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

From: Michael Ellerman <mpe@ellerman.id.au>
To: Vaibhav Jain <vaibhav@linux.ibm.com>, Ira Weiny <ira.weiny@intel.com>
Cc: "Aneesh Kumar K . V" <aneesh.kumar@linux.ibm.com>,
	linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org,
	Steven Rostedt <rostedt@goodmis.org>,
	linux-nvdimm@lists.01.org
Subject: Re: [RESEND PATCH v7 4/5] ndctl/papr_scm, uapi: Add support for PAPR nvdimm specific methods
Date: Tue, 26 May 2020 22:14:39 +1000	[thread overview]
Message-ID: <875zcigafk.fsf@mpe.ellerman.id.au> (raw)
In-Reply-To: <87a71ww7f9.fsf@linux.ibm.com>

Vaibhav Jain <vaibhav@linux.ibm.com> writes:
> Hi Ira, Mpe and Aneesh,
>
> Vaibhav Jain <vaibhav@linux.ibm.com> writes:
>
>> Michael Ellerman <mpe@ellerman.id.au> writes:
>>
>>> Ira Weiny <ira.weiny@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/include/uapi/linux/sched.h#n88
>>
>> 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.

I don't have strong opinions about the user API, it's really up to the
nvdimm folks.

cheers
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