Virtualization - Apr 2015 - [PATCH v4 7/8] vhost: feature to set the vring endianness

On Fri, Apr 10, 2015 at 12:19:16PM +0200, Greg Kurz
wrote:
> This patch brings cross-endian support to vhost when used to implement
> legacy virtio devices. Since it is a relatively rare situation, the
> feature availability is controlled by a kernel config option (not set
> by default).
> 
> The vq->is_le boolean field is added to cache the endianness to be
> used for ring accesses. It defaults to native endian, as expected
> by legacy virtio devices. When the ring gets active, we force little
> endian if the device is modern. When the ring is deactivated, we
> revert to the native endian default.
> 
> If cross-endian was compiled in, a vq->user_be boolean field is added
> so that userspace may request a specific endianness. This field is
> used to override the default when activating the ring of a legacy
> device. It has no effect on modern devices.
> 
> Signed-off-by: Greg Kurz <gkurz at linux.vnet.ibm.com>
> ---
>  drivers/vhost/Kconfig      |   10 ++++++
>  drivers/vhost/vhost.c      |   76
+++++++++++++++++++++++++++++++++++++++++++-
>  drivers/vhost/vhost.h      |   12 +++++--
>  include/uapi/linux/vhost.h |    9 +++++
>  4 files changed, 103 insertions(+), 4 deletions(-)
> 
> Changes since v3:
> - VHOST_SET_VRING_ENDIAN_LEGACY ioctl renamed to VHOST_SET_VRING_BIG_ENDIAN
> - ioctl API is now: 0 for le, 1 for be, other values are EINVAL
> - ioctl doesn't filter out modern devices
> - ioctl stubs return ENOIOCTLCMD
> - forbid endianness changes when vring is active
> - logic now handled with vq->is_le and vq->user_be according to
device
>   start/stop as suggested by Michael
> 
> diff --git a/drivers/vhost/Kconfig b/drivers/vhost/Kconfig
> index 017a1e8..0aec88c 100644
> --- a/drivers/vhost/Kconfig
> +++ b/drivers/vhost/Kconfig
> @@ -32,3 +32,13 @@ config VHOST
>  	---help---
>  	  This option is selected by any driver which needs to access
>  	  the core of vhost.
> +
> +config VHOST_SET_ENDIAN_LEGACY
I'd prefer namin this VHOST_CROSS_ENDIAN_LEGACY
> +	bool "Cross-endian support for host kernel accelerator"
> +	default n
> +	---help---
> +	  This option allows vhost to support guests with a different byte
> +	  ordering from host. It is disabled by default since it adds overhead
> +	  and it is only needed by a few platforms (powerpc and arm).
and is only useful on a few platforms (powerpc and arm).

"it" seems to refer to "overhead", which is rarely needed.
needed is a bit too strong, you can always e.g. run virtio
in userspace.
> +
> +	  If unsure, say "N".
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index 2ee2826..3eb756b 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -199,6 +199,10 @@ static void vhost_vq_reset(struct vhost_dev *dev,
>  	vq->call = NULL;
>  	vq->log_ctx = NULL;
>  	vq->memory = NULL;
> +	vq->is_le = virtio_legacy_is_little_endian();
> +#ifdef CONFIG_VHOST_SET_ENDIAN_LEGACY
> +	vq->user_be = !vq->is_le;
> +#endif
add a wrapper for this too?
