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Tue, 27 Aug 2019 11:52:21 +0000 From: Parav Pandit To: Cornelia Huck CC: "alex.williamson@redhat.com" , Jiri Pirko , "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 Thread-Topic: [PATCH 3/4] mdev: Expose mdev alias in sysfs tree Thread-Index: AQHVXE6v3Mg2zMVvBUG6schRq+9Lx6cO0OsAgAADYqCAAAnfAIAABFHg Date: Tue, 27 Aug 2019 11:52:21 +0000 Message-ID: References: <20190826204119.54386-1-parav@mellanox.com> <20190826204119.54386-4-parav@mellanox.com> <20190827124706.7e726794.cohuck@redhat.com> <20190827133432.156f7db3.cohuck@redhat.com> In-Reply-To: <20190827133432.156f7db3.cohuck@redhat.com> Accept-Language: en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: authentication-results: spf=none (sender IP is ) smtp.mailfrom=parav@mellanox.com; x-originating-ip: [106.51.18.188] x-ms-publictraffictype: Email x-ms-office365-filtering-correlation-id: e6ad6fcc-cf5b-4377-55f3-08d72ae504e1 x-ms-office365-filtering-ht: Tenant x-microsoft-antispam: BCL:0;PCL:0;RULEID:(2390118)(7020095)(4652040)(8989299)(5600166)(711020)(4605104)(1401327)(4618075)(4534185)(7168020)(4627221)(201703031133081)(201702281549075)(8990200)(2017052603328)(7193020);SRVR:AM0PR05MB4291; 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charset="us-ascii" Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 X-OriginatorOrg: Mellanox.com X-MS-Exchange-CrossTenant-Network-Message-Id: e6ad6fcc-cf5b-4377-55f3-08d72ae504e1 X-MS-Exchange-CrossTenant-originalarrivaltime: 27 Aug 2019 11:52:21.0890 (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: 59vPDxOBg50jziQeAhHdSy4ksQXdaWOfKh+g/ZqQW7CRxNiqUJ07sMa28pOQCpYpdMoyZTqTHAhvx410r0Sspw== X-MS-Exchange-Transport-CrossTenantHeadersStamped: AM0PR05MB4291 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org > -----Original Message----- > From: Cornelia Huck > Sent: Tuesday, August 27, 2019 5:05 PM > To: Parav Pandit > Cc: alex.williamson@redhat.com; Jiri Pirko ; > 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 11:07:37 +0000 > Parav Pandit wrote: >=20 > > > -----Original Message----- > > > From: Cornelia Huck > > > Sent: Tuesday, August 27, 2019 4:17 PM > > > To: Parav Pandit > > > Cc: alex.williamson@redhat.com; Jiri Pirko ; > > > 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 wrote: >=20 > > > > +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 ther= e 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 oth= er widely > used subsystem. > > > > > - Or not creating the attribute at all? That would match what userspa= ce > > > 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 th= is 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 u= sual > manner it's not recommended. > > So there is no old user space, new kernel issue here. >=20 > I'm not talking about old userspace/new kernel, but new userspace/old ker= nel. > Code that wants to consume this attribute needs to be able to cope with i= ts > absence anyway. >=20 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.