Re: [RFC PATCH 10/10] vhost/vdpa: return configuration bytes read and written to user space

[Jason Wang]

On 2021/3/2 10:06 下午, Stefano Garzarella wrote:
> On Tue, Mar 02, 2021 at 12:05:35PM +0800, Jason Wang wrote:
> > On 2021/2/16 5:44 下午, Stefano Garzarella wrote:
> > > vdpa_get_config() and vdpa_set_config() now return the amount
> > > of bytes read and written, so let's return them to the user space.
> > >
> > > We also modify vhost_vdpa_config_validate() to return 0 (bytes read
> > > or written) instead of an error, when the buffer length is 0.
> > >
> > > Signed-off-by: Stefano Garzarella <sgarzare@xxxxxxxxxx>
> > > ---
> > >  drivers/vhost/vdpa.c | 26 +++++++++++++++-----------
> > >  1 file changed, 15 insertions(+), 11 deletions(-)
> > >
> > > diff --git a/drivers/vhost/vdpa.c b/drivers/vhost/vdpa.c
> > > index 21eea2be5afa..b754c53171a7 100644
> > > --- a/drivers/vhost/vdpa.c
> > > +++ b/drivers/vhost/vdpa.c
> > > @@ -191,9 +191,6 @@ static ssize_t vhost_vdpa_config_validate(struct 
> > > vhost_vdpa *v,
> > >      struct vdpa_device *vdpa = v->vdpa;
> > >      u32 size = vdpa->config->get_config_size(vdpa);
> > > -    if (c->len == 0)
> > > -        return -EINVAL;
> > > -
> > >      return min(c->len, size);
> > >  }
> > > @@ -204,6 +201,7 @@ static long vhost_vdpa_get_config(struct 
> > > vhost_vdpa *v,
> > >      struct vhost_vdpa_config config;
> > >      unsigned long size = offsetof(struct vhost_vdpa_config, buf);
> > >      ssize_t config_size;
> > > +    long ret;
> > >      u8 *buf;
> > >      if (copy_from_user(&config, c, size))
> > > @@ -217,15 +215,18 @@ static long vhost_vdpa_get_config(struct 
> > > vhost_vdpa *v,
> > >      if (!buf)
> > >          return -ENOMEM;
> > > -    vdpa_get_config(vdpa, config.off, buf, config_size);
> > > -
> > > -    if (copy_to_user(c->buf, buf, config_size)) {
> > > -        kvfree(buf);
> > > -        return -EFAULT;
> > > +    ret = vdpa_get_config(vdpa, config.off, buf, config_size);
> > > +    if (ret < 0) {
> > > +        ret = -EFAULT;
> > > +        goto out;
> > >      }
> > > +    if (copy_to_user(c->buf, buf, config_size))
> > > +        ret = -EFAULT;
> > > +
> > > +out:
> > >      kvfree(buf);
> > > -    return 0;
> > > +    return ret;
> > >  }
> > >  static long vhost_vdpa_set_config(struct vhost_vdpa *v,
> > > @@ -235,6 +236,7 @@ static long vhost_vdpa_set_config(struct 
> > > vhost_vdpa *v,
> > >      struct vhost_vdpa_config config;
> > >      unsigned long size = offsetof(struct vhost_vdpa_config, buf);
> > >      ssize_t config_size;
> > > +    long ret;
> > >      u8 *buf;
> > >      if (copy_from_user(&config, c, size))
> > > @@ -248,10 +250,12 @@ static long vhost_vdpa_set_config(struct 
> > > vhost_vdpa *v,
> > >      if (IS_ERR(buf))
> > >          return PTR_ERR(buf);
> > > -    vdpa_set_config(vdpa, config.off, buf, config_size);
> > > +    ret = vdpa_set_config(vdpa, config.off, buf, config_size);
> > > +    if (ret < 0)
> > > +        ret = -EFAULT;
> > >      kvfree(buf);
> > > -    return 0;
> > > +    return ret;
> > >  }
> >
> >
> > So I wonder whether it's worth to return the number of bytes since we 
> > can't propogate the result to driver or driver doesn't care about that.
>
> Okay, but IIUC user space application that issue VHOST_VDPA_GET_CONFIG 
> ioctl can use the return value.


Yes, but it looks to it's too late to change since it's a userspace 
noticble behaviour.


> Should we change also 'struct virtio_config_ops' to propagate this 
> value also to virtio drivers?


I think not, the reason is the driver doesn't expect the get()/set() can 
fail...

Thanks


> Thanks,
> Stefano