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

[Michael S. Tsirkin]

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@xxxxxxxxxx> wrote:
> 
> > From: Rusty Russell <rusty@xxxxxxxxxxxxxxx>
> > 
> > Change the u32 to a u64, and make sure to use 1ULL everywhere!
> > 
> > Cc: Brian Swetland <swetland@xxxxxxxxxx>
> > Cc: Christian Borntraeger <borntraeger@xxxxxxxxxx>
> > [Thomas Huth: fix up virtio-ccw get_features]
> > Signed-off-by: Rusty Russell <rusty@xxxxxxxxxxxxxxx>
> > Signed-off-by: Cornelia Huck <cornelia.huck@xxxxxxxxxx>
> > Acked-by: Pawel Moll <pawel.moll@xxxxxxx>
> > Acked-by: Ohad Ben-Cohen <ohad@xxxxxxxxxx>
> > 
> > Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>

...

> > @@ -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?
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