Re: [PATCH v4 02/42] virtio: add support for 64 bit features.

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

On Wed, Nov 26, 2014 at 05:48:09PM +0100, Greg Kurz wrote:
> On Tue, 25 Nov 2014 18:41:22 +0200
> "Michael S. Tsirkin" <mst@redhat.com> wrote:
> 
> > From: Rusty Russell <rusty@rustcorp.com.au>
> > 
> > Change the u32 to a u64, and make sure to use 1ULL everywhere!
> > 
> > Cc: Brian Swetland <swetland@google.com>
> > Cc: Christian Borntraeger <borntraeger@de.ibm.com>
> > [Thomas Huth: fix up virtio-ccw get_features]
> > Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
> > Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
> > Acked-by: Pawel Moll <pawel.moll@arm.com>
> > Acked-by: Ohad Ben-Cohen <ohad@wizery.com>
> > 
> > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

...

> > @@ -86,14 +86,14 @@ static inline bool virtio_has_feature(const struct virtio_device *vdev,
> >  {
> >  	/* Did you forget to fix assumptions on max features? */
> >  	if (__builtin_constant_p(fbit))
> > -		BUILD_BUG_ON(fbit >= 32);
> > +		BUILD_BUG_ON(fbit >= 64);
> 
> 
> While you're here, maybe you could derive the max value from the .features field ?
> 
> -               BUILD_BUG_ON(fbit >= 64);
> +               BUILD_BUG_ON(fbit >= (sizeof(vdev->features) << 3));

I don't see how that will help.
All that 1ULL math only works up to 64 bit.
So this only makes it look like we support any size,
but doesn't really.

No?