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

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Date:   Thu, 26 May 2022 11:07:06 +0200
From:   Stefano Garzarella <sgarzare@...hat.com>
To:     Eugenio Perez Martin <eperezma@...hat.com>
Cc:     "Dawar, Gautam" <gautam.dawar@....com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        Jason Wang <jasowang@...hat.com>,
        Zhu Lingshan <lingshan.zhu@...el.com>,
        "martinh@...inx.com" <martinh@...inx.com>,
        "ecree.xilinx@...il.com" <ecree.xilinx@...il.com>,
        Eli Cohen <elic@...dia.com>,
        Dan Carpenter <dan.carpenter@...cle.com>,
        Parav Pandit <parav@...dia.com>,
        Wu Zongyong <wuzongyong@...ux.alibaba.com>,
        "dinang@...inx.com" <dinang@...inx.com>,
        Christophe JAILLET <christophe.jaillet@...adoo.fr>,
        Xie Yongji <xieyongji@...edance.com>,
        "lulu@...hat.com" <lulu@...hat.com>,
        "martinpo@...inx.com" <martinpo@...inx.com>,
        "pabloc@...inx.com" <pabloc@...inx.com>,
        Longpeng <longpeng2@...wei.com>,
        "Piotr.Uminski@...el.com" <Piotr.Uminski@...el.com>,
        "Kamde, Tanuj" <tanuj.kamde@....com>,
        Si-Wei Liu <si-wei.liu@...cle.com>,
        "habetsm.xilinx@...il.com" <habetsm.xilinx@...il.com>,
        "lvivier@...hat.com" <lvivier@...hat.com>,
        Zhang Min <zhang.min9@....com.cn>,
        "hanand@...inx.com" <hanand@...inx.com>
Subject: Re: [PATCH v3 2/4] vhost-vdpa: introduce STOP backend feature bit

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