>  }
>  
>  static int vhost_worker(void *data)
> @@ -630,6 +634,53 @@ static long vhost_set_memory(struct vhost_dev *d,
struct vhost_memory __user *m)
>  	return 0;
>  }
>  
> +#ifdef CONFIG_VHOST_SET_ENDIAN_LEGACY
> +static long vhost_set_vring_big_endian(struct vhost_virtqueue *vq,
> +				       int __user *argp)
> +{
> +	struct vhost_vring_state s;
> +
> +	if (vq->private_data)
> +		return -EBUSY;
> +
> +	if (copy_from_user(&s, argp, sizeof(s)))
> +		return -EFAULT;
> +
> +	if (s.num && s.num != 1)
s.num & ~0x1

> +		return -EINVAL;
> +
> +	vq->user_be = s.num;
> +
> +	return 0;
> +}
> +
> +static long vhost_get_vring_big_endian(struct vhost_virtqueue *vq, u32
idx,
> +				       int __user *argp)
> +{
> +	struct vhost_vring_state s = {
> +		.index = idx,
> +		.num = vq->user_be
> +	};
> +
> +	if (copy_to_user(argp, &s, sizeof(s)))
> +		return -EFAULT;
> +
> +	return 0;
> +}
> +#else
> +static long vhost_set_vring_big_endian(struct vhost_virtqueue *vq,
> +				       int __user *argp)
> +{
> +	return -ENOIOCTLCMD;
> +}
> +
> +static long vhost_get_vring_big_endian(struct vhost_virtqueue *vq, u32
idx,
> +				       int __user *argp)
> +{
> +	return -ENOIOCTLCMD;
> +}
> +#endif /* CONFIG_VHOST_SET_ENDIAN_LEGACY */
> +
>  long vhost_vring_ioctl(struct vhost_dev *d, int ioctl, void __user *argp)
>  {
>  	struct file *eventfp, *filep = NULL;
> @@ -806,6 +857,12 @@ long vhost_vring_ioctl(struct vhost_dev *d, int ioctl,
void __user *argp)
>  		} else
>  			filep = eventfp;
>  		break;
> +	case VHOST_SET_VRING_BIG_ENDIAN:
> +		r = vhost_set_vring_big_endian(vq, argp);
> +		break;
> +	case VHOST_GET_VRING_BIG_ENDIAN:
> +		r = vhost_get_vring_big_endian(vq, idx, argp);
> +		break;
>  	default:
>  		r = -ENOIOCTLCMD;
>  	}
> @@ -1040,12 +1097,29 @@ static int vhost_update_avail_event(struct
vhost_virtqueue *vq, u16 avail_event)
>  	return 0;
>  }
>  
> +#ifdef CONFIG_VHOST_SET_ENDIAN_LEGACY
> +static void vhost_init_is_le(struct vhost_virtqueue *vq)
> +{
> +	vq->is_le = vhost_has_feature(vq, VIRTIO_F_VERSION_1) ||
!vq->user_be;
> +}
> +#else
> +static void vhost_init_is_le(struct vhost_virtqueue *vq)
> +{
> +	if (vhost_has_feature(vq, VIRTIO_F_VERSION_1))
> +		vq->is_le = true;
> +}
> +#endif
> +
I'd prefer localizing ifdefery somewhere near top of file.
>  int vhost_init_used(struct vhost_virtqueue *vq)
>  {
>  	__virtio16 last_used_idx;
>  	int r;
> -	if (!vq->private_data)
> +	if (!vq->private_data) {
> +		vq->is_le = virtio_legacy_is_little_endian();
>  		return 0;
> +	}
> +
> +	vhost_init_is_le(vq);
>  
>  	r = vhost_update_used_flags(vq);
>  	if (r)
> diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
> index 4e9a186..04b2add 100644
> --- a/drivers/vhost/vhost.h
> +++ b/drivers/vhost/vhost.h
> @@ -106,6 +106,14 @@ struct vhost_virtqueue {
>  	/* Log write descriptors */
>  	void __user *log_base;
>  	struct vhost_log *log;
> +
> +	/* Ring endianness. Defaults to legacy native endianness.
