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

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Nov 25, 2011 at 09:23:42AM +0100, Paolo Bonzini wrote:
> 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.

I donn't think we want to exit on BAD_FEATURE...

> >>  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.

Ah, I understand, you mean helping debugging.
OK, but if we add exit() in the future we still do
not need to return a status from this function.

> > what would have prevented it.
> 
> exit(1) on unsupported features.
> 
> Paolo

This is somewhat problematic, for example this does not flush
out the cache and so might cause data corruption.
Yet I think it would be nice to have a 'safe_exit' functionality
to invoke on a guest bug. Or maybe it's better to stop the VM,
this way the monitor is available and debugging is easier.


-- 
MST