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;
From:   "Tian, Kevin" <kevin.tian@intel.com>
To:     Stefan Hajnoczi <stefanha@redhat.com>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>
CC:     "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "Alex Williamson" <alex.williamson@redhat.com>
Subject: RE: [PATCH v3] vfio: align capability structures
Thread-Topic: [PATCH v3] vfio: align capability structures
Thread-Index: AQHZywC7EI5rhN/SqE6vo66ghbV2tq/i1QWA
Date:   Thu, 10 Aug 2023 02:51:29 +0000
Message-ID: <BN9PR11MB527660D04574640B081E7AA48C13A@BN9PR11MB5276.namprd11.prod.outlook.com>
References: <20230809203144.2880050-1-stefanha@redhat.com>
In-Reply-To: <20230809203144.2880050-1-stefanha@redhat.com>
Accept-Language: en-US
Content-Language: en-US
x-ms-exchange-antispam-messagedata-chunkcount: 1
x-ms-exchange-antispam-messagedata-0: =?us-ascii?Q?HKP1k2kXyDiS5X8yJ8M/FGWpc4kIzzJgjoUXYMQIvZble2qlbOHGb8ImbWKU?=
 =?us-ascii?Q?Z5UgsL+etgmudtygNMNryV9uUS+QOqyPuOxtBrbPK8vbuv3RDMHQPHTMtV0c?=
 =?us-ascii?Q?CM6f1gabbehhgoMzVoTZpS+mRLKEqnubYcehKlIEH0L7S4wks5/dqFbUZVsb?=
 =?us-ascii?Q?wrVyZvfO/YM+ZBAPCCut8P1hCjPj03CkQn169P5Uos766FXodYqZcYB+S9eC?=
 =?us-ascii?Q?eKPf4xOPKkDe1NfYjj4GyNJROElpkpbAZ0YN+JqYLHWCe4Knm8pNXpV+N2PZ?=
 =?us-ascii?Q?90H0hmv6ikkghZK/drHfB3IArVo/Q3zoo3Desk96xlcvj1GpywfYvlolhu5B?=
 =?us-ascii?Q?muxW3WwCjKqh3a/ArW9S2hppKkWSz4nFZT1xKIG0fARQTHu9ked7gaPvJauo?=
 =?us-ascii?Q?ve0pbybwHlKbZpM6W3nbROxVWovjcETqmLbgxoSuzs0mjd3syAv1N3Zl6Q/T?=
 =?us-ascii?Q?VBh1o7RBscNRqixPDqwl0pEuzUuHlRIbnRYea+OmeOhIfinGNGRdqOMk3r3u?=
 =?us-ascii?Q?DVJZD2KXmtdveqzaVKIdQAmlSyRKDZjsWM9JbjHogE3JUlmAHDtzZ660DlMj?=
 =?us-ascii?Q?LN1ydL6G4BIX3O0+hMhwIQ51C/RnBnYzvg3uOKVE0ZGv8Ub585pmF6ky9D8r?=
 =?us-ascii?Q?BVTmhNmVWls20Yz5o+AJZyekujFN24Yoll8uxERreUOVKgAt0pzMLlFwA8JO?=
 =?us-ascii?Q?Mk0Kcgpauwug47Y4aTK1pXo278P5NjRrYYz7RGugQpizcuThRtudXKIP1wCV?=
 =?us-ascii?Q?fplwh+8HKKDYjsjGuvw5YPsYOVfwWDgQZbFSd/UUWYxOEMWF6XkzwenGoaVd?=
 =?us-ascii?Q?mWLPIs9f3hm55y/+GCXbvXgyr8HLmPoBDIB8Iy5kZeWHNy3m9B1AipQUuidh?=
 =?us-ascii?Q?5SeI6KUBZzS1es/B9+njd4IOiBIBma/W7sK9G/1OJsmdMnv71ULJypv/nI2j?=
 =?us-ascii?Q?hoiNZPmwVMIFvb+N0BSUVVed+pUgrM46ZLkCsT1TX0NUOnvZxL1rheZAeS9A?=
 =?us-ascii?Q?cj+fvSr6gJLtHUB6iSnEcARtsYcOWsydqjWONO6q2/zi7u3MQGrXr0vg2uOB?=
 =?us-ascii?Q?hpm3sp6d5im5j/3g2Nw6Gjbc82882vwTZ/brR7MyEdkgJHktvLKqivADpzWn?=
 =?us-ascii?Q?TdcMJRjI0vh5No1aHWLLUJSa/kE88WA0ZnZMdsegv1793QR4xq59uJpu8cQ2?=
 =?us-ascii?Q?oFJo00gHJGcCit8oTUQRx9lV9MNYhZyw9aXPQlNlEbLqr/b4lO9B9F07sp9j?=
 =?us-ascii?Q?iVnCkt2L2Rn/6kGvLwA1XFQr99Qh0/OdvgXaXilGDXJQIuigCmBoO+KLMU/8?=
 =?us-ascii?Q?f3XXPTkgmkyIcuObk/+Y0lkkTtrBZr3GLxzpTy+hg+1bExjy+TI+3kMNfUI1?=
 =?us-ascii?Q?kNsY9PVfTB03UIFYpozxdlLHvwbTwRYuzzUz7/5orV7+rfq4HwezYIbchwrF?=
 =?us-ascii?Q?nI4RUTkXIukeCQpOq3oN+n+RNwyDNLNg4T7OYPUOZJn6IVj0biru3TGGPgpT?=
 =?us-ascii?Q?1iivMg3BdQD9G4apycDzU+PAffCXuebThGIEBD7mqTGarrs+CXuLFZT5xdgy?=
 =?us-ascii?Q?WC8oURUBedGCcDW3QXzKeG0mpK6tGp5bGH4DqYJS?=
Content-Type: text/plain; charset="us-ascii"
Content-Transfer-Encoding: quoted-printable