> +	 * Set to true when starting a modern virtio device. */
> +	bool is_le;
> +#ifdef CONFIG_VHOST_SET_ENDIAN_LEGACY
> +	/* Ring endianness requested by userspace for cross-endian support. */
> +	bool user_be;
> +#endif
>  };
>  
>  struct vhost_dev {
> @@ -175,9 +183,7 @@ static inline bool vhost_has_feature(struct
vhost_virtqueue *vq, int bit)
>  
>  static inline bool vhost_is_little_endian(struct vhost_virtqueue *vq)
>  {
> -	if (vhost_has_feature(vq, VIRTIO_F_VERSION_1))
> -		return true;
> -	return virtio_legacy_is_little_endian();
> +	return vq->is_le;
>  }
>  
>  /* Memory accessors */
> diff --git a/include/uapi/linux/vhost.h b/include/uapi/linux/vhost.h
> index bb6a5b4..5cdebbc 100644
> --- a/include/uapi/linux/vhost.h
> +++ b/include/uapi/linux/vhost.h
> @@ -103,6 +103,15 @@ struct vhost_memory {
>  /* Get accessor: reads index, writes value in num */
>  #define VHOST_GET_VRING_BASE _IOWR(VHOST_VIRTIO, 0x12, struct
vhost_vring_state)
>  
> +/* Set the vring byte order in num. This is a legacy only API that is
simply
> + * ignored when VIRTIO_F_VERSION_1 is set.
> + * 0 to set to little-endian
> + * 1 to set to big-endian
How about defines for these?
> + * other values return EINVAL.
> + */
> +#define VHOST_SET_VRING_BIG_ENDIAN _IOW(VHOST_VIRTIO, 0x13, struct
vhost_vring_state)
> +#define VHOST_GET_VRING_BIG_ENDIAN _IOW(VHOST_VIRTIO, 0x14, struct
vhost_vring_state)
> +
>  /* The following ioctls use eventfd file descriptors to signal and poll
>   * for events. */
>

On Tue, 21 Apr 2015 16:04:23 +0200
"Michael S. Tsirkin" <mst at redhat.com> wrote:
> On Fri, Apr 10, 2015 at 12:19:16PM +0200, Greg Kurz wrote:
> > This patch brings cross-endian support to vhost when used to implement
> > legacy virtio devices. Since it is a relatively rare situation, the
> > feature availability is controlled by a kernel config option (not set
> > by default).
> > 
> > The vq->is_le boolean field is added to cache the endianness to be
> > used for ring accesses. It defaults to native endian, as expected
> > by legacy virtio devices. When the ring gets active, we force little
> > endian if the device is modern. When the ring is deactivated, we
> > revert to the native endian default.
> > 
> > If cross-endian was compiled in, a vq->user_be boolean field is
added
> > so that userspace may request a specific endianness. This field is
> > used to override the default when activating the ring of a legacy
> > device. It has no effect on modern devices.
> > 
> > Signed-off-by: Greg Kurz <gkurz at linux.vnet.ibm.com>
> > ---
> >  drivers/vhost/Kconfig      |   10 ++++++the
> >  drivers/vhost/vhost.c      |   76
+++++++++++++++++++++++++++++++++++++++++++-
> >  drivers/vhost/vhost.h      |   12 +++++--
> >  include/uapi/linux/vhost.h |    9 +++++
> >  4 files changed, 103 insertions(+), 4 deletions(-)
> > 
> > Changes since v3:
> > - VHOST_SET_VRING_ENDIAN_LEGACY ioctl renamed to
VHOST_SET_VRING_BIG_ENDIAN
> > - ioctl API is now: 0 for le, 1 for be, other values are EINVAL
> > - ioctl doesn't filter out modern devices
> > - ioctl stubs return ENOIOCTLCMD
> > - forbid endianness changes when vring is active
> > - logic now handled with vq->is_le and vq->user_be according to
device
> >   start/stop as suggested by Michael
> > 
> > diff --git a/drivers/vhost/Kconfig b/drivers/vhost/Kconfig
> > index 017a1e8..0aec88c 100644
> > --- a/drivers/vhost/Kconfig
> > +++ b/drivers/vhost/Kconfig
> > @@ -32,3 +32,13 @@ config VHOST
> >  	---help---
> >  	  This option is selected by any driver which needs to access
> >  	  the core of vhost.
> > +
> > +config VHOST_SET_ENDIAN_LEGACY
> 
> I'd prefer namin this VHOST_CROSS_ENDIAN_LEGACY
> 
Yes, makes sense. I'll make sure most of the cross-endian changes are
easy to grep.
