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

Hi Ira, Mpe and Aneesh, Vaibhav Jain <vaibhav(a)linux.ibm.com&gt; writes: > 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... > > 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. If you are ok with the explaination above please let me know. I will quickly spin off a v8 addressing your review comments.