[PATCH] KVM: s390: pci: Fix handling of AIF enable without AISB

[Matthew Rosato <mjrosato@linux.ibm.com>]

When a guest seeks to register IRQs without a summary bit specified,
ensure that the associated GAITE then stores 0 for the guest AISB
location instead of virt_to_phys(page_address(NULL)).

Fixes: 3c5a1b6f0a18 ("KVM: s390: pci: provide routines for enabling/disabling interrupt forwarding")
Cc: stable@vger.kernel.org
Reviewed-by: Farhan Ali <alifm@linux.ibm.com>
Signed-off-by: Matthew Rosato <mjrosato@linux.ibm.com>
---
 arch/s390/kvm/pci.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/arch/s390/kvm/pci.c b/arch/s390/kvm/pci.c
index 5b075c38998e..dc748cde4703 100644
--- a/arch/s390/kvm/pci.c
+++ b/arch/s390/kvm/pci.c
@@ -300,9 +300,14 @@ static int kvm_s390_pci_aif_enable(struct zpci_dev *zdev, struct zpci_fib *fib,
 
 	gaite->gisc = fib->fmt0.isc;
 	gaite->count++;
-	gaite->aisbo = fib->fmt0.aisbo;
-	gaite->aisb = virt_to_phys(page_address(aisb_page) + (fib->fmt0.aisb &
-							      ~PAGE_MASK));
+	if (fib->fmt0.sum == 1) {
+		gaite->aisbo = fib->fmt0.aisbo;
+		gaite->aisb = virt_to_phys(page_address(aisb_page) +
+					   (fib->fmt0.aisb & ~PAGE_MASK));
+	} else {
+		gaite->aisbo = 0;
+		gaite->aisb = 0;
+	}
 	aift->kzdev[zdev->aisb] = zdev->kzdev;
 	spin_unlock_irq(&aift->gait_lock);
 
-- 
2.54.0