> > +	bool "Cross-endian support for host kernel accelerator"
> > +	default n
> > +	---help---
> > +	  This option allows vhost to support guests with a different byte
> > +	  ordering from host. It is disabled by default since it adds
overhead
> > +	  and it is only needed by a few platforms (powerpc and arm).
> 
> and is only useful on a few platforms (powerpc and arm).
> 
> "it" seems to refer to "overhead", which is rarely
needed.
> needed is a bit too strong, you can always e.g. run virtio
> in userspace.
> 
My poor English again... it seems you understood what I wanted to
write though :)
> > +
> > +	  If unsure, say "N".
> > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> > index 2ee2826..3eb756b 100644the
> > --- a/drivers/vhost/vhost.c
> > +++ b/drivers/vhost/vhost.c
> > @@ -199,6 +199,10 @@ static void vhost_vq_reset(struct vhost_dev *dev,
> >  	vq->call = NULL;
> >  	vq->log_ctx = NULL;
> >  	vq->memory = NULL;
> > +	vq->is_le = virtio_legacy_is_little_endian();
> > +#ifdef CONFIG_VHOST_SET_ENDIAN_LEGACY
> > +	vq->user_be = !vq->is_le;
> > +#endif
> 
> add a wrapper for this too?
> 
Will do.
> >  }
> >  
> >  static int vhost_worker(void *data)
> > @@ -630,6 +634,53 @@ static long vhost_set_memory(struct vhost_dev *d,
struct vhost_memory __user *m)
> >  	return 0;
> >  }
> >  
> > +#ifdef CONFIG_VHOST_SET_ENDIAN_LEGACY
> > +static long vhost_set_vring_big_endian(struct vhost_virtqueue *vq,
> > +				       int __user *argp)
> > +{
> > +	struct vhost_vring_state s;
> > +
> > +	if (vq->private_data)
> > +		return -EBUSY;
> > +
> > +	if (copy_from_user(&s, argp, sizeof(s)))
> > +		return -EFAULT;
> > +
> > +	if (s.num && s.num != 1)
> 
> s.num & ~0x1
> 
Since s.num is unsigned and I assume this won't change, what about
s.num > 1 as suggested by Cornelia ?
> 
> > +		return -EINVAL;
> > +
> > +	vq->user_be = s.num;
> > +
> > +	return 0;
> > +}
> > +
> > +static long vhost_get_vring_big_endian(struct vhost_virtqueue *vq,
u32 idx,
> > +				       int __user *argp)
> > +{
> > +	struct vhost_vring_state s = {
> > +		.index = idx,
> > +		.num = vq->user_be
> > +	};
> > +
> > +	if (copy_to_user(argp, &s, sizeof(s)))
> > +		return -EFAULT;
> > +
> > +	return 0;
> > +}
> > +#else
> > +static long vhost_set_vring_big_endian(struct vhost_virtqueue *vq,
> > +				       int __user *argp)
> > +{
> > +	return -ENOIOCTLCMD;
> > +}
> > +
> > +static long vhost_get_vring_big_endian(struct vhost_virtqueue *vq,
u32 idx,
> > +				       int __user *argp)
> > +{
> > +	return -ENOIOCTLCMD;
> > +}
> > +#endif /* CONFIG_VHOST_SET_ENDIAN_LEGACY */
> > +
> >  long vhost_vring_ioctl(struct vhost_dev *d, int ioctl, void __user
*argp)
> >  {
> >  	struct file *eventfp, *filep = NULL;
> > @@ -806,6 +857,12 @@ long vhost_vring_ioctl(struct vhost_dev *d, int
ioctl, void __user *argp)
> >  		} else
> >  			filep = eventfp;the
> >  		break;
> > +	case VHOST_SET_VRING_BIG_ENDIAN:
> > +		r = vhost_set_vring_big_endian(vq, argp);
> > +		break;
> > +	case VHOST_GET_VRING_BIG_ENDIAN:
> > +		r = vhost_get_vring_big_endian(vq, idx, argp);
> > +		break;
> >  	default:
> >  		r = -ENOIOCTLCMD;
> >  	}
> > @@ -1040,12 +1097,29 @@ static int vhost_update_avail_event(struct
vhost_virtqueue *vq, u16 avail_event)
> >  	return 0;
> >  }
> >  
> > +#ifdef CONFIG_VHOST_SET_ENDIAN_LEGACY
> > +static void vhost_init_is_le(struct vhost_virtqueue *vq)
> > +{
> > +	vq->is_le = vhost_has_feature(vq, VIRTIO_F_VERSION_1) ||
!vq->user_be;
> > +}
> > +#else
> > +static void vhost_init_is_le(struct vhost_virtqueue *vq)
> > +{
> > +	if (vhost_has_feature(vq, VIRTIO_F_VERSION_1))
> > +		vq->is_le = true;
> > +}
> > +#endif
> > +
> 
> I'd prefer localizing ifdefery somewhere near top of file.
