Re: [PATCH 1/2] vhost: helpers to enable/disable vring endianness

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

On Wed, Feb 10, 2016 at 01:11:34PM +0100, Greg Kurz wrote:
> On Wed, 10 Feb 2016 13:21:22 +0200
> "Michael S. Tsirkin" <mst@redhat.com> wrote:
> 
> > On Wed, Jan 13, 2016 at 06:09:41PM +0100, Greg Kurz wrote:
> > > The default use case for vhost is when the host and the vring have the
> > > same endianness (default native endianness). But there are cases where
> > > they differ and vhost should byteswap when accessing the vring:
> > > - the host is big endian and the vring comes from a virtio 1.0 device
> > >   which is always little endian
> > > - the architecture is bi-endian and the vring comes from a legacy virtio
> > >   device with a different endianness than the endianness of the host (aka
> > >   legacy cross-endian)
> > > 
> > > These cases are handled by the vq->is_le and the optional vq->user_be,
> > > with the following logic:
> > > - if none of the fields is enabled, vhost access the vring without byteswap
> > > - if the vring is virtio 1.0 and the host is big endian, vq->is_le is
> > >   enabled to enforce little endian access to the vring
> > > - if the vring is legacy cross-endian, userspace enables vq->user_be
> > >   to inform vhost about the vring endianness. This endianness is then
> > >   enforced for vring accesses through vq->is_le again
> > > 
> > > The logic is unclear in the current code.
> > > 
> > > This patch introduces helpers with explicit enable and disable semantics,
> > > for better clarity.
> > > 
> > > No behaviour change.
> > > 
> > > Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
> > > ---
> > >  drivers/vhost/vhost.c |   28 +++++++++++++++++++---------
> > >  1 file changed, 19 insertions(+), 9 deletions(-)
> > > 
> > > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> > > index ad2146a9ab2d..e02e06755ab7 100644
> > > --- a/drivers/vhost/vhost.c
> > > +++ b/drivers/vhost/vhost.c
> > > @@ -43,11 +43,16 @@ enum {
> > >  #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)
> > > +static void vhost_disable_user_be(struct vhost_virtqueue *vq)
> > >  {
> > >  	vq->user_be = !virtio_legacy_is_little_endian();
> > >  }
> > >    
> > 
> > Hmm this doesn't look like an improvement to me.
> > What does it mean to disable big endian? Make it little endian?
> 
> The default behavior for the device is native endian.
> 
> The SET_VRING_ENDIAN ioctl is used to make the device big endian
> on little endian hosts, hence "enabling" cross-endian mode...
> 
> > Existing reset seems to make sense.
> > 
> 
> ... and we "disable" cross-endian mode on reset.

OK but that's enable cross-endian. Not enable be.  We could have
something like vhost_disable_user_byte_swap though each time we try,
the result makes my head hurt: "swap" is a relative thing and hard to
keep track of.

> > > +static void vhost_enable_user_be(struct vhost_virtqueue *vq, bool user_be)
> > > +{
> > > +	vq->user_be = user_be;
> > > +}
> > > +  
> > 
> > And this is maybe "init_user_be"?
> > 
> 
> Anyway I don't mind changing the names to reset/init_user_be if you think it
> is clearer.
> 
> > >  static long vhost_set_vring_endian(struct vhost_virtqueue *vq, int __user *argp)
> > >  {
> > >  	struct vhost_vring_state s;
> > > @@ -62,7 +67,7 @@ static long vhost_set_vring_endian(struct vhost_virtqueue *vq, int __user *argp)
> > >  	    s.num != VHOST_VRING_BIG_ENDIAN)
> > >  		return -EINVAL;
> > >  
> > > -	vq->user_be = s.num;
> > > +	vhost_enable_user_be(vq, !!s.num);
> > >  
> > >  	return 0;
> > >  }
> > > @@ -81,7 +86,7 @@ static long vhost_get_vring_endian(struct vhost_virtqueue *vq, u32 idx,
> > >  	return 0;
> > >  }
> > >  
> > > -static void vhost_init_is_le(struct vhost_virtqueue *vq)
> > > +static void vhost_enable_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  
> > 
> > Same thing really. I'd rather add "reset_is_le".
> > 
> > > @@ -91,7 +96,7 @@ 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_vq_reset_user_be(struct vhost_virtqueue *vq)
> > > +static void vhost_disable_user_be(struct vhost_virtqueue *vq)
> > >  {
> > >  }
> > >  
> > > @@ -106,13 +111,18 @@ static long vhost_get_vring_endian(struct vhost_virtqueue *vq, u32 idx,
> > >  	return -ENOIOCTLCMD;
> > >  }
> > >  
> > > -static void vhost_init_is_le(struct vhost_virtqueue *vq)
> > > +static void vhost_enable_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_disable_is_le(struct vhost_virtqueue *vq)
> > > +{
> > > +	vq->is_le = virtio_legacy_is_little_endian();
> > > +}
> > > +
> > >  static void vhost_poll_func(struct file *file, wait_queue_head_t *wqh,
> > >  			    poll_table *pt)
> > >  {
> > > @@ -276,8 +286,8 @@ 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();
> > > -	vhost_vq_reset_user_be(vq);
> > > +	vhost_disable_is_le(vq);
> > > +	vhost_disable_user_be(vq);
> > >  }
> > >  
> > >  static int vhost_worker(void *data)
> > > @@ -1157,11 +1167,11 @@ int vhost_init_used(struct vhost_virtqueue *vq)
> > >  	__virtio16 last_used_idx;
> > >  	int r;
> > >  	if (!vq->private_data) {
> > > -		vq->is_le = virtio_legacy_is_little_endian();
> > > +		vhost_disable_is_le(vq);
> > >  		return 0;
> > >  	}
> > >  
> > > -	vhost_init_is_le(vq);
> > > +	vhost_enable_is_le(vq);
> > >  
> > >  	r = vhost_update_used_flags(vq);
> > >  	if (r)  
> >