From:   Nathan Lynch <nathanl@linux.ibm.com>
To:     Laurent Dufour <ldufour@linux.ibm.com>
Cc:     linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org
Subject: Re: [PATCH v3] powerpc/pseries: read the lpar name from the firmware
In-Reply-To: <21eb4749-42b1-da78-8833-00d360fa36e5@linux.ibm.com>
References: <20211203154321.13168-1-ldufour@linux.ibm.com>
 <87bl1so588.fsf@linux.ibm.com>
 <bbaa0d78-a09f-3ce3-25a9-67434039b741@linux.ibm.com>
 <878rwwny1l.fsf@linux.ibm.com>
 <21eb4749-42b1-da78-8833-00d360fa36e5@linux.ibm.com>
Date:   Wed, 08 Dec 2021 09:21:29 -0600
Message-ID: <874k7jnmva.fsf@linux.ibm.com>
MIME-Version: 1.0
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Laurent Dufour <ldufour@linux.ibm.com> writes:
> On 07/12/2021, 18:07:50, Nathan Lynch wrote:
>> Laurent Dufour <ldufour@linux.ibm.com> writes:
>>> On 07/12/2021, 15:32:39, Nathan Lynch wrote:
>>>> Is there a reasonable fallback for VMs where this parameter doesn't
>>>> exist? PowerVM partitions should always have it, but what do we want the
>>>> behavior to be on other hypervisors?
>>>
>>> In that case, there is no value displayed in the /proc/powerpc/lparcfg and
>>> the lparstat -i command will fall back to the device tree value. I can't
>>> see any valid reason to report the value defined in the device tree
>>> here.
>> 
>> Here's a valid reason :-)
>> 
>> lparstat isn't the only possible consumer of the interface, and the
>> 'ibm,partition-name' property and the dynamic system parameter clearly
>> serve a common purpose. 'ibm,partition-name' is provided by qemu.
>
> If the hypervisor is not providing this value, this is not the goal of this
> interface to fetch it from the device tree.
>
> Any consumer should be able to fall back on the device tree value, and
> there is no added value to do such a trick in the kernel when it can be
> done in the user space.

There is value in imposing a level of abstraction so that the semantics
are:

* Report the name assigned to the guest by the hosting environment, if
  available

as opposed to

* Return the string returned by a RTAS call to ibm,get-system-parameter
  with token 55, if implemented

The benefit is that consumers of lparcfg do not have to be coded with
the knowledge that "if a partition_name= line is absent, the
ibm,get-system-parameter RTAS call must have failed, so now I should
read /sys/firmware/devicetree/base/ibm,partition_name." That's the sort
of esoterica that is appropriate for the kernel to encapsulate.

And I'd say the effort involved (falling back to a root node property
lookup) is proportional to the benefit.