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X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1 X-MS-Exchange-AntiSpam-MessageData-0: =?us-ascii?Q?xP3lzca/3RdxktLkZTGuAO9tHdfbbOaE382WrBqp2LB4D4mQHcnlmd2n7sVv?= =?us-ascii?Q?cSC22cjcWsNGl5CTSxa2hENZINtPk49Ba7e+o6wIKQfGFh+AgXQB0vKnHp1s?= =?us-ascii?Q?xHpJRRE2e+Z61EuPGxb3rmfCttu4JTa0Pj0VfmCvKgDi+ijWkfUWPpI8Ry/F?= =?us-ascii?Q?HLOPuIw3pZhI/m9xmEE7WGKZtKrPkNxT9rRuHn8nPpzxusd9FsAwA+9jpsgo?= =?us-ascii?Q?vEOu7vjSO3y76d9IoJCDCOvG5Wd+p8CbOWrrBesmvXdfnC+lBNQD1nRXNOty?= =?us-ascii?Q?VGaj3LxGP5HUhntRUl9AzcbqxO0cetCMTyBUKUmrHLfdd08PQok4GibROsTj?= =?us-ascii?Q?sZYWpzAhsMOhrV+8IqPAHxn16xRH1t/y8TMC8bbT0RWAQoHjCkmEM4Nb4p0q?= =?us-ascii?Q?OOX7Ic4+N1eZMo2o7H6Xq764y1bm3ERkCTij5GlrnhZaCB6CTv2Tx6V70qg6?= =?us-ascii?Q?yMJJRb/2JDQ9UBH0SyYr1jxyZdsUxIDTGqxWSYwXXcxbkllrS0zeD6jKfEQ2?= =?us-ascii?Q?PopN1vtq6JLog+iiCKP2VM4r5BWG/Gkx+p9Q7/jPBQ0Clw4RR5xsCr4RO6/1?= =?us-ascii?Q?Nn08CeerWyzesLpJy0HlriGfe96QYe/tT90KsstUnnap8yAa/uO9p+p/Kqor?= =?us-ascii?Q?tuUrCmLyD491cyB6OnaGkraNUW1T/wYlKiqdUL4kbaUXEN2+/7C3XOGT2akW?= =?us-ascii?Q?W5s92qJy/4krhFlRRCdBWY9m5BThvMV2e06T/qruDqhfuUZSVl+NhhwFjFMg?= =?us-ascii?Q?WkUacmPrlfOgZ0io5FMUExlmxtwqkqKMRoozvwWIFXuyNOfNm75PoSLGSOlC?= =?us-ascii?Q?aaamHO39+tJtgZrIUp8uKs/lzS8HvSbl8oBKsGyA2VCKa+sb3F9dmc50WSjn?= =?us-ascii?Q?JIrLYhcRgZSwRpiLcGu+/4nv+wOxYflY/5idUAJab8ujVgFdi9PjJc+CNB3W?= =?us-ascii?Q?fYzQnLyQrlv7UF9md9O/phaybgbbyl44upSDBncLmeOWHLCqmEJ0GfPP37Ru?= =?us-ascii?Q?DhRZFGsW3hsYxkJh9+ng27+LN7GKO8FzS8DDUUN5XLgGUrS+e1Ru18ktAAlW?= =?us-ascii?Q?KNPFjiscvH1hmEXsCAPR8zjnSqTm2trIOFvzzzEgFIlY5eeJ+qoFEdXTKYAx?= =?us-ascii?Q?Q0AGtH6v30OMnHU4aoxhh1OA7vp7nFtvRZJzrDiYdNDNh7yCUQYeH/T9HMgA?= =?us-ascii?Q?a92GpvAiMv3ploeZ5R9ZcqFMjLstVBUDxxZNy5LJ4P/3YoIr76BN4TCRBUJk?= =?us-ascii?Q?dNnSKU6ulAUnubkLKkhW9oz4nnAJ/25/4k14WaY6swsVAmKBf8QwTrP2g07S?= =?us-ascii?Q?L/R57fXHj+M8zwB74gNYhVRhwnOL+JAFMSpc37XzTFYiJfUs3Mx0rBR5VVAY?= =?us-ascii?Q?VYkd3poubcXpPgJ2+C8RumT5ypk0r+2hMz6wBgH7Gku9IrEll2TnFhyNe8xN?= =?us-ascii?Q?ElPaafmiWPhIlgTxesRUElW5pTVGUydz9LxqzaczYz0z1shFh6Dr923I2IO4?= =?us-ascii?Q?fbNhyzWN1mjcSrijEjW+xWVZqTK9RmN0lU5mUDeW7Bxs9Lcm9CFe2f9plZBb?= =?us-ascii?Q?SqZiy3BEcOFDuJYDb2QXJ0wWhbcIIZ+MSh+E4GFO?= X-OriginatorOrg: Nvidia.com X-MS-Exchange-CrossTenant-Network-Message-Id: 95f26ac3-d55a-4609-c065-08dcb22aa978 X-MS-Exchange-CrossTenant-AuthSource: CH3PR12MB7763.namprd12.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-OriginalArrivalTime: 01 Aug 2024 13:05:48.3339 (UTC) X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted X-MS-Exchange-CrossTenant-Id: 43083d15-7273-40c1-b7db-39efd9ccc17a X-MS-Exchange-CrossTenant-MailboxType: HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: lgcuJESmNSK7jyk6Et/rFe81oLqaooWvyi5OlPfpUyP1IuBaLU3d/yUpdRa+F/wN X-MS-Exchange-Transport-CrossTenantHeadersStamped: PH7PR12MB6785 On Tue, Jul 30, 2024 at 06:28:08PM +0100, Jonathan Cameron wrote: > > And the basic userspace pattern is: > > > > struct fwctl_info info = {.size = sizeof(info), ...); > > ioctl(fd, FWCTL_INFO, &info); > > > > This works today and generates the 24 byte command. > > > > Tomorrow the kernel adds a new member: > > > > struct fwctl_info { > > __u32 size; > > __u32 flags; > > __u32 out_device_type; > > __u32 device_data_len; > > __aligned_u64 out_device_data; > > __aligned_u64 new_thing; > > }; > > > > Current builds of the userpace use a 24 byte command. A new kernel > > will see the 24 bytes and behave as before. > > > > When I recompile the userspace with the updated header it will issue a > > 32 byte command with no source change. > > > > Old kernel will see a 32 byte command with the trailing bytes it > > doesn't understand as 0 and keep working. > > > > The new kernel will see the new_thing bytes are zero and behave the > > same as before. > > > > If then the userspace decides to set new_thing the old kernel will > > stop working. Userspace can use some 'try and fail' approach to try > > again with new_thing = 0. > > I'm not keen on try and fail interfaces because they become messy > if this has potentially be extended multiple times. Rest > of argument is fair enough. Thanks for the explanation. I'd say try-and-fail is just the universal option, if there is merit we can put cap bits and other things to positively indicate increased kernel capability. We have quite a deep experiance on this topic now in RDMA, and there we've been doing both options, depending on the situation. For instance you might introduce a new API that returns FOO and extend a prior API to optionally accept FOO as well. A cap flag that the new API exists is useful [1], but it is not for the prior API. The userspace can just blindly pass FOO to the prior API, and if it happened to get a non-zero FOO somehow then the kernel must also support it.. [1] "try and fail" works well here too you can invoke the IOCTL with a 0 size and you get ENOTTY if the IOCTL is not understood, and another error code if it is. Jason