_GUEST;
 		break;
+	case BOOK3S_INTERRUPT_TRACE:
+		if (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP) {
+			run->exit_reason = KVM_EXIT_DEBUG;
+			r = RESUME_HOST;
+		} else {
+			kvmppc_book3s_queue_irqprio(vcpu, exit_nr);
+			r = RESUME_GUEST;
+		}
+		break;
 	default:
 	{
 		ulong shadow_srr1 = vcpu->arch.shadow_srr1;
@@ -1479,6 +1505,8 @@ static int kvmppc_vcpu_run_pr(struct kvm_run *kvm_run, struct kvm_vcpu *vcpu)
 		goto out;
 	}
 
+	kvmppc_setup_debug(vcpu);
+
 	/*
 	 * Interrupts could be timers for the guest which we have to inject
 	 * again, so let's postpone them until we're in the guest and if we
@@ -1501,6 +1529,8 @@ static int kvmppc_vcpu_run_pr(struct kvm_run *kvm_run, struct kvm_vcpu *vcpu)
 
 	ret = __kvmppc_vcpu_run(kvm_run, vcpu);
 
+	kvmppc_clear_debug(vcpu);
+
 	/* No need for kvm_guest_exit. It's done in handle_exit.
 	   We also get here with interrupts enabled. */