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

[Stefano Garzarella <sgarzare@redhat.com>]

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@amd.com> wrote:
>>
>> [AMD Official Use Only - General]
>>
>> -----Original Message-----
>> From: Eugenio Pérez <eperezma@redhat.com>
>> Sent: Wednesday, May 25, 2022 4:29 PM
>> To: Michael S. Tsirkin <mst@redhat.com>; netdev@vger.kernel.org; linux-kernel@vger.kernel.org; kvm@vger.kernel.org; virtualization@lists.linux-foundation.org; Jason Wang <jasowang@redhat.com>
>> Cc: Zhu Lingshan <lingshan.zhu@intel.com>; martinh@xilinx.com; Stefano Garzarella <sgarzare@redhat.com>; ecree.xilinx@gmail.com; Eli Cohen <elic@nvidia.com>; Dan Carpenter <dan.carpenter@oracle.com>; Parav Pandit <parav@nvidia.com>; Wu Zongyong <wuzongyong@linux.alibaba.com>; dinang@xilinx.com; Christophe JAILLET <christophe.jaillet@wanadoo.fr>; Xie Yongji <xieyongji@bytedance.com>; Dawar, Gautam <gautam.dawar@amd.com>; lulu@redhat.com; martinpo@xilinx.com; pabloc@xilinx.com; Longpeng <longpeng2@huawei.com>; Piotr.Uminski@intel.com; Kamde, Tanuj <tanuj.kamde@amd.com>; Si-Wei Liu <si-wei.liu@oracle.com>; habetsm.xilinx@gmail.com; lvivier@redhat.com; Zhang Min <zhang.min9@zte.com.cn>; hanand@xilinx.com
>> 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@redhat.com>
>> ---
>>  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