[PATCH 4.14.y] virtio/s390: implement virtio-ccw revision 2 correctly

[backport of commit 182f709c5cff683e6732d04c78e328de0532284f to
 4.14-stable; context diff in second hunk]

CCW_CMD_READ_STATUS was introduced with revision 2 of virtio-ccw,
and drivers should only rely on it being implemented when they
negotiated at least that revision with the device.

However, virtio_ccw_get_status() issued READ_STATUS for any
device operating at least at revision 1. If the device accepts
READ_STATUS regardless of the negotiated revision (which some
implementations like QEMU do, even though the spec currently does
not allow it), everything works as intended. While a device
rejecting the command should also be handled gracefully, we will
not be able to see any changes the device makes to the status,
such as setting NEEDS_RESET or setting the status to zero after
a completed reset.

We negotiated the revision to at most 1, as we never bumped the
maximum revision; let's do that now and properly send READ_STATUS
only if we are operating at least at revision 2.

Cc: stable@vger.kernel.org
Fixes: 7d3ce5ab9430 ("virtio/s390: support READ_STATUS command for virtio-ccw")
Reviewed-by: Halil Pasic <pasic@linux.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Link: https://lore.kernel.org/r/20210216110645.1087321-1-cohuck@redhat.com (local)
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>
---
 drivers/s390/virtio/virtio_ccw.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/s390/virtio/virtio_ccw.c b/drivers/s390/virtio/virtio_ccw.c
index f9cf676a0469..56794b13bb12 100644
--- a/drivers/s390/virtio/virtio_ccw.c
+++ b/drivers/s390/virtio/virtio_ccw.c

@@ -106,7 +106,7 @@ struct virtio_rev_info {
 };
 
 /* the highest virtio-ccw revision we support */
-#define VIRTIO_CCW_REV_MAX 1
+#define VIRTIO_CCW_REV_MAX 2
 
 struct virtio_ccw_vq_info {
 	struct virtqueue *vq;

@@ -911,7 +911,7 @@ static u8 virtio_ccw_get_status(struct virtio_device *vdev)
 	u8 old_status = *vcdev->status;
 	struct ccw1 *ccw;
 
-	if (vcdev->revision < 1)
+	if (vcdev->revision < 2)
 		return *vcdev->status;
 
 	ccw = kzalloc(sizeof(*ccw), GFP_DMA | GFP_KERNEL);

-- 
2.26.2