KVM: s390: interrupt: Fix single-stepping userspace-emulated instructions

Single-stepping a userspace-emulated instruction that generates an
interrupt causes GDB to land on the instruction following it instead of
the respective interrupt handler.

The reason is that after arranging a KVM_EXIT_S390_SIEIC exit,
kvm_handle_sie_intercept() calls kvm_s390_handle_per_ifetch_icpt(),
which sets KVM_GUESTDBG_EXIT_PENDING. This bit, however, is not
processed immediately, but rather persists until the next ioctl(),
causing a spurious single-step exit.

Fix by clearing this bit in ioctl().

Reviewed-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: <20230725143857.228626-5-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/kvm-s390.c

@@ -5383,14 +5383,16 @@ long kvm_arch_vcpu_async_ioctl(struct file *filp,
 {
 	struct kvm_vcpu *vcpu = filp->private_data;
 	void __user *argp = (void __user *)arg;
+	int rc;
 
 	switch (ioctl) {
 	case KVM_S390_IRQ: {
 		struct kvm_s390_irq s390irq;
 
 		if (copy_from_user(&s390irq, argp, sizeof(s390irq)))
 			return -EFAULT;
-		return kvm_s390_inject_vcpu(vcpu, &s390irq);
+		rc = kvm_s390_inject_vcpu(vcpu, &s390irq);
+		break;
 	}
 	case KVM_S390_INTERRUPT: {
 		struct kvm_s390_interrupt s390int;
@@ -5400,10 +5402,25 @@ long kvm_arch_vcpu_async_ioctl(struct file *filp,
 			return -EFAULT;
 		if (s390int_to_s390irq(&s390int, &s390irq))
 			return -EINVAL;
-		return kvm_s390_inject_vcpu(vcpu, &s390irq);
+		rc = kvm_s390_inject_vcpu(vcpu, &s390irq);
+		break;
 	}
+	default:
+		rc = -ENOIOCTLCMD;
+		break;
 	}
-	return -ENOIOCTLCMD;
+
+	/*
+	 * To simplify single stepping of userspace-emulated instructions,
+	 * KVM_EXIT_S390_SIEIC exit sets KVM_GUESTDBG_EXIT_PENDING (see
+	 * should_handle_per_ifetch()). However, if userspace emulation injects
+	 * an interrupt, it needs to be cleared, so that KVM_EXIT_DEBUG happens
+	 * after (and not before) the interrupt delivery.
+	 */
+	if (!rc)
+		vcpu->guest_debug &= ~KVM_GUESTDBG_EXIT_PENDING;
+
+	return rc;
 }
 
 static int kvm_s390_handle_pv_vcpu_dump(struct kvm_vcpu *vcpu,