Re: [PATCH V3 1/2] virtio: introduce methods of sanitizing device features

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Nov 17, 2014 at 10:44:30AM +0100, Cornelia Huck wrote:
> On Mon, 17 Nov 2014 11:37:01 +0200
> "Michael S. Tsirkin" <mst@redhat.com> wrote:
> 
> > On Mon, Nov 17, 2014 at 05:17:17PM +0800, Jason Wang wrote:
> > > Buggy host may advertised buggy host features (a usual case is that host
> > > advertise a feature whose dependencies were missed). In this case, driver
> > > should detect and disable the buggy features by itself.
> > > 
> > > This patch introduces driver specific sanitize_features() method which is
> > > called just before features finalizing to detect and disable buggy features
> > > advertised by host.
> > > 
> > > Virtio-net will be the first user.
> > > 
> > > Cc: Rusty Russell <rusty@rustcorp.com.au>
> > > Cc: Michael S. Tsirkin <mst@redhat.com>
> > > Cc: Cornelia Huck <cornelia.huck@de.ibm.com>
> > > Cc: Wanlong Gao <gaowanlong@cn.fujitsu.com>
> > > Signed-off-by: Jason Wang <jasowang@redhat.com>
> > 
> > Hmm this conflicts with virtio 1.0 work: we drop
> > features as bitmap there.
> 
> But that's an implementation detail, no? We'll still need a way for the
> driver to sanitize features, and I think this interface works just fine.

Now that you mention it, I don't think we do.

The spec is quite explicit that devices must not expose invalid
combinations of features.

Admittedly, BUG_ON isn't very friendly to hypervisors.

But e.g. failing probe seems better than trying to work around
hypervisor bugs - otherwise we'll be stuck maintaining compatibility
with hypervisors forever.

-- 
MST