Re: [Qemu-devel] [PATCH] net: don't set native endianness

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Oct 14, 2015 at 03:54:32PM +0200, Greg Kurz wrote:
> On Wed, 14 Oct 2015 12:26:58 +0300
> "Michael S. Tsirkin" <mst@redhat.com> wrote:
> 
> > commit 5be7d9f1b1452613b95c6ba70b8d7ad3d0797991
> >     vhost-net: tell tap backend about the vnet endianness
> > makes vhost net always try to set LE - even if that matches the
> > native endian-ness.
> > 
> > This makes it fail on older kernels on x86 without TUNSETVNETLE support.
> > 
> 
> Since qemu_set_vnet_le() is only called from vhost_net_set_vnet_endian():
> 
> 
>     if (virtio_vdev_has_feature(dev, VIRTIO_F_VERSION_1) ||
>         (virtio_legacy_is_cross_endian(dev) && !virtio_is_big_endian(dev))) {
>         r = qemu_set_vnet_le(peer, set);
>         if (r) {
>             error_report("backend does not support LE vnet headers");
>         }
> 
> and virtio_legacy_is_cross_endian() is { return false; } on x86, I guess this
> is about virtio 1.0, right ?
> 
> > To fix, make qemu_set_vnet_le/qemu_set_vnet_be skip the
> > ioctl if it matches the host endian-ness.
> > 
> 
> The qemu_set_vnet_le() change indeed makes sense since it fixes a bug.
> 
> I am not so sure about qemu_set_vnet_be() since it is only called in the
> case we have a ppc64le host and a legacy ppc64 guest, in which case
> HOST_WORDS_BIGENDIAN is not defined...

You are right, but it's there for symmetry.


> IMHO it is better to rework the logic so that we only call qemu_set_vnet_*
> when it is needed. There are only 3 cases actually:
> - BE host with virtio 1 needs vnet_le
> - BE host with legacy LE needs vnet_le
> - LE host with legacy BE needs vnet_be
> 
> Something like:
> 
> static inline bool vhost_net_needs_vnet_le(VirtIODevice *dev)
> {
> #if defined(HOST_WORDS_BIGENDIAN)
>     return
>         virtio_vdev_has_feature(dev, VIRTIO_F_VERSION_1) ||
>         !virtio_is_big_endian(dev);
> #else
>     return false;
> #endif
> }
> 
> static inline bool vhost_net_needs_vnet_be(VirtIODevice *dev)
> {
> #if defined(HOST_WORDS_BIGENDIAN)
>     return false;
> #else
>     return virtio_is_big_endian(dev);
> #endif
> }
> 
> static int vhost_net_set_vnet_endian(VirtIODevice *dev, NetClientState *peer,
>                                      bool set)
> {
>     int r = 0;
> 
>     if (vhost_net_needs_vnet_le(dev)) {
>         r = qemu_set_vnet_le(peer, set);
>         if (r) {
>             error_report("backend does not support LE vnet headers");
>         }
>     } else if (vhost_net_needs_vnet_be(dev)) {
>         r = qemu_set_vnet_be(peer, set);
>         if (r) {
>             error_report("backend does not support BE vnet headers");
>         }
>     }
> 
>     return r;
> }
> 
> Cheers.

To me this looks wrong: why not call it unconditionally?
The reason is that qemu_set_vnet_be/qemu_set_vnet_le fails on
some backends and that is a low-level detail that should be
hidden.


> --
> Greg




> > Reported-by: Marcel Apfelbaum <marcel@redhat.com>
> > Cc: Greg Kurz <gkurz@linux.vnet.ibm.com>
> > Cc: qemu-stable@nongnu.org
> > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> > ---
> >  net/net.c | 8 ++++++++
> >  1 file changed, 8 insertions(+)
> > 
> > diff --git a/net/net.c b/net/net.c
> > index 28a5597..8e96011 100644
> > --- a/net/net.c
> > +++ b/net/net.c
> > @@ -517,20 +517,28 @@ void qemu_set_vnet_hdr_len(NetClientState *nc, int len)
> > 
> >  int qemu_set_vnet_le(NetClientState *nc, bool is_le)
> >  {
> > +#ifdef HOST_WORDS_BIGENDIAN
> >      if (!nc || !nc->info->set_vnet_le) {
> >          return -ENOSYS;
> >      }
> > 
> >      return nc->info->set_vnet_le(nc, is_le);
> > +#else
> > +    return 0;
> > +#endif
> >  }
> > 
> >  int qemu_set_vnet_be(NetClientState *nc, bool is_be)
> >  {
> > +#ifdef HOST_WORDS_BIGENDIAN
> > +    return 0;
> > +#else
> >      if (!nc || !nc->info->set_vnet_be) {
> >          return -ENOSYS;
> >      }
> > 
> >      return nc->info->set_vnet_be(nc, is_be);
> > +#endif
> >  }
> > 
> >  int qemu_can_send_packet(NetClientState *sender)