> 
Will do.
> >  int vhost_init_used(struct vhost_virtqueue *vq)
> >  {
> >  	__virtio16 last_used_idx;
> >  	int r;
> > -	if (!vq->private_data)
> > +	if (!vq->private_data) {
> > +		vq->is_le = virtio_legacy_is_little_endian();
> >  		return 0;
> > +	}
> > +
> > +	vhost_init_is_le(vq);
> >  
> >  	r = vhost_update_used_flags(vq);
> >  	if (r)
> > diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
> > index 4e9a186..04b2add 100644
> > --- a/drivers/vhost/vhost.h
> > +++ b/drivers/vhost/vhost.h
> > @@ -106,6 +106,14 @@ struct vhost_virtqueue {
> >  	/* Log write descriptors */
> >  	void __user *log_base;
> >  	struct vhost_log *log;
> > +
> > +	/* Ring endianness. Defaults to legacy native endianness.
> > +	 * Set to true when starting a modern virtio device. */
> > +	bool is_le;
> > +#ifdef CONFIG_VHOST_SET_ENDIAN_LEGACY
> > +	/* Ring endianness requested by userspace for cross-endian support.
*/
> > +	bool user_be;
> > +#endif
> >  };
> >  
> >  struct vhost_dev {
> > @@ -175,9 +183,7 @@ static inline bool vhost_has_feature(struct
vhost_virtqueue *vq, int bit)
> >  
> >  static inline bool vhost_is_little_endian(struct vhost_virtqueue *vq)
> >  {
> > -	if (vhost_has_feature(vq, VIRTIO_F_VERSION_1))
> > -		return true;
> > -	return virtio_legacy_is_little_endian();
> > +	return vq->is_le;
> >  }
> >  
> >  /* Memory accessors */
> > diff --git a/include/uapi/linux/vhost.h b/include/uapi/linux/vhost.h
> > index bb6a5b4..5cdebbc 100644
> > --- a/include/uapi/linux/vhost.h
> > +++ b/include/uapi/linux/vhost.h
> > @@ -103,6 +103,15 @@ struct vhost_memory {
> >  /* Get accessor: reads index, writes value in num */
> >  #define VHOST_GET_VRING_BASE _IOWR(VHOST_VIRTIO, 0x12, struct
vhost_vring_state)
> >  
> > +/* Set the vring byte order in num. This is a legacy only API that is
simply
> > + * ignored when VIRTIO_F_VERSION_1 is set.
> > + * 0 to set to little-endian
> > + * 1 to set to big-endian
> 
> How about defines for these?
> 
Ok. I'll put the defines here so that all the cross-endian stuff
lies in the same hunk. Is it ok for you ?
> > + * other values return EINVAL.
