Re: [PATCH v4 8/8] macvtap/tun: add VNET_BE flag

[Greg Kurz]

On Tue, 21 Apr 2015 16:06:33 +0200
"Michael S. Tsirkin" <mst@xxxxxxxxxx> wrote:

> On Fri, Apr 10, 2015 at 12:20:21PM +0200, Greg Kurz wrote:
> > The VNET_LE flag was introduced to fix accesses to virtio 1.0 headers
> > that are always little-endian. It can also be used to handle the special
> > case of a legacy little-endian device implemented by a big-endian host.
> > 
> > Let's add a flag and ioctls for big-endian devices as well. If both flags
> > are set, little-endian wins.
> > 
> > Since this is isn't a common usecase, the feature is controlled by a kernel
> > config option (not set by default).
> > 
> > Both macvtap and tun are covered by this patch since they share the same
> > API with userland.
> > 
> > Signed-off-by: Greg Kurz <gkurz@xxxxxxxxxxxxxxxxxx>
> > ---
> >  drivers/net/Kconfig         |   12 ++++++++
> >  drivers/net/macvtap.c       |   60 +++++++++++++++++++++++++++++++++++++++++-
> >  drivers/net/tun.c           |   62 ++++++++++++++++++++++++++++++++++++++++++-
> >  include/uapi/linux/if_tun.h |    2 +
> >  4 files changed, 134 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/net/Kconfig b/drivers/net/Kconfig
> > index df51d60..f0e23a0 100644
> > --- a/drivers/net/Kconfig
> > +++ b/drivers/net/Kconfig
> > @@ -244,6 +244,18 @@ config TUN
> >  
> >  	  If you don't know what to use this for, you don't need it.
> >  
> > +config TUN_VNET_BE
> > +	bool "Support for big-endian vnet headers"
> > +	default n
> > +	---help---
> > +	  This option allows TUN/TAP and MACVTAP device drivers to parse
> > +	  vnet headers that are in big-endian byte order. It is useful
> > +	  when the headers come from a big-endian legacy virtio driver and
> > +	  the host is little-endian.
> > +
> > +	  Unless you have a little-endian system hosting a big-endian virtual
> > +	  machine with a virtio NIC, you should say N.
> > +
> 
> should mention cross-endian, not big-endian, right?
> 

The current TUN_VNET_LE related code is already doing cross-endian: without
this patch, one can already run a LE guest on a BE host... wouldn't it be
confusing to mention cross-endian only when the guest is BE ?

What about having a completely distinct implementation for cross-endian that
don't reuse the existing code and defines then ?

