Re: [PATCHv2 0/4] tools/memory-model: Define more of LKMM in tools/memory-model

[Hernan Ponce de Leon]

On 7/29/2024 4:45 PM, Jonas Oberhauser wrote:
> 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 I would argue that if people are using outdated models, 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?

Yes, my bad.

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

IIRC I have seen (non lkmm related) patches being backported to stable 
kernel versions. Why can't we do this for lkmm if backward compatibility 
is really a requirement? Otherwise I don't see a way of preventing 
developers to use old models with the new option (since I plan to keep 
the "old variant" as default, this would have to be done on purpose, but 
still).

> > 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