MIME-Version: 1.0
X-MS-Exchange-CrossTenant-AuthAs: Internal
X-MS-Exchange-CrossTenant-AuthSource: BN9PR11MB5276.namprd11.prod.outlook.com
X-MS-Exchange-CrossTenant-Network-Message-Id: 82b432e2-f9ab-4fa5-6a40-08db994cb2c8
X-MS-Exchange-CrossTenant-originalarrivaltime: 10 Aug 2023 02:51:29.8710
 (UTC)
X-MS-Exchange-CrossTenant-fromentityheader: Hosted
X-MS-Exchange-CrossTenant-id: 46c98d88-e344-4ed4-8496-4ed7712e255d
X-MS-Exchange-CrossTenant-mailboxtype: HOSTED
X-MS-Exchange-CrossTenant-userprincipalname: Mh2Bx71LfN3lSTbySrdSnaTaqjeKwbPp671SXiwn46qIlL45TaaOSYuwnU8axK/b2Xu6Bycizhkkk31NzNMURQ==
X-MS-Exchange-Transport-CrossTenantHeadersStamped: PH7PR11MB7594
X-OriginatorOrg: intel.com
X-Spam-Status: No, score=-4.4 required=5.0 tests=BAYES_00,DKIMWL_WL_HIGH,
        DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,RCVD_IN_DNSWL_MED,
        SPF_HELO_NONE,SPF_NONE autolearn=ham autolearn_force=no version=3.4.6
X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on
        lindbergh.monkeyblade.net
Precedence: bulk
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

> From: Stefan Hajnoczi <stefanha@redhat.com>
> Sent: Thursday, August 10, 2023 4:32 AM
>=20
> The VFIO_DEVICE_GET_INFO, VFIO_DEVICE_GET_REGION_INFO, and
> VFIO_IOMMU_GET_INFO ioctls fill in an info struct followed by capability
> structs:
>=20
>   +------+---------+---------+-----+
>   | info | caps[0] | caps[1] | ... |
>   +------+---------+---------+-----+
>=20
> Both the info and capability struct sizes are not always multiples of
> sizeof(u64), leaving u64 fields in later capability structs misaligned.
>=20
> Userspace applications currently need to handle misalignment manually in
> order to support CPU architectures and programming languages with strict
> alignment requirements.
>=20
> Make life easier for userspace by ensuring alignment in the kernel. This
> is done by padding info struct definitions and by copying out zeroes
> after capability structs that are not aligned.
>=20
> The new layout is as follows:
>=20
>   +------+---------+---+---------+-----+
>   | info | caps[0] | 0 | caps[1] | ... |
>   +------+---------+---+---------+-----+
>=20
> In this example caps[0] has a size that is not multiples of sizeof(u64),
> so zero padding is added to align the subsequent structure.
>=20
> Adding zero padding between structs does not break the uapi. The memory
> layout is specified by the info.cap_offset and caps[i].next fields
> filled in by the kernel. Applications use these field values to locate
> structs and are therefore unaffected by the addition of zero padding.
>=20
> Note that code that copies out info structs with padding is updated to
> always zero the struct and copy out as many bytes as userspace
> requested. This makes the code shorter and avoids potential information
> leaks by ensuring padding is initialized.
>=20
> Originally-by: Alex Williamson <alex.williamson@redhat.com>
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

Reviewed-by: Kevin Tian <kevin.tian@intel.com>