Re: [PATCH RFC v6 12/20] virtio: disallow late feature changes for virtio-1

On Fri, 12 Dec 2014 11:55:38 +0100
Thomas Huth [off-list ref] wrote:

On Thu, 11 Dec 2014 14:25:14 +0100
Cornelia Huck [off-list ref] wrote:

quoted

For virtio-1 devices, the driver must not attempt to set feature bits
after it set FEATURES_OK in the device status. Simply reject it in
that case.

Signed-off-by: Cornelia Huck <redacted>
---
 hw/virtio/virtio.c         |   16 ++++++++++++++--
 include/hw/virtio/virtio.h |    2 ++
 2 files changed, 16 insertions(+), 2 deletions(-)

diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
index 57190ba..a3dd67b 100644
--- a/hw/virtio/virtio.c
+++ b/hw/virtio/virtio.c

@@ -978,7 +978,7 @@ void virtio_save(VirtIODevice *vdev, QEMUFile *f)
     vmstate_save_state(f, &vmstate_virtio, vdev);
 }

-int virtio_set_features(VirtIODevice *vdev, uint64_t val)
+static int __virtio_set_features(VirtIODevice *vdev, uint64_t val)

Maybe avoid the double underscores here? But unfortunately, I also fail
to come up with a better suggestion for a name here ...

virtio_set_features_nocheck()?

This function is only called within virtio.c anyway...

quoted

 {
     BusState *qbus = qdev_get_parent_bus(DEVICE(vdev));
     VirtioBusClass *vbusk = VIRTIO_BUS_GET_CLASS(qbus);

@@ -994,6 +994,18 @@ int virtio_set_features(VirtIODevice *vdev, uint64_t val)
     return bad ? -1 : 0;
 }

+int virtio_set_features(VirtIODevice *vdev, uint64_t val)
+{
+   /*
+     * The driver must not attempt to set features after feature negotiation
+     * has finished.
+     */
+    if (vdev->status & VIRTIO_CONFIG_S_FEATURES_OK) {
+        return -EINVAL;
+    }

Hmm, according to your patch description, the FEATURES_OK check only
applies to virtio-1.0 devices ... so shouldn't there be a check for
virtio-1 here? Or did I miss something?

A device in legacy mode will never have FEATURES_OK set. But it is a
bit non-obvious - maybe adding a check for VERSION_1 does not hurt.

quoted

+    return __virtio_set_features(vdev, val);
+}

 Thomas