Re: [Qemu-devel] [PATCH RFC 1/7] virtio: relax feature check

[Greg Kurz <gkurz@linux.vnet.ibm.com>]

On Tue, 12 May 2015 15:14:53 +0200
Cornelia Huck <cornelia.huck@de.ibm.com> wrote:
> On Wed, 06 May 2015 14:07:37 +0200
> Greg Kurz <gkurz@linux.vnet.ibm.com> wrote:
> 
> > Unlike with add and clear, there is no valid reason to abort when checking
> > for a feature. It makes more sense to return false (i.e. the feature bit
> > isn't set). This is exactly what __virtio_has_feature() does if fbit >= 32.
> > 
> > This allows to introduce code that is aware about new 64-bit features like
> > VIRTIO_F_VERSION_1, even if they are still not implemented.
> > 
> > Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
> > ---
> >  include/hw/virtio/virtio.h |    1 -
> >  1 file changed, 1 deletion(-)
> > 
> > diff --git a/include/hw/virtio/virtio.h b/include/hw/virtio/virtio.h
> > index d95f8b6..6ef70f1 100644
> > --- a/include/hw/virtio/virtio.h
> > +++ b/include/hw/virtio/virtio.h
> > @@ -233,7 +233,6 @@ static inline void virtio_clear_feature(uint32_t *features, unsigned int fbit)
> > 
> >  static inline bool __virtio_has_feature(uint32_t features, unsigned int fbit)
> >  {
> > -    assert(fbit < 32);
> >      return !!(features & (1 << fbit));
> >  }
> > 
> > 
> > 
> 
> I must say I'm not very comfortable with knowingly passing out-of-rage
> values to this function.
> 

I take that as a valid reason then :)

> Can we perhaps apply at least the feature-bit-size extending patches
> prior to your patchset, if the remainder of the virtio-1 patchset still
> takes some time?

Hmm... if I remember well, it still lacks migration support.

--
Greg