linux-kernel - Re: [PATCH RFC v3 09/16] virtio: set FEATURES

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Message-ID: <20141023125139.GA10033@redhat.com>
Date:	Thu, 23 Oct 2014 15:51:39 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Cornelia Huck <cornelia.huck@...ibm.com>
Cc:	linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
	virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH RFC v3 09/16] virtio: set FEATURES_OK

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@...hat.com> wrote:
> 
> > set FEATURES_OK as per virtio 1.0 spec
> > 
> > Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> > ---
> >  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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