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



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