Re: [PATCH 2/2] vhost: disentangle vring endianness stuff from the core code

[Greg Kurz]

On Wed, 10 Feb 2016 13:48:09 +0200
"Michael S. Tsirkin" <mst@xxxxxxxxxx> wrote:

> On Wed, Jan 13, 2016 at 06:09:47PM +0100, Greg Kurz wrote:
> > The way vring endianness is being handled currently obfuscates
> > the code in vhost_init_used().
> > 
> > This patch tries to fix that by doing the following:
> > - move the the code that adjusts endianness to a dedicated helper
> > - export this helper so that backends explicitely call it
> > 
> > No behaviour change.
> > 
> > Signed-off-by: Greg Kurz <gkurz@xxxxxxxxxxxxxxxxxx>
> > ---
> >  drivers/vhost/net.c   |    3 +++
> >  drivers/vhost/scsi.c  |    3 +++
> >  drivers/vhost/test.c  |    2 ++
> >  drivers/vhost/vhost.c |   16 +++++++++++-----
> >  drivers/vhost/vhost.h |    1 +
> >  5 files changed, 20 insertions(+), 5 deletions(-)
> > 
> > diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> > index 9eda69e40678..df01c939cd00 100644
> > --- a/drivers/vhost/net.c
> > +++ b/drivers/vhost/net.c
> > @@ -917,6 +917,9 @@ static long vhost_net_set_backend(struct vhost_net *n, unsigned index, int fd)
> >  
> >  		vhost_net_disable_vq(n, vq);
> >  		vq->private_data = sock;
> > +
> > +		vhost_adjust_vring_endian(vq);
> > +
> >  		r = vhost_init_used(vq);
> >  		if (r)
> >  			goto err_used;  
> 
> 
> This is in fact a bug in existing code: if vhost_init_used
> fails, it preferably should not have side-effects.
> It's best to update it last thing.
> 

I'm afraid we can't because the following path needs the
vring endianness:

vhost_init_used()->vhost_update_used_flags()->cpu_to_vhost16()

But you are right, there is a bug: we should rollback if vhost_init_used()
fails. Something like below:

 err_used:
        vq->private_data = oldsock;
        vhost_net_enable_vq(n, vq);
+       vhost_adjust_vring_endian(vq);
        if (ubufs)
                vhost_net_ubuf_put_wait_and_free(ubufs);
 err_ubufs:

> > diff --git a/drivers/vhost/scsi.c b/drivers/vhost/scsi.c
> > index 29cfc57d496e..5a8363bfcb74 100644
> > --- a/drivers/vhost/scsi.c
> > +++ b/drivers/vhost/scsi.c
> > @@ -1274,6 +1274,9 @@ vhost_scsi_set_endpoint(struct vhost_scsi *vs,
> >  			vq = &vs->vqs[i].vq;
> >  			mutex_lock(&vq->mutex);
> >  			vq->private_data = vs_tpg;
> > +
> > +			vhost_adjust_vring_endian(vq);
> > +
> >  			vhost_init_used(vq);
> >  			mutex_unlock(&vq->mutex);
> >  		}
> > diff --git a/drivers/vhost/test.c b/drivers/vhost/test.c
> > index f2882ac98726..75e3e0e9f5a8 100644
> > --- a/drivers/vhost/test.c
> > +++ b/drivers/vhost/test.c
> > @@ -196,6 +196,8 @@ static long vhost_test_run(struct vhost_test *n, int test)
> >  		oldpriv = vq->private_data;
> >  		vq->private_data = priv;
> >  
> > +		vhost_adjust_vring_endian(vq);
> > +
> >  		r = vhost_init_used(&n->vqs[index]);
> >  
> >  		mutex_unlock(&vq->mutex);
> > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> > index e02e06755ab7..b0a00340309e 100644
> > --- a/drivers/vhost/vhost.c
> > +++ b/drivers/vhost/vhost.c
> > @@ -123,6 +123,15 @@ static void vhost_disable_is_le(struct vhost_virtqueue *vq)
> >  	vq->is_le = virtio_legacy_is_little_endian();
> >  }
> >  
> > +void vhost_adjust_vring_endian(struct vhost_virtqueue *vq)
> > +{
> > +	if (!vq->private_data)
> > +		vhost_disable_is_le(vq);
> > +	else
> > +		vhost_enable_is_le(vq);
> > +}
> > +EXPORT_SYMBOL_GPL(vhost_adjust_vring_endian);
> > +
> >  static void vhost_poll_func(struct file *file, wait_queue_head_t *wqh,
> >  			    poll_table *pt)
> >  {  
> 
> I'd prefer "vhost_update_is_le" here. "endian" might also mean
> "user_be". But see below pls.
> 

Ok, I will rename if this patch survives the review.

> 
> > @@ -1166,12 +1175,9 @@ int vhost_init_used(struct vhost_virtqueue *vq)
> >  {
> >  	__virtio16 last_used_idx;
> >  	int r;
> > -	if (!vq->private_data) {
> > -		vhost_disable_is_le(vq);
> > -		return 0;
> > -	}
> >  
> > -	vhost_enable_is_le(vq);
> > +	if (!vq->private_data)
> > +		return 0;
> >  
> >  	r = vhost_update_used_flags(vq);
> >  	if (r)  
> 
> Looking at how callers use this, maybe we should just rename init_used
> to vhost_vq_init_access. The _used suffix was a hint that we
> access the vq used ring. But maybe what callers care about is
> that it must be called after access_ok.
> 

And so we would keep the current logic where it is up to the core
code to update is_le... that is basically getting rid of this patch :)

So IIUC you're asking for:
- fix the side-effect in vhost_init_used()
- rename vhost_init_used() to vhost_vq_init_access()

> > diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
> > index d3f767448a72..88d86f45f756 100644
> > --- a/drivers/vhost/vhost.h
> > +++ b/drivers/vhost/vhost.h
> > @@ -162,6 +162,7 @@ bool vhost_enable_notify(struct vhost_dev *, struct vhost_virtqueue *);
> >  
> >  int vhost_log_write(struct vhost_virtqueue *vq, struct vhost_log *log,
> >  		    unsigned int log_num, u64 len);
> > +void vhost_adjust_vring_endian(struct vhost_virtqueue *vq);
> >  
> >  #define vq_err(vq, fmt, ...) do {                                  \
> >  		pr_debug(pr_fmt(fmt), ##__VA_ARGS__);       \  
>