On Mon, 3 Nov 2014 16:29:31 +0100
Cornelia Huck <cornelia.h...@de.ibm.com> wrote:
> On Wed, 29 Oct 2014 09:42:09 +0100
> Cédric Le Goater <c...@fr.ibm.com> wrote:
>
> > When the guest and the host have a different endian order, the data
> > being accessed in the vring queues needs to be byteswapped.
> >
> > This patch adds a VHOST_VRING_F_BYTESWAP flag to inform the vhost
> > kernel backend to byteswap vring data.
> >
> > Signed-off-by: Cédric Le Goater <c...@fr.ibm.com>
> > ---
> > hw/virtio/vhost.c | 25 ++++++++++++++++++++++++-
> > include/hw/virtio/vhost.h | 1 +
> > linux-headers/linux/vhost.h | 3 +++
> > 3 files changed, 28 insertions(+), 1 deletion(-)
>
> > +static bool vhost_virtqueue_needs_byteswap(VirtIODevice *vdev)
> > +{
> > +#ifdef TARGET_IS_BIENDIAN
> > +#ifdef HOST_WORDS_BIGENDIAN
> > + return !virtio_is_big_endian(vdev);
> > +#else
> > + return virtio_is_big_endian(vdev);
> > +#endif
> > +
> > +#else
> > + return false;
> > +#endif
> > +}
>
> *thinks aloud*
>
> We call this function after features have been negotiated, so we should
> be able to reuse this interface for virtio-1 by checking for
> VIRTIO_F_VERSION_1, right?
>
>
Yes. Moreover, we're not on a hot path, so we could simply ignore
TARGET_IS_BIENDIAN and only have:
static bool vhost_virtqueue_needs_byteswap(VirtIODevice *vdev)
{
#ifdef HOST_WORDS_BIGENDIAN
return !virtio_is_big_endian(vdev);
#else
return virtio_is_big_endian(vdev);
#endif
}
in which case, this would work right away with the changes brought by
Conny's virtio-1 series:
static inline bool virtio_device_is_legacy(VirtIODevice *vdev)
{
return !(vdev->guest_features[1] & (1 << (VIRTIO_F_VERSION_1 - 32)));
}
static inline bool virtio_is_big_endian(VirtIODevice *vdev)
{
if (virtio_device_is_legacy(vdev)) {
assert(vdev->device_endian != VIRTIO_DEVICE_ENDIAN_UNKNOWN);
return vdev->device_endian == VIRTIO_DEVICE_ENDIAN_BIG;
}
/* Devices conforming to VIRTIO 1.0 or later are always LE. */
return false;
}
Cheers.
--
Greg
