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

[Michael S. Tsirkin]

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@xxxxxxxxxx> 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@xxxxxxxxxxxxxxx>
> >>>> Cc: Michael S. Tsirkin <mst@xxxxxxxxxx>
> >>>> Cc: Cornelia Huck <cornelia.huck@xxxxxxxxxx>
> >>>> Cc: Wanlong Gao <gaowanlong@xxxxxxxxxxxxxx>
> >>>> Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
> >>> 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?

-- 
MST
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