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

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Date:	Mon, 17 Nov 2014 11:20:48 +0100
From:	Cornelia Huck <cornelia.huck@...ibm.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Jason Wang <jasowang@...hat.com>, rusty@...tcorp.com.au,
	virtualization@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org,
	Wanlong Gao <gaowanlong@...fujitsu.com>
Subject: Re: [PATCH V3 1/2] virtio: introduce methods of sanitizing device
 features

On Mon, 17 Nov 2014 12:11:39 +0200
"Michael S. Tsirkin" <mst@...hat.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@...hat.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@...tcorp.com.au>
> > > > Cc: Michael S. Tsirkin <mst@...hat.com>
> > > > Cc: Cornelia Huck <cornelia.huck@...ibm.com>
> > > > Cc: Wanlong Gao <gaowanlong@...fujitsu.com>
> > > > Signed-off-by: Jason Wang <jasowang@...hat.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.

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