Re: [PATCH v5 7/8] vhost: cross-endian support for legacy devices

[Greg Kurz]

On Fri, 24 Apr 2015 09:19:26 +0200
Cornelia Huck <cornelia.huck@xxxxxxxxxx> wrote:

> On Thu, 23 Apr 2015 17:29:42 +0200
> Greg Kurz <gkurz@xxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxx>
> > ---
> > 
> > Changes since v4:
> > - rewrote patch title to mention cross-endian
> > - renamed config to VHOST_CROSS_ENDIAN_LEGACY
> > - rewrote config description and help
> > - moved ifdefery to top of vhost.c
> > - added a detailed comment about the lifecycle of vq->user_be in
> >   vhost_init_is_le()
> > - renamed ioctls to VHOST_[GS]ET_VRING_ENDIAN
> > - added LE/BE defines to the ioctl API
> > - rewrote ioctl sanity check with the LE/BE defines
> > - updated comment in <uapi/linux/vhost.h> to mention that the availibility
> >   of both SET and GET ioctls depends on the kernel config
> > 
> >  drivers/vhost/Kconfig      |   15 ++++++++
> >  drivers/vhost/vhost.c      |   86 +++++++++++++++++++++++++++++++++++++++++++-
> >  drivers/vhost/vhost.h      |   10 +++++
> >  include/uapi/linux/vhost.h |   12 ++++++
> >  4 files changed, 121 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/vhost/Kconfig b/drivers/vhost/Kconfig
> > index 017a1e8..74d7380 100644
> > --- a/drivers/vhost/Kconfig
> > +++ b/drivers/vhost/Kconfig
> > @@ -32,3 +32,18 @@ config VHOST
> >  	---help---
> >  	  This option is selected by any driver which needs to access
> >  	  the core of vhost.
> > +
> > +config VHOST_CROSS_ENDIAN_LEGACY
> > +	bool "Cross-endian support for vhost"
> > +	default n
> > +	---help---
> > +	  This option allows vhost to support guests with a different byte
> > +	  ordering from host.
> 
> "...while using legacy virtio."
> 
> Might help to explain the "LEGACY" in the config option ;)
> 

Makes sense indeed !

> > +
> > +	  Userspace programs can control the feature using the
> > +	  VHOST_SET_VRING_ENDIAN and VHOST_GET_VRING_ENDIAN ioctls.
> > +
> > +	  This is only useful on a few platforms (ppc64 and arm64). Since it
> > +	  adds some overhead, it is disabled default.
> 
> s/default/by default/
> 

Ok.

> > +
> > +	  If unsure, say "N".
> > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> > index 2ee2826..8c4390d 100644
> > --- a/drivers/vhost/vhost.c
> > +++ b/drivers/vhost/vhost.c
> > @@ -36,6 +36,78 @@ enum {
> >  #define vhost_used_event(vq) ((__virtio16 __user *)&vq->avail->ring[vq->num])
> >  #define vhost_avail_event(vq) ((__virtio16 __user *)&vq->used->ring[vq->num])
> > 
> > +#ifdef CONFIG_VHOST_CROSS_ENDIAN_LEGACY
> > +static void vhost_vq_reset_user_be(struct vhost_virtqueue *vq)
> > +{
> > +	vq->user_be = !virtio_legacy_is_little_endian();
> > +}
> > +
> > +static long vhost_set_vring_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 != VHOST_VRING_LITTLE_ENDIAN &&
> > +	    s.num != VHOST_VRING_BIG_ENDIAN)
> > +		return -EINVAL;
> > +
> > +	vq->user_be = s.num;
> > +
> > +	return 0;
> > +}
> > +
> > +static long vhost_get_vring_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;
> > +}
> > +
> > +static void vhost_init_is_le(struct vhost_virtqueue *vq)
> > +{
> > +	/* Note for legacy virtio: user_be is initialized at reset time
> > +	 * according to the host endianness. If userspace does not set an
> > +	 * explicit endianness, the default behavior is native endian, as
> > +	 * expected by legacy virtio.
> > +	 */
> > +	vq->is_le = vhost_has_feature(vq, VIRTIO_F_VERSION_1) || !vq->user_be;
> > +}
> > +#else
> > +static void vhost_vq_reset_user_be(struct vhost_virtqueue *vq)
> > +{
> > +	;
> 
> Just leave the function body empty?
> 

Ok.

> > +}
> > +
> > +static long vhost_set_vring_endian(struct vhost_virtqueue *vq, int __user *argp)
> > +{
> > +	return -ENOIOCTLCMD;
> > +}
> > +
> > +static long vhost_get_vring_endian(struct vhost_virtqueue *vq, u32 idx,
> > +				   int __user *argp)
> > +{
> > +	return -ENOIOCTLCMD;
> > +}
> > +
> > +static void vhost_init_is_le(struct vhost_virtqueue *vq)
> > +{
> > +	if (vhost_has_feature(vq, VIRTIO_F_VERSION_1))
> > +		vq->is_le = true;
> > +}
> > +#endif /* CONFIG_VHOST_CROSS_ENDIAN_LEGACY */
> > +
> >  static void vhost_poll_func(struct file *file, wait_queue_head_t *wqh,
> >  			    poll_table *pt)
> >  {
> (...)
> > diff --git a/include/uapi/linux/vhost.h b/include/uapi/linux/vhost.h
> > index bb6a5b4..b980b53 100644
> > --- a/include/uapi/linux/vhost.h
> > +++ b/include/uapi/linux/vhost.h
> > @@ -103,6 +103,18 @@ 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. Valid values are VHOST_VRING_LITTLE_ENDIAN
> > + * or VHOST_VRING_BIG_ENDIAN (other values return EINVAL).
> 
> -EINVAL?
> 

Oops, yes. :)

> Should you also mention when you return -EBUSY?
> 

Indeed... what about:

"The byte order cannot be changed when the device is active: trying to do so
returns -EBUSY."

> > + * This is a legacy only API that is simply ignored when VIRTIO_F_VERSION_1 is
> > + * set.
> > + * Not all kernel configurations support this ioctl, but all configurations that
> > + * support SET also support GET.
> > + */
> > +#define VHOST_VRING_LITTLE_ENDIAN 0
> > +#define VHOST_VRING_BIG_ENDIAN 1
> > +#define VHOST_SET_VRING_ENDIAN _IOW(VHOST_VIRTIO, 0x13, struct vhost_vring_state)
> > +#define VHOST_GET_VRING_ENDIAN _IOW(VHOST_VIRTIO, 0x14, struct vhost_vring_state)
> > +
> >  /* The following ioctls use eventfd file descriptors to signal and poll
> >   * for events. */
> 
> Apart from style nitpicks, looks good.
> 

Cool ! :)

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