> > + */
> > +#define VHOST_SET_VRING_BIG_ENDIAN _IOW(VHOST_VIRTIO, 0x13, struct
vhost_vring_state)
> > +#define VHOST_GET_VRING_BIG_ENDIAN _IOW(VHOST_VIRTIO, 0x14, struct
vhost_vring_state)
> > +
> >  /* The following ioctls use eventfd file descriptors to signal and
poll
> >   * for events. */
> >  
>

On Tue, Apr 21, 2015 at 05:48:20PM +0200, Greg Kurz
wrote:
> On Tue, 21 Apr 2015 16:04:23 +0200
> "Michael S. Tsirkin" <mst at redhat.com> wrote:
> 
> > On Fri, Apr 10, 2015 at 12:19:16PM +0200, Greg Kurz wrote:
> > > This patch brings cross-endian support to vhost when used to
implement
> > > legacy virtio devices. Since it is a relatively rare situation,
the
> > > feature availability is controlled by a kernel config option (not
set
> > > by default).
> > > 
> > > The vq->is_le boolean field is added to cache the endianness
to be
> > > used for ring accesses. It defaults to native endian, as expected
> > > by legacy virtio devices. When the ring gets active, we force
little
> > > endian if the device is modern. When the ring is deactivated, we
> > > revert to the native endian default.
> > > 
> > > If cross-endian was compiled in, a vq->user_be boolean field
is added
> > > so that userspace may request a specific endianness. This field
is
> > > used to override the default when activating the ring of a legacy
> > > device. It has no effect on modern devices.
> > > 
> > > Signed-off-by: Greg Kurz <gkurz at linux.vnet.ibm.com>
> > > ---
> > >  drivers/vhost/Kconfig      |   10 ++++++the
> > >  drivers/vhost/vhost.c      |   76
+++++++++++++++++++++++++++++++++++++++++++-
> > >  drivers/vhost/vhost.h      |   12 +++++--
> > >  include/uapi/linux/vhost.h |    9 +++++
> > >  4 files changed, 103 insertions(+), 4 deletions(-)
> > > 
> > > Changes since v3:
> > > - VHOST_SET_VRING_ENDIAN_LEGACY ioctl renamed to
VHOST_SET_VRING_BIG_ENDIAN
> > > - ioctl API is now: 0 for le, 1 for be, other values are EINVAL
> > > - ioctl doesn't filter out modern devices
> > > - ioctl stubs return ENOIOCTLCMD
> > > - forbid endianness changes when vring is active
> > > - logic now handled with vq->is_le and vq->user_be
according to device
> > >   start/stop as suggested by Michael
> > > 
> > > diff --git a/drivers/vhost/Kconfig b/drivers/vhost/Kconfig
> > > index 017a1e8..0aec88c 100644
> > > --- a/drivers/vhost/Kconfig
> > > +++ b/drivers/vhost/Kconfig
> > > @@ -32,3 +32,13 @@ config VHOST
> > >  	---help---
> > >  	  This option is selected by any driver which needs to access
> > >  	  the core of vhost.
> > > +
> > > +config VHOST_SET_ENDIAN_LEGACY
> > 
> > I'd prefer namin this VHOST_CROSS_ENDIAN_LEGACY
> > 
> 
> Yes, makes sense. I'll make sure most of the cross-endian changes are
> easy to grep.
> 
> > > +	bool "Cross-endian support for host kernel
accelerator"
> > > +	default n
> > > +	---help---
> > > +	  This option allows vhost to support guests with a different
byte
> > > +	  ordering from host. It is disabled by default since it adds
overhead
> > > +	  and it is only needed by a few platforms (powerpc and arm).
> > 
> > and is only useful on a few platforms (powerpc and arm).
> > 
> > "it" seems to refer to "overhead", which is rarely
needed.
> > needed is a bit too strong, you can always e.g. run virtio
> > in userspace.
> > 
> 
> My poor English again... it seems you understood what I wanted to
> write though :)
> 
> > > +
> > > +	  If unsure, say "N".
> > > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> > > index 2ee2826..3eb756b 100644the
> > > --- a/drivers/vhost/vhost.c
> > > +++ b/drivers/vhost/vhost.c
> > > @@ -199,6 +199,10 @@ static void vhost_vq_reset(struct vhost_dev
*dev,
> > >  	vq->call = NULL;
> > >  	vq->log_ctx = NULL;
> > >  	vq->memory = NULL;
> > > +	vq->is_le = virtio_legacy_is_little_endian();
> > > +#ifdef CONFIG_VHOST_SET_ENDIAN_LEGACY
> > > +	vq->user_be = !vq->is_le;
> > > +#endif
> > 
> > add a wrapper for this too?
