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

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

On Tue, 14 Apr 2015 16:20:23 +0200
Cornelia Huck <cornelia.huck@de.ibm.com> wrote:

> On Fri, 10 Apr 2015 12:19:16 +0200
> Greg Kurz <gkurz@linux.vnet.ibm.com> wrote:
> 
> > This patch brings cross-endian support to vhost when used to implement
> > legacy virtio devices. Since it is a relatively rare situation, the
> > feature availability is controlled by a kernel config option (not set
> > by default).
> > 
> > The vq->is_le boolean field is added to cache the endianness to be
> > used for ring accesses. It defaults to native endian, as expected
> > by legacy virtio devices. When the ring gets active, we force little
> > endian if the device is modern. When the ring is deactivated, we
> > revert to the native endian default.
> > 
> > If cross-endian was compiled in, a vq->user_be boolean field is added
> > so that userspace may request a specific endianness. This field is
> > used to override the default when activating the ring of a legacy
> > device. It has no effect on modern devices.
> > 
> > Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
> > ---
> >  drivers/vhost/Kconfig      |   10 ++++++
> >  drivers/vhost/vhost.c      |   76 +++++++++++++++++++++++++++++++++++++++++++-
> >  drivers/vhost/vhost.h      |   12 +++++--
> >  include/uapi/linux/vhost.h |    9 +++++
> >  4 files changed, 103 insertions(+), 4 deletions(-)
> 
> > +#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)
> 
> Checking for s.num > 1 might be more obvious at a glance?
> 

Sure since s.num is unsigned.

> > +		return -EINVAL;
> > +
> > +	vq->user_be = s.num;
> > +
> > +	return 0;
> > +}
> > +
> 
> (...)
> 
> > +#ifdef CONFIG_VHOST_SET_ENDIAN_LEGACY
> > +static void vhost_init_is_le(struct vhost_virtqueue *vq)
> > +{
> > +	vq->is_le = vhost_has_feature(vq, VIRTIO_F_VERSION_1) || !vq->user_be;
> 
> So if the endianness is not explicitly set to BE, it will be forced to
> LE (instead of native endian)? Won't that break userspace that does not
> yet use the new interface when CONFIG_VHOST_SET_ENDIAN_LEGACY is set?
> 

If userspace doesn't use the new interface, then vq->user_be will retain its
default value that was set in vhost_vq_reset(), i.e. :

 vq->user_be = !virtio_legacy_is_little_endian();

This means default is native endian.

What about adding this comment ?

static void vhost_init_is_le(struct vhost_virtqueue *vq)
{
	/* Note for legacy virtio: user_be is initialized in vhost_vq_reset()
	 * according to the host endianness. If userspace does not set an
	 * explicit endianness, the default behavior is native endian, as
	 * expected by legacy virtio.
	 */
	vq->is_le = vhost_has_feature(vq, VIRTIO_F_VERSION_1) || !vq->user_be;
}

> > +}
> > +#else
> > +static void vhost_init_is_le(struct vhost_virtqueue *vq)
> > +{
> > +	if (vhost_has_feature(vq, VIRTIO_F_VERSION_1))
> > +		vq->is_le = true;
> > +}
> > +#endif
> > +
> >  int vhost_init_used(struct vhost_virtqueue *vq)
> >  {
> >  	__virtio16 last_used_idx;
> >  	int r;
> > -	if (!vq->private_data)
> > +	if (!vq->private_data) {
> > +		vq->is_le = virtio_legacy_is_little_endian();
> >  		return 0;
> > +	}
> > +
> > +	vhost_init_is_le(vq);
> > 
> >  	r = vhost_update_used_flags(vq);
> >  	if (r)