Re: [PATCH v4 7/8] vhost: feature to set the vring endianness

["Michael S. Tsirkin" <mst-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

On Wed, Apr 22, 2015 at 11:08:54AM +0200, Greg Kurz wrote:
> On Tue, 21 Apr 2015 20:25:03 +0200
> "Michael S. Tsirkin" <mst-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> wrote:
> [ ... ]
> > > > > @@ -630,6 +634,53 @@ static long vhost_set_memory(struct vhost_dev *d, struct vhost_memory __user *m)
> > > > >  	return 0;
> > > > >  }
> > > > >  
> > > > > +#ifdef CONFIG_VHOST_SET_ENDIAN_LEGACY
> > > > > +static long vhost_set_vring_big_endian(struct vhost_virtqueue *vq,
> > > > > +				       int __user *argp)
> > > > > +{
> > > > > +	struct vhost_vring_state s;
> > > > > +
> > > > > +	if (vq->private_data)
> > > > > +		return -EBUSY;
> > > > > +
> > > > > +	if (copy_from_user(&s, argp, sizeof(s)))
> > > > > +		return -EFAULT;
> > > > > +
> > > > > +	if (s.num && s.num != 1)
> > > > 
> > > > s.num & ~0x1
> > > > 
> > > 
> > > Since s.num is unsigned and I assume this won't change, what about
> > > s.num > 1 as suggested by Cornelia ?
> > 
> > I just tried and gcc optimizes
> > s.num != 0 && s.num != 1 to s.num > 1
> > 
> > The former will be more readable once we
> > replace 0 and 1 with defines.
> > 
> > So ignore my advice, keep code as is but use defines.
> > 
> 
> Ok.
> 
> [ ... ] 
> > > > > --- a/include/uapi/linux/vhost.h
> > > > > +++ b/include/uapi/linux/vhost.h
> > > > > @@ -103,6 +103,15 @@ struct vhost_memory {
> > > > >  /* Get accessor: reads index, writes value in num */
> > > > >  #define VHOST_GET_VRING_BASE _IOWR(VHOST_VIRTIO, 0x12, struct vhost_vring_state)
> > > > >  
> > > > > +/* Set the vring byte order in num. This is a legacy only API that is simply
> > > > > + * ignored when VIRTIO_F_VERSION_1 is set.
> > > > > + * 0 to set to little-endian
> > > > > + * 1 to set to big-endian
> > > > 
> > > > How about defines for these?
> > > > 
> > > 
> > > Ok. I'll put the defines here so that all the cross-endian stuff
> > > lies in the same hunk. Is it ok for you ?
> > 
> > Fine.
> > 
> > > > > + * other values return EINVAL.
> > 
> > Pls also add a note saying that not all kernel configurations support this ioctl,
> > but all configurations that support SET also support GET.
> > 
> 
> Ok.
> 
> > > > > + */
> > > > > +#define VHOST_SET_VRING_BIG_ENDIAN _IOW(VHOST_VIRTIO, 0x13, struct vhost_vring_state)
> > > > > +#define VHOST_GET_VRING_BIG_ENDIAN _IOW(VHOST_VIRTIO, 0x14, struct vhost_vring_state)
> > > > > +
> > > > >  /* The following ioctls use eventfd file descriptors to signal and poll
> > > > >   * for events. */
> > > > >  
> > > > 
> > 
> > I'm inclined to think VHOST_SET_VRING_ENDIAN is a slightly better name.
> > What do you think?
> > 
> 
> Or VHOST_SET_VRING_CROSS_ENDIAN ? I like the idea to keep a hint that this
> API is for cross-endian only... like the rest of this series.
> 
> --
> Greg

I think VHOST_SET_VRING_CROSS_ENDIAN is not a good name -
it would imply 1 for cross endian, 0 for native endian.

-- 
MST