Re: [Qemu-devel] [PATCH 1.0] virtio: add and use virtio_set_features

[Paolo Bonzini]

On 11/24/2011 06:55 PM, Michael S. Tsirkin wrote:
> >  Could some backend make it a hard failure?
>
> I don't see how, there's no way to report a failure from
> an io port write.

You can exit(1), or fall back to a restricted set of features like we do 
for BAD_FEATURE.  BAD_FEATURE is a special case of features that is not 
exposed by the host, but requested by the guest.

> >  If I understand
> >  correctly, this would have prevented the BAD_FEATURE bug too.
>
> Which bug?

VIRTIO_F_BAD_FEATURE(30)
This feature should never be negotiated by the guest; doing so is an 
indication that the guest is faulty.  An experimental virtio PCI driver 
contained in Linux version 2.6.25 had this problem, and this feature bit 
can be used to detect it.

>  what would have prevented it.

exit(1) on unsupported features.

Paolo