Re: [PATCH v3 2/4] vhost-vdpa: introduce STOP backend feature bit

[Stefano Garzarella]

On Thu, May 26, 2022 at 10:57:03AM +0200, Eugenio Perez Martin wrote:
> On Wed, May 25, 2022 at 1:23 PM Dawar, Gautam <gautam.dawar@xxxxxxx> wrote:
> > [AMD Official Use Only - General]
> >
> > -----Original Message-----
> > From: Eugenio Pérez <eperezma@xxxxxxxxxx>
> > Sent: Wednesday, May 25, 2022 4:29 PM
> > To: Michael S. Tsirkin <mst@xxxxxxxxxx>; netdev@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; kvm@xxxxxxxxxxxxxxx; virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx; Jason Wang <jasowang@xxxxxxxxxx>
> > Cc: Zhu Lingshan <lingshan.zhu@xxxxxxxxx>; martinh@xxxxxxxxxx; Stefano Garzarella <sgarzare@xxxxxxxxxx>; ecree.xilinx@xxxxxxxxx; Eli Cohen <elic@xxxxxxxxxx>; Dan Carpenter <dan.carpenter@xxxxxxxxxx>; Parav Pandit <parav@xxxxxxxxxx>; Wu Zongyong <wuzongyong@xxxxxxxxxxxxxxxxx>; dinang@xxxxxxxxxx; Christophe JAILLET <christophe.jaillet@xxxxxxxxxx>; Xie Yongji <xieyongji@xxxxxxxxxxxxx>; Dawar, Gautam <gautam.dawar@xxxxxxx>; lulu@xxxxxxxxxx; martinpo@xxxxxxxxxx; pabloc@xxxxxxxxxx; Longpeng <longpeng2@xxxxxxxxxx>; Piotr.Uminski@xxxxxxxxx; Kamde, Tanuj <tanuj.kamde@xxxxxxx>; Si-Wei Liu <si-wei.liu@xxxxxxxxxx>; habetsm.xilinx@xxxxxxxxx; lvivier@xxxxxxxxxx; Zhang Min <zhang.min9@xxxxxxxxxx>; hanand@xxxxxxxxxx
> > Subject: [PATCH v3 2/4] vhost-vdpa: introduce STOP backend feature bit
> >
> > [CAUTION: External Email]
> >
> > Userland knows if it can stop the device or not by checking this feature bit.
> >
> > It's only offered if the vdpa driver backend implements the stop() operation callback, and try to set it if the backend does not offer that callback is an error.
> >
> > Signed-off-by: Eugenio Pérez <eperezma@xxxxxxxxxx>
> > ---
> >  drivers/vhost/vdpa.c             | 16 +++++++++++++++-
> >  include/uapi/linux/vhost_types.h |  2 ++
> >  2 files changed, 17 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/vhost/vdpa.c b/drivers/vhost/vdpa.c index 1f1d1c425573..32713db5831d 100644
> > --- a/drivers/vhost/vdpa.c
> > +++ b/drivers/vhost/vdpa.c
> > @@ -347,6 +347,14 @@ static long vhost_vdpa_set_config(struct vhost_vdpa *v,
> >         return 0;
> >  }
> >
> > +static bool vhost_vdpa_can_stop(const struct vhost_vdpa *v) {
> > +       struct vdpa_device *vdpa = v->vdpa;
> > +       const struct vdpa_config_ops *ops = vdpa->config;
> > +
> > +       return ops->stop;
> > [GD>>] Would it be better to explicitly return a bool to match the return type?
>
> I'm not sure about the kernel code style regarding that casting. Maybe
> it's better to return !!ops->stop here. The macros likely and unlikely
> do that.

IIUC `ops->stop` is a function pointer, so what about

    return ops->stop != NULL;

Thanks,
Stefano