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

[Jason Wang <jasowang@redhat.com>]

On 11/18/2014 07:04 PM, Michael S. Tsirkin wrote:
> On Tue, Nov 18, 2014 at 11:23:49AM +0800, Jason Wang wrote:
>> 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.
> From experience, second-guessing user always adds maintainance.
>
>> 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.
> That code is common for all drivers, so it was moved to core.
>
>> There seems no way to get rid of maintaining compatibility, e.g  the
>> workarounds for the buggy hypervisor without VIRTIO_F_ANY_LAYOUT support.
> Right - because too many hypervisors shipped without it, it's too
> much work to fix them all.
> No such motivation here, right?
>

Right, will post the patch that just fail the probe of virtio-net.

Thanks