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From: Vaibhav Jain <vaibhav@linux.ibm.com>
To: Michael Ellerman <mpe@ellerman.id.au>, 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: Fri, 22 May 2020 13:08:05 +0530
Message-ID: <87ftbswhb6.fsf@linux.ibm.com> (raw)
In-Reply-To: <87367t941j.fsf@mpe.ellerman.id.au>

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
>
> 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
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Thread overview: 20+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-05-19 19:00 [RESEND PATCH v7 0/5] powerpc/papr_scm: Add support for reporting nvdimm health Vaibhav Jain
2020-05-19 19:00 ` [RESEND PATCH v7 1/5] powerpc: Document details on H_SCM_HEALTH hcall Vaibhav Jain
2020-05-19 19:00 ` [RESEND PATCH v7 2/5] seq_buf: Export seq_buf_printf() to external modules Vaibhav Jain
2020-05-20 17:01   ` Christoph Hellwig
2020-05-19 19:00 ` [RESEND PATCH v7 3/5] powerpc/papr_scm: Fetch nvdimm health information from PHYP Vaibhav Jain
2020-05-20 14:54   ` Ira Weiny
2020-05-20 17:15     ` Vaibhav Jain
2020-05-21 14:31       ` Michael Ellerman
2020-05-21 16:59         ` Vaibhav Jain
2020-05-21 23:32       ` Ira Weiny
2020-05-19 19:00 ` [RESEND PATCH v7 4/5] ndctl/papr_scm,uapi: Add support for PAPR nvdimm specific methods Vaibhav Jain
2020-05-20  7:09   ` Aneesh Kumar K.V
2020-05-20 10:19     ` Vaibhav Jain
2020-05-20 15:32   ` Ira Weiny
2020-05-20 18:37     ` Vaibhav Jain
2020-05-21  6:48     ` Michael Ellerman
2020-05-22  7:38       ` Vaibhav Jain [this message]
2020-05-25 12:00         ` [RESEND PATCH v7 4/5] ndctl/papr_scm, uapi: " Vaibhav Jain
2020-05-26 12:14           ` Michael Ellerman
2020-05-19 19:00 ` [RESEND PATCH v7 5/5] powerpc/papr_scm: Implement support for PAPR_SCM_PDSM_HEALTH Vaibhav Jain

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