> > 
> 
> Will do.
> 
> > >  }
> > >  
> > >  static int vhost_worker(void *data)
> > > @@ -630,6 +634,53 @@ static long vhost_set_memory(struct
vhost_dev *d, struct vhost_memory __user *m)
> > >  	return 0;
> > >  }
> > >  
> > > +#ifdef CONFIG_VHOST_SET_ENDIAN_LEGACY
> > > +static long vhost_set_vring_big_endian(struct vhost_virtqueue
*vq,
> > > +				       int __user *argp)
> > > +{
> > > +	struct vhost_vring_state s;
> > > +
> > > +	if (vq->private_data)
> > > +		return -EBUSY;
> > > +
> > > +	if (copy_from_user(&s, argp, sizeof(s)))
> > > +		return -EFAULT;
> > > +
> > > +	if (s.num && s.num != 1)
> > 
> > s.num & ~0x1
> > 
> 
> Since s.num is unsigned and I assume this won't change, what about
> s.num > 1 as suggested by Cornelia ?
I just tried and gcc optimizes
s.num != 0 && s.num != 1 to s.num > 1

The former will be more readable once we
replace 0 and 1 with defines.

So ignore my advice, keep code as is but use defines.


> > 
> > > +		return -EINVAL;
> > > +
> > > +	vq->user_be = s.num;
> > > +
> > > +	return 0;
> > > +}
> > > +
> > > +static long vhost_get_vring_big_endian(struct vhost_virtqueue
*vq, u32 idx,
> > > +				       int __user *argp)
> > > +{
> > > +	struct vhost_vring_state s = {
> > > +		.index = idx,
> > > +		.num = vq->user_be
> > > +	};
> > > +
> > > +	if (copy_to_user(argp, &s, sizeof(s)))
> > > +		return -EFAULT;
> > > +
> > > +	return 0;
> > > +}
> > > +#else
> > > +static long vhost_set_vring_big_endian(struct vhost_virtqueue
*vq,
> > > +				       int __user *argp)
> > > +{
> > > +	return -ENOIOCTLCMD;
> > > +}
> > > +
> > > +static long vhost_get_vring_big_endian(struct vhost_virtqueue
*vq, u32 idx,
> > > +				       int __user *argp)
> > > +{
> > > +	return -ENOIOCTLCMD;
> > > +}
> > > +#endif /* CONFIG_VHOST_SET_ENDIAN_LEGACY */
> > > +
> > >  long vhost_vring_ioctl(struct vhost_dev *d, int ioctl, void
__user *argp)
> > >  {
> > >  	struct file *eventfp, *filep = NULL;
> > > @@ -806,6 +857,12 @@ long vhost_vring_ioctl(struct vhost_dev *d,
int ioctl, void __user *argp)
> > >  		} else
> > >  			filep = eventfp;the
> > >  		break;
> > > +	case VHOST_SET_VRING_BIG_ENDIAN:
> > > +		r = vhost_set_vring_big_endian(vq, argp);
> > > +		break;
> > > +	case VHOST_GET_VRING_BIG_ENDIAN:
> > > +		r = vhost_get_vring_big_endian(vq, idx, argp);
> > > +		break;
> > >  	default:
> > >  		r = -ENOIOCTLCMD;
> > >  	}
> > > @@ -1040,12 +1097,29 @@ static int
vhost_update_avail_event(struct vhost_virtqueue *vq, u16 avail_event)
> > >  	return 0;
> > >  }
> > >  
> > > +#ifdef CONFIG_VHOST_SET_ENDIAN_LEGACY
> > > +static void vhost_init_is_le(struct vhost_virtqueue *vq)
> > > +{
> > > +	vq->is_le = vhost_has_feature(vq, VIRTIO_F_VERSION_1) ||
!vq->user_be;
> > > +}
> > > +#else
> > > +static void vhost_init_is_le(struct vhost_virtqueue *vq)
> > > +{
> > > +	if (vhost_has_feature(vq, VIRTIO_F_VERSION_1))
> > > +		vq->is_le = true;
> > > +}
> > > +#endif
> > > +
> > 
> > I'd prefer localizing ifdefery somewhere near top of file.
