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

[Jason Wang <jasowang@redhat.com>]

On 11/17/2014 06:11 PM, Michael S. Tsirkin 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.
>
> 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.
>

I'm ok with failing the probe.

But it won't cost big effort to workaround only features dependencies
issue. I don't see how this block any further features implementation.
Looking at virtio-net, it also depends on network core to fix NETIF_F_*
dependencies.

There seems no way to get rid of maintaining compatibility, e.g  the
workarounds for the buggy hypervisor without VIRTIO_F_ANY_LAYOUT support.