> >  config VETH
> >  	tristate "Virtual ethernet pair device"
> >  	---help---
> > diff --git a/drivers/net/macvtap.c b/drivers/net/macvtap.c
> > index 0a03a66..e0ab1b7 100644
> > --- a/drivers/net/macvtap.c
> > +++ b/drivers/net/macvtap.c
> > @@ -48,12 +48,27 @@ struct macvtap_queue {
> >  #define MACVTAP_FEATURES (IFF_VNET_HDR | IFF_MULTI_QUEUE)
> >  
> >  #define MACVTAP_VNET_LE 0x80000000
> > +#define MACVTAP_VNET_BE 0x40000000
> > +
> > +#ifdef CONFIG_TUN_VNET_BE
> > +static inline bool macvtap_legacy_is_little_endian(struct macvtap_queue *q)
> > +{
> > +	if (q->flags & MACVTAP_VNET_BE)
> > +		return false;
> > +	return virtio_legacy_is_little_endian();
> > +}
> > +#else
> > +static inline bool macvtap_legacy_is_little_endian(struct macvtap_queue *q)
> > +{
> > +	return virtio_legacy_is_little_endian();
> > +}
> > +#endif
> >  
> >  static inline bool macvtap_is_little_endian(struct macvtap_queue *q)
> >  {
> >  	if (q->flags & MACVTAP_VNET_LE)
> >  		return true;
> > -	return virtio_legacy_is_little_endian();
> > +	return macvtap_legacy_is_little_endian(q);
> >  }
> >  
> >  static inline u16 macvtap16_to_cpu(struct macvtap_queue *q, __virtio16 val)
> > @@ -1000,6 +1015,43 @@ static int set_offload(struct macvtap_queue *q, unsigned long arg)
> >  	return 0;
> >  }
> >  
> > +#ifdef CONFIG_TUN_VNET_BE
> > +static long macvtap_get_vnet_be(struct macvtap_queue *q, int __user *sp)
> > +{
> > +	int s = !!(q->flags & MACVTAP_VNET_BE);
> > +
> > +	if (put_user(s, sp))
> > +		return -EFAULT;
> > +
> > +	return 0;
> > +}
> > +
> > +static long macvtap_set_vnet_be(struct macvtap_queue *q, int __user *sp)
> > +{
> > +	int s;
> > +
> > +	if (get_user(s, sp))
> > +		return -EFAULT;
> > +
> > +	if (s)
> > +		q->flags |= MACVTAP_VNET_BE;
> > +	else
> > +		q->flags &= ~MACVTAP_VNET_BE;
> > +
> > +	return 0;
> > +}
> > +#else
> > +static long macvtap_get_vnet_be(struct macvtap_queue *q, int __user *argp)
> > +{
> > +	return -EINVAL;
> > +}
> > +
> > +static long macvtap_set_vnet_be(struct macvtap_queue *q, int __user *argp)
> > +{
> > +	return -EINVAL;
> > +}
> > +#endif /* CONFIG_TUN_VNET_BE */
> > +
> >  /*
> >   * provide compatibility with generic tun/tap interface
> >   */
> > @@ -1097,6 +1149,12 @@ static long macvtap_ioctl(struct file *file, unsigned int cmd,
> >  			q->flags &= ~MACVTAP_VNET_LE;
> >  		return 0;
> >  
> > +	case TUNGETVNETBE:
> > +		return macvtap_get_vnet_be(q, sp);
> > +
> > +	case TUNSETVNETBE:
> > +		return macvtap_set_vnet_be(q, sp);
> > +
> >  	case TUNSETOFFLOAD:
> >  		/* let the user check for future flags */
> >  		if (arg & ~(TUN_F_CSUM | TUN_F_TSO4 | TUN_F_TSO6 |
> > diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> > index 053f9b6..4e12488 100644
> > --- a/drivers/net/tun.c
> > +++ b/drivers/net/tun.c
> > @@ -111,6 +111,7 @@ do {								\
> >  #define TUN_FASYNC	IFF_ATTACH_QUEUE
> >  /* High bits in flags field are unused. */
> >  #define TUN_VNET_LE     0x80000000
> > +#define TUN_VNET_BE     0x40000000
> >  
> >  #define TUN_FEATURES (IFF_NO_PI | IFF_ONE_QUEUE | IFF_VNET_HDR | \
> >  		      IFF_MULTI_QUEUE)
> > @@ -206,11 +207,25 @@ struct tun_struct {
> >  	u32 flow_count;
> >  };
> >  
> > +#ifdef CONFIG_TUN_VNET_BE
> > +static inline bool tun_legacy_is_little_endian(struct tun_struct *tun)
> > +{
> > +	if (tun->flags & TUN_VNET_BE)
> > +		return false;
> > +	return virtio_legacy_is_little_endian();
> > +}
> > +#else
> > +static inline bool tun_legacy_is_little_endian(struct tun_struct *tun)
> > +{
> > +	return virtio_legacy_is_little_endian();
> > +}
> > +#endif
> > +
> >  static inline bool tun_is_little_endian(struct tun_struct *tun)
> >  {
> >  	if (tun->flags & TUN_VNET_LE)
> >  		return true;
> > -	return virtio_legacy_is_little_endian();
> > +	return tun_legacy_is_little_endian(tun);
> >  }
> >  
> >  static inline u16 tun16_to_cpu(struct tun_struct *tun, __virtio16 val)
> > @@ -1836,6 +1851,43 @@ unlock:
> >  	return ret;
> >  }
> >  
> > +#ifdef CONFIG_TUN_VNET_BE
> > +static long tun_get_vnet_be(struct tun_struct *tun, int __user *argp)
> > +{
> > +	int be = !!(tun->flags & TUN_VNET_BE);
> > +
> > +	if (put_user(be, argp))
> > +		return EFAULT;
> > +
> > +	return 0;
> > +}
> > +
> > +static long tun_set_vnet_be(struct tun_struct *tun, int __user *argp)
> > +{
> > +	int be;
> > +
> > +	if (get_user(be, argp))
> > +		return -EFAULT;
> > +
> > +	if (be)
> > +		tun->flags |= TUN_VNET_BE;
> > +	else
> > +		tun->flags &= ~TUN_VNET_BE;
> > +
> > +	return 0;
> > +}
> > +#else
> > +static long tun_get_vnet_be(struct tun_struct *tun, int __user *argp)
> > +{
> > +	return -EINVAL;
> > +}
> > +
> > +static long tun_set_vnet_be(struct tun_struct *tun, int __user *argp)
> > +{
> > +	return -EINVAL;
> > +}
> > +#endif /* CONFIG_TUN_VNET_BE */
> > +
> >  static long __tun_chr_ioctl(struct file *file, unsigned int cmd,
> >  			    unsigned long arg, int ifreq_len)
> >  {
> > @@ -2065,6 +2117,14 @@ static long __tun_chr_ioctl(struct file *file, unsigned int cmd,
> >  			tun->flags &= ~TUN_VNET_LE;
> >  		break;
> >  
> > +	case TUNGETVNETBE:
> > +		ret = tun_get_vnet_be(tun, argp);
> > +		break;
> > +
> > +	case TUNSETVNETBE:
> > +		ret = tun_set_vnet_be(tun, argp);
> > +		break;
> > +
> >  	case TUNATTACHFILTER:
> >  		/* Can be set only for TAPs */
> >  		ret = -EINVAL;
> > diff --git a/include/uapi/linux/if_tun.h b/include/uapi/linux/if_tun.h
> > index 50ae243..bcac4c0 100644
> > --- a/include/uapi/linux/if_tun.h
> > +++ b/include/uapi/linux/if_tun.h
> > @@ -50,6 +50,8 @@
> >  #define TUNGETFILTER _IOR('T', 219, struct sock_fprog)
> >  #define TUNSETVNETLE _IOW('T', 220, int)
> >  #define TUNGETVNETLE _IOR('T', 221, int)
> > +#define TUNSETVNETBE _IOW('T', 222, int)
> > +#define TUNGETVNETBE _IOR('T', 223, int)
> >  
> >  /* TUNSETIFF ifr flags */
> >  #define IFF_TUN		0x0001
> 

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