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

[Greg Kurz]

On Wed, 10 Feb 2016 13:21:22 +0200
"Michael S. Tsirkin" <mst@xxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxx>
> > ---
> >  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.

> > +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)  
>