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

[Cornelia Huck <cornelia.huck@de.ibm.com>]

On Mon, 17 Nov 2014 12:28:49 +0200
"Michael S. Tsirkin" <mst@redhat.com> wrote:

> On Mon, Nov 17, 2014 at 11:20:48AM +0100, Cornelia Huck wrote:
> > On Mon, 17 Nov 2014 12:11:39 +0200
> > "Michael S. Tsirkin" <mst@redhat.com> wrote:
> > 
> > > 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.
> > 
> > Unfortunately, this does not ensure that there won't be buggy
> > hypervisors out there, just as there's buggy hardware floating around.
> > 
> > > 
> > > 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.
> > 
> > Good point. Failing probe is still much better than hitting BUG_ONs.
> > 
> > We'll still need a driver callback, though, that can return an error on
> > bogus feature bit combinations.
> 
> Why bother?  Just check features at start of probe, and return an error.

So we'd fail probing due to bogus features after setting FEATURES_OK in
the virtio-1 case, won't we? Feels a bit weird, but seems to be covered
by the spec.