Message-ID: <00f58d20-f92d-461e-beac-b307905cab3b@huaweicloud.com>
Date: Mon, 29 Jul 2024 16:45:58 +0200
From: Jonas Oberhauser <jonas.oberhauser@...weicloud.com>
To: Hernan Ponce de Leon <hernan.poncedeleon@...weicloud.com>,
 Alan Stern <stern@...land.harvard.edu>
Cc: paulmck@...nel.org, parri.andrea@...il.com, will@...nel.org,
 peterz@...radead.org, boqun.feng@...il.com, npiggin@...il.com,
 dhowells@...hat.com, j.alglave@....ac.uk, luc.maranget@...ia.fr,
 akiyks@...il.com, dlustig@...dia.com, joel@...lfernandes.org,
 urezki@...il.com, quic_neeraju@...cinc.com, frederic@...nel.org,
 linux-kernel@...r.kernel.org, lkmm@...ts.linux.dev
Subject: Re: [PATCHv2 0/4] tools/memory-model: Define more of LKMM in
 tools/memory-model



Am 7/29/2024 um 3:30 PM schrieb Hernan Ponce de Leon:
> On 7/12/2024 10:06 AM, Hernan Ponce de Leon wrote:
>> On 6/10/2024 10:38 AM, Hernan Ponce de Leon wrote:
>>> On 6/8/2024 3:00 AM, Alan Stern wrote:
>>>> On Wed, Jun 05, 2024 at 09:58:42PM +0200, Jonas Oberhauser wrote:
>>>>>
>>>>>
>>>>> Am 6/4/2024 um 7:56 PM schrieb Alan Stern:
>>>>>> Just to clarify: Your first step encompasses patches 1 - 3, and the
>>>>>> second step is patch 4.  The first three patches can be applied 
>>>>>> now, but
>>>>>> the last one needs to wait until herd7 has been updated.  Is this all
>>>>>> correct?
>>>>>
>>>>> Exactly.
>>>>
>>>> With regard to patch 4, how much thought have you and Hernan given to
>>>> backward compatibility?  Once herd7 is changed, old memory model files
>>>> will no longer work correctly.
>>>>
>>>
>>> Honestly, I did not think much about this (at least until Akira 
>>> mentioned in my PR). My hope was that changes to the model could be 
>>> back-ported to previous kernel versions. However that would not work 
>>> for existing out-of-tree files.
>>>
>>> My question is: is compatibility with out-of-tree files really a 
>>> requirement? I would argue that if people are using outdated models, 
>>> they may get wrong results anyway. This is because some of the 
>>> changes done to lkmm during the last few years change the expected 
>>> result for some litmus tests.
>>>
>>> Hernan
>>
>> I pushed some new changes to the code for backward compatibility [1]. 
>> The series also needs the patch at the bottom to properly deal with 
>> the ordering of failing CAses and non-returning operations. With it, 
>> all litmus tests return the correct result (the script needs to pass 
>> option -lkmm-legacy false to herd).
> 
> I have been playing around with an alternative to this.
> 
> Rather than implementing this as an "option", I can implemented it as a 
> "model variant (*)" and add this to the model

How exactly do these model variants get selected?

I was thinking that another good approach could be to have a new generic 
C model which doesn't know anything about LKMM. I believe this would be 
specified in the header of the .litmus files?


> flag ~empty (if "lkmmlatest" then 0 else _)
>    as new-lkmm-models-require-variant-lkmmlatest
> 
> If the user forgets to set the variant for the new model, herd7 will 
> flag the executions showing that something is off.
> 
> To be fully backward compatible, we would need to backport this to old 
> models
> 
> flag ~empty (if "lkmmlatest" then 1 else _)
>    as new-lkmm-models-require-variant-lkmmlatest

should this be then _ else 0  ? or what does the _ do here?

I also don't think we can backport things to old models

> If the user (wrongly) sets the variant for an old model, the the 
> executions will be flagged.
> 
> Any thoughts?
> 
> Hernan
> 
> (*) This trick seems to be used for some arm models
> 
> https://github.com/herd/herdtools7/blob/master/herd/libdir/arm-models/mixed/ec.cat#L66C1-L67C67

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