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

From: Vaibhav Jain <[email-masked]>
Date: Mon, 25 May 2020 17:30:42 +0530

Hi Ira, Mpe and Aneesh,

Vaibhav Jain <[email-masked]> writes:

> Michael Ellerman <[email-masked]> writes:
>
>> Ira Weiny <[email-masked]> 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.

If you are ok with the explaination above please let me know. I will
quickly spin off a v8 addressing your review comments.

Thanks,
-- 
Cheers
~ Vaibhav