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

[Vaibhav Jain]

Michael Ellerman <mpe@xxxxxxxxxxxxxx> writes:

> Ira Weiny <ira.weiny@xxxxxxxxx> 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/include/uapi/linux/sched.h#n88
>
> 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.
  
-- 
Cheers
~ Vaibhav