"Michael S. Tsirkin" [off-list ref] writes:

On Fri, Sep 07, 2012 at 09:15:46AM +0930, Rusty Russell wrote:

quoted

Paolo Bonzini [off-list ref] writes:

quoted

Instead, VIRTIO_NET_F_HOST_MUST_SEND_GARP would be a "negative" feature:
if set, the host _may not_ rely on the guest to send a gARP.  Similarly
if VIRTIO_BALLOON_F_MUST_TELL_HOST is set, the guest _may not_ use
ballooned pages directly.

There are _no_ other negative features besides
VIRTIO_BALLOON_F_MUST_TELL_HOST in the spec, and for a good
reason---because they're broken.

(Hmm, actually we have one, VIRTIO_BLK_F_RO.  It is also a bit broken,
but it is not so important because it depends on user input more than
hypervisor version).

Yes, this is the key observation, and an important lesson for the
future.  Thanks!
Note that these two negative features were in the original spec, where
it's assumed that every device supports them.  That's not explicitly
documented, however.

I'm curious what would we do for the future? I tried to imagine that _RO
was not in the original spec, so virtio-blk expects a r/w device.
Now we can not add _RW - old hypervisors do not set it, and old
drivers do not ack it.
What would a new flag with equivalent functionality be?

Backwards compatibility in the R/O case would actually work: just fail
writes.  Because it's just friendly advice to the OS, really.

The final test is always: does it break users?  If there are no users
who will notice, we can do anything.  If there are users, we have to
keep backwards compatibility, and that implies we can't add "must know"
features.

So it looks like a bug: we should teach driver to tell host first on leak?
Yan, Vadim, can you comment please?

Also if true, looks like this bit will be useful to detect a fixed driver on
the hypervisor side - to avoid unmapping such pages? Rusty what do you
think?

So, feature is unimplemented in qemu, and broken in drivers.  I starting
to share Paolo's dislike of it.

Don't understand why we'd care about fixed drivers though, if we remove
the feature bit....

Cheers,
Rusty.