> > 
> 
> Will do.
> 
> > >  int vhost_init_used(struct vhost_virtqueue *vq)
> > >  {
> > >  	__virtio16 last_used_idx;
> > >  	int r;
> > > -	if (!vq->private_data)
> > > +	if (!vq->private_data) {
> > > +		vq->is_le = virtio_legacy_is_little_endian();
> > >  		return 0;
> > > +	}
> > > +
> > > +	vhost_init_is_le(vq);
> > >  
> > >  	r = vhost_update_used_flags(vq);
> > >  	if (r)
> > > diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
> > > index 4e9a186..04b2add 100644
> > > --- a/drivers/vhost/vhost.h
> > > +++ b/drivers/vhost/vhost.h
> > > @@ -106,6 +106,14 @@ struct vhost_virtqueue {
> > >  	/* Log write descriptors */
> > >  	void __user *log_base;
> > >  	struct vhost_log *log;
> > > +
> > > +	/* Ring endianness. Defaults to legacy native endianness.
> > > +	 * Set to true when starting a modern virtio device. */
> > > +	bool is_le;
> > > +#ifdef CONFIG_VHOST_SET_ENDIAN_LEGACY
> > > +	/* Ring endianness requested by userspace for cross-endian
support. */
> > > +	bool user_be;
> > > +#endif
> > >  };
> > >  
> > >  struct vhost_dev {
> > > @@ -175,9 +183,7 @@ static inline bool vhost_has_feature(struct
vhost_virtqueue *vq, int bit)
> > >  
> > >  static inline bool vhost_is_little_endian(struct vhost_virtqueue
*vq)
> > >  {
> > > -	if (vhost_has_feature(vq, VIRTIO_F_VERSION_1))
> > > -		return true;
> > > -	return virtio_legacy_is_little_endian();
> > > +	return vq->is_le;
> > >  }
> > >  
> > >  /* Memory accessors */
> > > diff --git a/include/uapi/linux/vhost.h
b/include/uapi/linux/vhost.h
> > > index bb6a5b4..5cdebbc 100644
> > > --- a/include/uapi/linux/vhost.h
> > > +++ b/include/uapi/linux/vhost.h
> > > @@ -103,6 +103,15 @@ struct vhost_memory {
> > >  /* Get accessor: reads index, writes value in num */
> > >  #define VHOST_GET_VRING_BASE _IOWR(VHOST_VIRTIO, 0x12, struct
vhost_vring_state)
> > >  
> > > +/* Set the vring byte order in num. This is a legacy only API
that is simply
> > > + * ignored when VIRTIO_F_VERSION_1 is set.
> > > + * 0 to set to little-endian
> > > + * 1 to set to big-endian
> > 
> > How about defines for these?
> > 
> 
> Ok. I'll put the defines here so that all the cross-endian stuff
> lies in the same hunk. Is it ok for you ?
Fine.
> > > + * other values return EINVAL.
Pls also add a note saying that not all kernel configurations support this
ioctl,
but all configurations that support SET also support GET.
> > > + */
> > > +#define VHOST_SET_VRING_BIG_ENDIAN _IOW(VHOST_VIRTIO, 0x13,
struct vhost_vring_state)
> > > +#define VHOST_GET_VRING_BIG_ENDIAN _IOW(VHOST_VIRTIO, 0x14,
struct vhost_vring_state)
> > > +
> > >  /* The following ioctls use eventfd file descriptors to signal
and poll
> > >   * for events. */
> > >  
> > 
I'm inclined to think VHOST_SET_VRING_ENDIAN is a slightly better name.
What do you think?

-- 
MST