[PATCH v4 3/6] KVM: s390: interrupt: Fix single-stepping kernel-emulated instructions

[Ilya Leoshkevich <iii@linux.ibm.com>]

Single-stepping a kernel-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 kvm_handle_sie_intercept(), after injecting the
interrupt, also processes the PER event and arranges a KVM_SINGLESTEP
exit. The interrupt is not yet delivered, however, so the userspace
sees the next instruction.

Fix by avoiding the KVM_SINGLESTEP exit when there is a pending
interrupt. The next __vcpu_run() loop iteration will arrange a
KVM_SINGLESTEP exit after delivering the interrupt.

Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
---
 arch/s390/kvm/intercept.c | 17 ++++++++++++++---
 1 file changed, 14 insertions(+), 3 deletions(-)

diff --git a/arch/s390/kvm/intercept.c b/arch/s390/kvm/intercept.c
index e54496740859..db222c749e5e 100644
--- a/arch/s390/kvm/intercept.c
+++ b/arch/s390/kvm/intercept.c
@@ -583,6 +583,19 @@ static int handle_pv_notification(struct kvm_vcpu *vcpu)
 	return handle_instruction(vcpu);
 }
 
+static bool should_handle_per_ifetch(const struct kvm_vcpu *vcpu, int rc)
+{
+	/* Process PER, also if the instruction is processed in user space. */
+	if (!(vcpu->arch.sie_block->icptstatus & 0x02))
+		return false;
+	if (rc != 0 && rc != -EOPNOTSUPP)
+		return false;
+	if (guestdbg_sstep_enabled(vcpu) && vcpu->arch.local_int.pending_irqs)
+		/* __vcpu_run() will exit after delivering the interrupt. */
+		return false;
+	return true;
+}
+
 int kvm_handle_sie_intercept(struct kvm_vcpu *vcpu)
 {
 	int rc, per_rc = 0;
@@ -645,9 +658,7 @@ int kvm_handle_sie_intercept(struct kvm_vcpu *vcpu)
 		return -EOPNOTSUPP;
 	}
 
-	/* process PER, also if the instruction is processed in user space */
-	if (vcpu->arch.sie_block->icptstatus & 0x02 &&
-	    (!rc || rc == -EOPNOTSUPP))
+	if (should_handle_per_ifetch(vcpu, rc))
 		per_rc = kvm_s390_handle_per_ifetch_icpt(vcpu);
 	return per_rc ? per_rc : rc;
 }
-- 
2.41.0