KVM: s390: interrupt: Fix single-stepping into interrupt handlers

After single-stepping an instruction that generates an interrupt, GDB
ends up on the second instruction of the respective interrupt handler.

The reason is that vcpu_pre_run() manually delivers the interrupt, and
then __vcpu_run() runs the first handler instruction using the
CPUSTAT_P flag. This causes a KVM_SINGLESTEP exit on the second handler
instruction.

Fix by delaying the KVM_SINGLESTEP exit until after the manual
interrupt delivery.

Acked-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Message-ID: <20230725143857.228626-2-iii@linux.ibm.com>
Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Signed-off-by: Janosch Frank <frankja@linux.ibm.com>

Author: iii-i

arch/s390/kvm/interrupt.c

@@ -1392,6 +1392,7 @@ int __must_check kvm_s390_deliver_pending_interrupts(struct kvm_vcpu *vcpu)
 {
 	struct kvm_s390_local_interrupt *li = &vcpu->arch.local_int;
 	int rc = 0;
+	bool delivered = false;
 	unsigned long irq_type;
 	unsigned long irqs;
 
@@ -1465,6 +1466,19 @@ int __must_check kvm_s390_deliver_pending_interrupts(struct kvm_vcpu *vcpu)
 			WARN_ONCE(1, "Unknown pending irq type %ld", irq_type);
 			clear_bit(irq_type, &li->pending_irqs);
 		}
+		delivered |= !rc;
+	}
+
+	/*
+	 * We delivered at least one interrupt and modified the PC. Force a
+	 * singlestep event now.
+	 */
+	if (delivered && guestdbg_sstep_enabled(vcpu)) {
+		struct kvm_debug_exit_arch *debug_exit = &vcpu->run->debug.arch;
+
+		debug_exit->addr = vcpu->arch.sie_block->gpsw.addr;
+		debug_exit->type = KVM_SINGLESTEP;
+		vcpu->guest_debug |= KVM_GUESTDBG_EXIT_PENDING;
 	}
 
 	set_intercept_indicators(vcpu);

arch/s390/kvm/kvm-s390.c

@@ -4611,7 +4611,7 @@ static int vcpu_pre_run(struct kvm_vcpu *vcpu)
 
 	if (!kvm_is_ucontrol(vcpu->kvm)) {
 		rc = kvm_s390_deliver_pending_interrupts(vcpu);
-		if (rc)
+		if (rc || guestdbg_exit_pending(vcpu))
 			return rc;
 	}
 
@@ -4738,7 +4738,7 @@ static int __vcpu_run(struct kvm_vcpu *vcpu)
 
 	do {
 		rc = vcpu_pre_run(vcpu);
-		if (rc)
+		if (rc || guestdbg_exit_pending(vcpu))
 			break;
 
 		kvm_vcpu_srcu_read_unlock(vcpu);