[PULL 5/8] virtio-ccw: commands on revision-less devices

[Cornelia Huck <cohuck@redhat.com>]

The virtio standard specifies that any non-transitional device must
reject commands prior to revision setting (which we do). Devices
that are transitional need to assume revision 0 (legacy) if the
driver sends a non-revision-setting command first in order to
support legacy drivers. We neglected to do the latter.

Fortunately, nearly everything worked as intended anyway; the only
problem was not properly rejecting revision setting after some other
command had been issued. Easy to fix by setting revision to 0 if
we see a non-revision command on a legacy-capable revision-less
device.

Found by code inspection, not observed in the wild.

Signed-off-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Halil Pasic <pasic@linux.ibm.com>
Message-Id: <20210216111830.1087847-1-cohuck@redhat.com>
---
 hw/s390x/virtio-ccw.c | 21 ++++++++++++++-------
 1 file changed, 14 insertions(+), 7 deletions(-)

diff --git a/hw/s390x/virtio-ccw.c b/hw/s390x/virtio-ccw.c
index 4582e94ae7dc..06c06056814b 100644
--- a/hw/s390x/virtio-ccw.c
+++ b/hw/s390x/virtio-ccw.c
@@ -327,13 +327,20 @@ static int virtio_ccw_cb(SubchDev *sch, CCW1 ccw)
                                    ccw.cmd_code);
     check_len = !((ccw.flags & CCW_FLAG_SLI) && !(ccw.flags & CCW_FLAG_DC));
 
-    if (dev->force_revision_1 && dev->revision < 0 &&
-        ccw.cmd_code != CCW_CMD_SET_VIRTIO_REV) {
-        /*
-         * virtio-1 drivers must start with negotiating to a revision >= 1,
-         * so post a command reject for all other commands
-         */
-        return -ENOSYS;
+    if (dev->revision < 0 && ccw.cmd_code != CCW_CMD_SET_VIRTIO_REV) {
+        if (dev->force_revision_1) {
+            /*
+             * virtio-1 drivers must start with negotiating to a revision >= 1,
+             * so post a command reject for all other commands
+             */
+            return -ENOSYS;
+        } else {
+            /*
+             * If the driver issues any command that is not SET_VIRTIO_REV,
+             * we'll have to operate the device in legacy mode.
+             */
+            dev->revision = 0;
+        }
     }
 
     /* Look at the command. */
-- 
2.26.2