Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
From:   Parav Pandit <parav@mellanox.com>
To:     Cornelia Huck <cohuck@redhat.com>
CC:     "alex.williamson@redhat.com" <alex.williamson@redhat.com>,
        Jiri Pirko <jiri@mellanox.com>,
        "kwankhede@nvidia.com" <kwankhede@nvidia.com>,
        "davem@davemloft.net" <davem@davemloft.net>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "netdev@vger.kernel.org" <netdev@vger.kernel.org>
Subject: RE: [PATCH 3/4] mdev: Expose mdev alias in sysfs tree
Thread-Topic: [PATCH 3/4] mdev: Expose mdev alias in sysfs tree
Thread-Index: AQHVXE6v3Mg2zMVvBUG6schRq+9Lx6cO0OsAgAADYqCAAAnfAIAABFHggAABh4CAAAC7kA==
Date:   Tue, 27 Aug 2019 12:00:34 +0000
Message-ID: <AM0PR05MB4866E222E33DBD4C40A7EA9CD1A00@AM0PR05MB4866.eurprd05.prod.outlook.com>
References: <20190826204119.54386-1-parav@mellanox.com>
        <20190826204119.54386-4-parav@mellanox.com>
        <20190827124706.7e726794.cohuck@redhat.com>
        <AM0PR05MB4866BDA002F2C6566492244ED1A00@AM0PR05MB4866.eurprd05.prod.outlook.com>
        <20190827133432.156f7db3.cohuck@redhat.com>
        <AM0PR05MB4866FD2DB357C5EB4A7A75ADD1A00@AM0PR05MB4866.eurprd05.prod.outlook.com>
 <20190827135527.7c9d3940.cohuck@redhat.com>
In-Reply-To: <20190827135527.7c9d3940.cohuck@redhat.com>
Accept-Language: en-US
Content-Language: en-US
received-spf: None (protection.outlook.com: mellanox.com does not designate
 permitted sender hosts)
Content-Type: text/plain; charset="us-ascii"
Content-Transfer-Encoding: quoted-printable
MIME-Version: 1.0
X-MS-Exchange-CrossTenant-Network-Message-Id: 5612809c-fc47-42cc-c16e-08d72ae62acb
X-MS-Exchange-CrossTenant-originalarrivaltime: 27 Aug 2019 12:00:34.4256
 (UTC)
X-MS-Exchange-CrossTenant-fromentityheader: Hosted
X-MS-Exchange-CrossTenant-id: a652971c-7d2e-4d9b-a6a4-d149256f461b
X-MS-Exchange-CrossTenant-mailboxtype: HOSTED
X-MS-Exchange-CrossTenant-userprincipalname: Jja7uYs026N8z9mkKdZwoxLCsOgyEr7WCviC5i8Dbogu+++vx0edG47TBwOHTlj+6U/X5FQVU45XHp7KJtTSsw==
X-MS-Exchange-Transport-CrossTenantHeadersStamped: AM0PR05MB6723
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org


> -----Original Message-----
> From: Cornelia Huck <cohuck@redhat.com>
> Sent: Tuesday, August 27, 2019 5:25 PM
> To: Parav Pandit <parav@mellanox.com>
> Cc: alex.williamson@redhat.com; Jiri Pirko <jiri@mellanox.com>;
> kwankhede@nvidia.com; davem@davemloft.net; kvm@vger.kernel.org; linux-
> kernel@vger.kernel.org; netdev@vger.kernel.org
> Subject: Re: [PATCH 3/4] mdev: Expose mdev alias in sysfs tree
>=20
> On Tue, 27 Aug 2019 11:52:21 +0000
> Parav Pandit <parav@mellanox.com> wrote:
>=20
> > > -----Original Message-----
> > > From: Cornelia Huck <cohuck@redhat.com>
> > > Sent: Tuesday, August 27, 2019 5:05 PM
> > > To: Parav Pandit <parav@mellanox.com>
> > > Cc: alex.williamson@redhat.com; Jiri Pirko <jiri@mellanox.com>;
> > > kwankhede@nvidia.com; davem@davemloft.net; kvm@vger.kernel.org;
> > > linux- kernel@vger.kernel.org; netdev@vger.kernel.org
> > > Subject: Re: [PATCH 3/4] mdev: Expose mdev alias in sysfs tree
> > >
> > > On Tue, 27 Aug 2019 11:07:37 +0000
> > > Parav Pandit <parav@mellanox.com> wrote:
> > >
> > > > > -----Original Message-----
> > > > > From: Cornelia Huck <cohuck@redhat.com>
> > > > > Sent: Tuesday, August 27, 2019 4:17 PM
> > > > > To: Parav Pandit <parav@mellanox.com>
> > > > > Cc: alex.williamson@redhat.com; Jiri Pirko <jiri@mellanox.com>;
> > > > > kwankhede@nvidia.com; davem@davemloft.net; kvm@vger.kernel.org;
> > > > > linux- kernel@vger.kernel.org; netdev@vger.kernel.org
> > > > > Subject: Re: [PATCH 3/4] mdev: Expose mdev alias in sysfs tree
> > > > >
> > > > > On Mon, 26 Aug 2019 15:41:18 -0500 Parav Pandit
> > > > > <parav@mellanox.com> wrote:
> > >
> > > > > > +static ssize_t alias_show(struct device *device,
> > > > > > +			  struct device_attribute *attr, char *buf) {
> > > > > > +	struct mdev_device *dev =3D mdev_from_dev(device);
> > > > > > +
> > > > > > +	if (!dev->alias)
> > > > > > +		return -EOPNOTSUPP;
> > > > >
> > > > > I'm wondering how to make this consumable by userspace in the
> > > > > easiest
> > > way.
> > > > > - As you do now (userspace gets an error when trying to read)?
> > > > > - Returning an empty value (nothing to see here, move along)?
> > > > No. This is confusing, to return empty value, because it says that
> > > > there is an
> > > alias but it is some weird empty string.
> > > > If there is alias, it shows exactly what it is.
> > > > If no alias it returns an error code =3D unsupported -> inline with
> > > > other widely
> > > used subsystem.
> > > >
> > > > > - Or not creating the attribute at all? That would match what use=
rspace
> > > > >   sees on older kernels, so it needs to be able to deal with
> > > > > that
> > > > New sysfs files can appear. Tool cannot say that I was not
> > > > expecting this file
> > > here.
> > > > User space is supposed to work with the file they are off interest.
> > > > Mdev interface has option to specify vendor specific files, though
> > > > in usual
> > > manner it's not recommended.
> > > > So there is no old user space, new kernel issue here.
> > >
> > > I'm not talking about old userspace/new kernel, but new userspace/old
> kernel.
> > > Code that wants to consume this attribute needs to be able to cope
> > > with its absence anyway.
> > >
> > Old kernel doesn't have alias file.
> > If some tool tries to read this file it will fail to open non existing =
file; open()
> system call is already taking care of it.
>=20
> Yes, that was exactly my argument?
I misunderstood probably.
I re-read all 3 options you posted.
I do not see any issue in reporting error code similar to other widely used=
 netdev subsystem, hence propose what is posted in the patch.