On Tue, 12 Jan 2016 01:23:56 +0100
Laurent Vivier <lviv...@redhat.com> wrote:
> On 11/01/2016 17:13, Greg Kurz wrote:
> > Indeed vhost doesn't need to ask for vring endian fixing if the device is
> > virtio 1.0, since it is already handled by the in-kernel vhost driver. This
> > patch simply consolidates the logic into the existing helper.
> >
> > Signed-off-by: Greg Kurz <gk...@linux.vnet.ibm.com>
> > Reviewed-by: Cornelia Huck <cornelia.h...@de.ibm.com>
> > ---
> > hw/virtio/vhost.c | 9 +++++----
> > 1 file changed, 5 insertions(+), 4 deletions(-)
> >
> > diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
> > index 2e1e792d599e..aef750df22ad 100644
> > --- a/hw/virtio/vhost.c
> > +++ b/hw/virtio/vhost.c
> > @@ -750,6 +750,9 @@ static void vhost_log_stop(MemoryListener *listener,
> >
> > static inline bool vhost_needs_vring_endian(VirtIODevice *vdev)
> > {
> > + if (virtio_vdev_has_feature(vdev, VIRTIO_F_VERSION_1)) {
> > + return false;
> > + }
> > #ifdef TARGET_IS_BIENDIAN
> > #ifdef HOST_WORDS_BIGENDIAN
> > return !virtio_is_big_endian(vdev);
>
> I'm wondering if in term of performance you can remove the call of
> virtio_vdev_has_feature() as it is done in virtio_is_big_endian().
>
We cannot do this: in the case of a big endian host and a virtio 1.0 device,
virtio_is_big_endian() would return false and vhost_needs_vring_endian()
would return true. This is wrong: the byteswapping for virtio 1.0 is already
handled in the linux vhost driver since it has the feature bits. Only legacy
devices may need to call vhost_virtqueue_set_vring_endian_legacy() in a
cross-endian situation.
This isn't really a performance issue since this function is never called on
a hot path.
But indeed, your comment raises a concern about clarity. After the the call
to virtio_vdev_has_feature(), we are only caring for legacy devices: we
don't need the extra check in virtio_is_big_endian().
Also the device_endian field is always set (VIRTIO_DEVICE_ENDIAN_UNKNOWN may
only happen on a virtio_load() path that cannot lead here). So we don't need
the assert() either.
We simply need to open code the device_endian checking instead of calling
virtio_is_big_endian(). Like this:
/* Only legacy devices offered to an endian ambivalent guest may
* face a cross-endian situation. All other cases return false.
*/
static inline bool vhost_needs_vring_endian(VirtIODevice *vdev)
{
if (virtio_vdev_has_feature(vdev, VIRTIO_F_VERSION_1)) {
return false;
}
#ifdef TARGET_IS_BIENDIAN
#ifdef HOST_WORDS_BIGENDIAN
return vdev->device_endian == VIRTIO_DEVICE_ENDIAN_LITTLE;
#else
return vdev->device_endian == VIRTIO_DEVICE_ENDIAN_BIG;
#endif
#else
return false;
#endif
}
I'll make it a separate patch.
Thanks !
--
Greg
> > @@ -811,8 +814,7 @@ static int vhost_virtqueue_start(struct vhost_dev *dev,
> > return -errno;
> > }
> >
> > - if (!virtio_vdev_has_feature(vdev, VIRTIO_F_VERSION_1) &&
> > - vhost_needs_vring_endian(vdev)) {
> > + if (vhost_needs_vring_endian(vdev)) {
> > r = vhost_virtqueue_set_vring_endian_legacy(dev,
> >
> > virtio_is_big_endian(vdev),
> > vhost_vq_index);
> > @@ -908,8 +910,7 @@ static void vhost_virtqueue_stop(struct vhost_dev *dev,
> > /* In the cross-endian case, we need to reset the vring endianness to
> > * native as legacy devices expect so by default.
> > */
> > - if (!virtio_vdev_has_feature(vdev, VIRTIO_F_VERSION_1) &&
> > - vhost_needs_vring_endian(vdev)) {
> > + if (vhost_needs_vring_endian(vdev)) {
> > r = vhost_virtqueue_set_vring_endian_legacy(dev,
> >
> > !virtio_is_big_endian(vdev),
> > vhost_vq_index);
> >
> >
>
