5:38 p.m.
Michael Ellerman <mpe(a)ellerman.id.au> writes:
Ira Weiny <ira.weiny(a)intel.com> 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.
--
Cheers
~ Vaibhav