Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
MIME-Version: 1.0
References: <20220525105922.2413991-1-eperezma@redhat.com> <20220525105922.2413991-3-eperezma@redhat.com>
 <BL1PR12MB582520CC9CE024149141327499D69@BL1PR12MB5825.namprd12.prod.outlook.com>
 <CAJaqyWc9_ErCg4whLKrjNyP5z2DZno-LJm7PN=-9uk7PUT4fJw@mail.gmail.com> <20220526090706.maf645wayelb7mcp@sgarzare-redhat>
In-Reply-To: <20220526090706.maf645wayelb7mcp@sgarzare-redhat>
From:   Eugenio Perez Martin <eperezma@redhat.com>
Date:   Thu, 26 May 2022 14:44:02 +0200
Message-ID: <CAJaqyWf7PumZXy1g3PbbTNCdn3u1XH3XQF73tw2w8Py5yLkSAg@mail.gmail.com>
Subject: Re: [PATCH v3 2/4] vhost-vdpa: introduce STOP backend feature bit
To:     Stefano Garzarella <sgarzare@redhat.com>
Cc:     "Dawar, Gautam" <gautam.dawar@amd.com>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        "netdev@vger.kernel.org" <netdev@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        "virtualization@lists.linux-foundation.org" 
        <virtualization@lists.linux-foundation.org>,
        Jason Wang <jasowang@redhat.com>,
        Zhu Lingshan <lingshan.zhu@intel.com>,
        "martinh@xilinx.com" <martinh@xilinx.com>,
        "ecree.xilinx@gmail.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" <dinang@xilinx.com>,
        Christophe JAILLET <christophe.jaillet@wanadoo.fr>,
        Xie Yongji <xieyongji@bytedance.com>,
        "lulu@redhat.com" <lulu@redhat.com>,
        "martinpo@xilinx.com" <martinpo@xilinx.com>,
        "pabloc@xilinx.com" <pabloc@xilinx.com>,
        Longpeng <longpeng2@huawei.com>,
        "Piotr.Uminski@intel.com" <Piotr.Uminski@intel.com>,
        "Kamde, Tanuj" <tanuj.kamde@amd.com>,
        Si-Wei Liu <si-wei.liu@oracle.com>,
        "habetsm.xilinx@gmail.com" <habetsm.xilinx@gmail.com>,
        "lvivier@redhat.com" <lvivier@redhat.com>,
        Zhang Min <zhang.min9@zte.com.cn>,
        "hanand@xilinx.com" <hanand@xilinx.com>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
Precedence: bulk

On Thu, May 26, 2022 at 11:07 AM Stefano Garzarella <sgarzare@redhat.com> w=
rote:
>
> 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> wro=
te:
> >>
> >> [AMD Official Use Only - General]
> >>
> >> -----Original Message-----
> >> From: Eugenio P=C3=A9rez <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-fo=
undation.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; mart=
inpo@xilinx.com; pabloc@xilinx.com; Longpeng <longpeng2@huawei.com>; Piotr.=
Uminski@intel.com; Kamde, Tanuj <tanuj.kamde@amd.com>; Si-Wei Liu <si-wei.l=
iu@oracle.com>; habetsm.xilinx@gmail.com; lvivier@redhat.com; Zhang Min <zh=
ang.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 featu=
re bit.
> >>
> >> It's only offered if the vdpa driver backend implements the stop() ope=
ration callback, and try to set it if the backend does not offer that callb=
ack is an error.
> >>
> >> Signed-off-by: Eugenio P=C3=A9rez <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 1f1d1c4=
25573..32713db5831d 100644
> >> --- a/drivers/vhost/vdpa.c
> >> +++ b/drivers/vhost/vdpa.c
> >> @@ -347,6 +347,14 @@ static long vhost_vdpa_set_config(struct vhost_vd=
pa *v,
> >>         return 0;
> >>  }
> >>
> >> +static bool vhost_vdpa_can_stop(const struct vhost_vdpa *v) {
> >> +       struct vdpa_device *vdpa =3D v->vdpa;
> >> +       const struct vdpa_config_ops *ops =3D vdpa->config;
> >> +
> >> +       return ops->stop;
> >> [GD>>] Would it be better to explicitly return a bool to match the ret=
urn 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 !=3D NULL;
>

I'm ok with any method proposed. Both three ways can be found in the
kernel so I think they are all valid (although the double negation is
from bool to integer in (0,1) set actually).

Maybe Jason or Michael (as maintainers) can state the preferred method here=
.

Generally I prefer explicit conversions, both signed and from/to
different types length. But I find conversion to bool to be simple
enough to be an exception to the rule. Same with void *. Let's see!

Sending v4 without this changed, waiting for answers.

Thanks!