Subject: Re: [PATCH] net: tuntap: Fix tun_net_fix_features()

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 17 May 2011 17:54:28 +0300

On Tue, May 17, 2011 at 04:46:35PM +0200, Michał Mirosław wrote:
> On Tue, May 17, 2011 at 05:29:43PM +0300, Michael S. Tsirkin wrote:
> > On Tue, May 17, 2011 at 10:19:54AM +0200, Michał Mirosław wrote:
> > > tun->set_features are meant to limit not force the features.
> > > 
> > > Signed-off-by: Michał Mirosław <[email-masked]>
> > > ---
> > >  drivers/net/tun.c |    2 +-
> > >  1 files changed, 1 insertions(+), 1 deletions(-)
> > > 
> > > diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> > > index 74e9405..f77c6d0 100644
> > > --- a/drivers/net/tun.c
> > > +++ b/drivers/net/tun.c
> > > @@ -458,7 +458,7 @@ static u32 tun_net_fix_features(struct net_device *dev, u32 features)
> > >  {
> > >  	struct tun_struct *tun = netdev_priv(dev);
> > >  
> > > -	return (features & tun->set_features) | (features & ~TUN_USER_FEATURES);
> > > +	return features & (tun->set_features | ~TUN_USER_FEATURES);
> > >  }
> > >  
> > >  static const struct net_device_ops tun_netdev_ops = {
> > > -- 
> > > 1.7.2.5
> > 
> > One thing that this will do though: previously, if
> > ethtool disables offloads, then an application enables
> > them, the application will have the last say.
> > With this patch, the most conservative approach wins.
> > Right?
> 
> Exactly.
> 
> On device creation, wanted_features default to all offloads
> enabled, so unless an admin changes the flags, the application controls
> what is enabled. This matters only when using persistent tun/tap and
> admin and user are two different people. If the admin is using queues
> and doesn't want to handle e.g. TSO packets (I'm not sure if they are
> properly accounted in all queuing disciplines), then the feature should
> not be enabled by user.
> 
> > If we want to have the existing behaviour
> > I think the following would do this (untested). What do you think?
> > 
> > diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> > index 74e9405..1d6c7bc 100644
> > --- a/drivers/net/tun.c
> > +++ b/drivers/net/tun.c
> > @@ -1199,6 +1199,8 @@ static int set_offload(struct tun_struct *tun, unsigned long arg)
> >  		return -EINVAL;
> >  
> >  	tun->set_features = features;
> > +	tun->dev->features &= TUN_USER_FEATURES;
> > +	tun->dev->features |= (features & TUN_USER_FEATURES);
> >  	netdev_update_features(tun->dev);
> 
> tun->dev->features will be recalculated by netdev_update_features()
> anyway. For this to work as you described it would need to alter
> wanted_features. I don't like the idea that something other than one
> of ethtool_ops is changing this field, as it then becomes something
> else that what the admin wants (even if that is not what he gets).
> 
> Best Regards,
> Michał Mirosław

Yes, with virtualization admin and the app are two different people
usually.  The device doesn't have to be persistent though I think -
what limits this to persistent devices?
I agree this behaviour seems more consistent, I just hope this change
does not break any setups.

-- 
MST