Re: [RESEND PATCH v7 4/5] ndctl/papr_scm, uapi: Add support for PAPR nvdimm specific methods

Michael Ellerman <mpe(a)ellerman.id.au&gt; writes: > Ira Weiny <ira.weiny(a)intel.com&gt; writes: >> On Wed, May 20, 2020 at 12:30:57AM +0530, Vaibhav Jain wrote: >>> Introduce support for Papr nvDimm Specific Methods (PDSM) in papr_scm >>> modules and add the command family to the white list of NVDIMM command >>> sets. Also advertise support for ND_CMD_CALL for the dimm >>> command mask and implement necessary scaffolding in the module to >>> handle ND_CMD_CALL ioctl and PDSM requests that we receive. > ... >>> + * >>> + * Payload Version: >>> + * >>> + * A 'payload_version' field is present in PDSM header that indicates a specific >>> + * version of the structure present in PDSM Payload for a given PDSM command. >>> + * This provides backward compatibility in case the PDSM Payload structure >>> + * evolves and different structures are supported by 'papr_scm' and 'libndctl'. >>> + * >>> + * When sending a PDSM Payload to 'papr_scm', 'libndctl' should send the version >>> + * of the payload struct it supports via 'payload_version' field. The 'papr_scm' >>> + * module when servicing the PDSM envelope checks the 'payload_version' and then >>> + * uses 'payload struct version' == MIN('payload_version field', >>> + * 'max payload-struct-version supported by papr_scm') to service the PDSM. >>> + * After servicing the PDSM, 'papr_scm' put the negotiated version of payload >>> + * struct in returned 'payload_version' field. >> >> FWIW many people believe using a size rather than version is more sustainable. >> It is expected that new payload structures are larger (more features) than the >> previous payload structure. >> >> I can't find references at the moment through. > > I think clone_args is a good modern example: > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/i... > > cheers Thank Ira and Mpe for pointing this out. I looked into how clone3 sycall handles clone_args and few differences came out: * Unlike clone_args that are always transferred in one direction from user-space to kernel, payload contents of pdsms are transferred in both directions. Having a single version number makes it easier for user-space and kernel to determine what data will be exchanged. * For PDSMs, the version number is negotiated between libndctl and kernel. For example in case kernel only supports an older version of a structure, its free to send a lower version number back to libndctl. Such negotiations doesnt happen with clone3 syscall.