On Thu, Oct 23, 2014 at 02:28:08PM +0200, Cornelia Huck wrote:
> On Wed, 22 Oct 2014 21:44:44 +0300
> "Michael S. Tsirkin" <mst@xxxxxxxxxx> wrote:
>
> > set FEATURES_OK as per virtio 1.0 spec
> >
> > Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
> > ---
> > include/uapi/linux/virtio_config.h | 2 ++
> > drivers/virtio/virtio.c | 29 ++++++++++++++++++++++-------
> > 2 files changed, 24 insertions(+), 7 deletions(-)
> >
>
> > dev->config->finalize_features(dev);
> >
> > + if (virtio_has_feature(dev, VIRTIO_F_VERSION_1)) {
> > + add_status(dev, VIRTIO_CONFIG_S_FEATURES_OK);
> > + status = dev->config->get_status(dev);
> > + if (!(status & VIRTIO_CONFIG_S_FEATURES_OK)) {
> > + printk(KERN_ERR "virtio: device refuses features: %x\n",
> > + status);
> > + err = -ENODEV;
> > + goto err;
> > + }
> > + }
> > +
>
> Ugh, I just realize that virtio-ccw has a problem with that mechanism :(
>
> Up to now, the driver only propagated status to the device: For
> virtio-ccw, this was easily implemented via a ccw that transmitted
> "status" to the device. However, the "read back status" part now
> actually requires that the driver can get "status" from the device, or
> has a comparable way to find out that the device won't accept the
> status it tried to write.
Ugh, it actually caches the status in the transport :(
> I can think of two solutions:
>
> (1) Introduce a new ccw that actually reads the device status.
> (2) Make the WRITE_STATUS ccw fail (with a unit check) if the driver
> sets FEATURES_OK after it tried to set features the device won't
> accept.
>
> (1) is probably more generic, while (2) is more straightforward to
> implement.
>
> Good thing we actually try to finally implement this,
> I did not notice
> this problem during the review :(
Well, it's a nuisance, but the spec is out.
It seems to me a new command would be a substantive change so we can't
do this in errata.
Option (2) would require two statements for drivers and devices,
but since it's clearly the case for correct drivers/devices
that command does not fail, it follows that this
is not a substantive change so it can be fixed
in an errata.
So the new command would have to be optional, please open
two issues in the TC: one documenting that driver must check
WRITE_STATUS and device can fail WRITE_STATUS, and another
for adding READ_STATUS (which will have to wait until
the next CS).
--
MST
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