[PATCH v4 2/6] KVM: s390: interrupt: Fix single-stepping into program interrupt handlers

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

Currently, after single-stepping an instruction that generates a
specification exception, GDB ends up on the instruction immediately
following it.

The reason is that vcpu_post_run() injects the interrupt and sets
KVM_GUESTDBG_EXIT_PENDING, causing a KVM_SINGLESTEP exit. The
interrupt is not delivered, however, therefore userspace sees the
address of the next instruction.

Fix by letting the __vcpu_run() loop go into the next iteration,
where vcpu_pre_run() delivers the interrupt and sets
KVM_GUESTDBG_EXIT_PENDING.

Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
---
 arch/s390/kvm/intercept.c | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/arch/s390/kvm/intercept.c b/arch/s390/kvm/intercept.c
index 954d39adf85c..e54496740859 100644
--- a/arch/s390/kvm/intercept.c
+++ b/arch/s390/kvm/intercept.c
@@ -228,6 +228,21 @@ static int handle_itdb(struct kvm_vcpu *vcpu)
 
 #define per_event(vcpu) (vcpu->arch.sie_block->iprcc & PGM_PER)
 
+static bool should_handle_per_event(const struct kvm_vcpu *vcpu)
+{
+	if (!guestdbg_enabled(vcpu) || !per_event(vcpu))
+		return false;
+	if (guestdbg_sstep_enabled(vcpu) &&
+	    vcpu->arch.sie_block->iprcc != PGM_PER) {
+		/*
+		 * __vcpu_run() will exit after delivering the concurrently
+		 * indicated condition.
+		 */
+		return false;
+	}
+	return true;
+}
+
 static int handle_prog(struct kvm_vcpu *vcpu)
 {
 	psw_t psw;
@@ -242,7 +257,7 @@ static int handle_prog(struct kvm_vcpu *vcpu)
 	if (kvm_s390_pv_cpu_is_protected(vcpu))
 		return -EOPNOTSUPP;
 
-	if (guestdbg_enabled(vcpu) && per_event(vcpu)) {
+	if (should_handle_per_event(vcpu)) {
 		rc = kvm_s390_handle_per_event(vcpu);
 		if (rc)
 			return rc;
-- 
2.41.0