On Thu, Oct 23, 2014 at 03:30:06PM +0200, Cornelia Huck wrote:

On Thu, 23 Oct 2014 15:51:39 +0300
"Michael S. Tsirkin" [off-list ref] wrote:

quoted

On Thu, Oct 23, 2014 at 02:28:08PM +0200, Cornelia Huck wrote:

quoted

On Wed, 22 Oct 2014 21:44:44 +0300
"Michael S. Tsirkin" [off-list ref] wrote:

quoted

set FEATURES_OK as per virtio 1.0 spec

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
 include/uapi/linux/virtio_config.h |  2 ++
 drivers/virtio/virtio.c            | 29 ++++++++++++++++++++++-------
 2 files changed, 24 insertions(+), 7 deletions(-)

quoted

 	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 :(

Well, it worked as long as this was unidirectional...

quoted

quoted

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,

quoted

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.

It only adds a new failure case, so it's not really magic, agreed.

quoted

So the new command would have to be optional, 

I think a new command should be tied to a new virtio-ccw revision.

Or a feature bit.

quoted

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).

I think I need to contemplate that a bit more. The problem with a new
READ_STATUS is that it is, by nature, an asynchronous command (as all
ccw commands are - the Linux guest driver uses a simple wrapper that
makes it appear synchronous). This means, for example, that every
status update now involves two channel programs instead of one, and
that reading e.g. the status sysfs attribute in Linux now triggers
channel I/O, which means several exits (at least ssch, interrupt
injection, tsch; probably more depending on what the guest does)...

Well it seems more robust than just caching it within guest.
Anyway, let's start with issue tracker, it does not force any specific
solution.

-- 
MST