Re: [PATCH] Revert "powerpc/rtas: Implement reentrant rtas call"

["Leonardo Brás" <leobras.c@gmail.com>]

On Mon, 2022-09-12 at 14:58 -0500, Nathan Lynch wrote:
> Leonardo Brás <leobras.c@gmail.com> writes:
> > On Fri, 2022-09-09 at 09:04 -0500, Nathan Lynch wrote:
> > > Leonardo Brás <leobras.c@gmail.com> writes:
> > > > On Wed, 2022-09-07 at 17:01 -0500, Nathan Lynch wrote:
> > > > > At the time this was submitted by Leonardo, I confirmed -- or thought
> > > > > I had confirmed -- with PowerVM partition firmware development that
> > > > > the following RTAS functions:
> > > > > 
> > > > > - ibm,get-xive
> > > > > - ibm,int-off
> > > > > - ibm,int-on
> > > > > - ibm,set-xive
> > > > > 
> > > > > were safe to call on multiple CPUs simultaneously, not only with
> > > > > respect to themselves as indicated by PAPR, but with arbitrary other
> > > > > RTAS calls:
> > > > > 
> > > > > https://lore.kernel.org/linuxppc-dev/875zcy2v8o.fsf@linux.ibm.com/
> > > > > 
> > > > > Recent discussion with firmware development makes it clear that this
> > > > > is not true, and that the code in commit b664db8e3f97 ("powerpc/rtas:
> > > > > Implement reentrant rtas call") is unsafe, likely explaining several
> > > > > strange bugs we've seen in internal testing involving DLPAR and
> > > > > LPM. These scenarios use ibm,configure-connector, whose internal state
> > > > > can be corrupted by the concurrent use of the "reentrant" functions,
> > > > > leading to symptoms like endless busy statuses from RTAS.
> > > > 
> > > > Oh, does not it means PowerVM is not compliant to the PAPR specs?
> > > 
> > > No, it means the premise of commit b664db8e3f97 ("powerpc/rtas:
> > > Implement reentrant rtas call") change is incorrect. The "reentrant"
> > > property described in the spec applies only to the individual RTAS
> > > functions. The OS can invoke (for example) ibm,set-xive on multiple CPUs
> > > simultaneously, but it must adhere to the more general requirement to
> > > serialize with other RTAS functions.
> > > 
> > 
> > I see. Thanks for explaining that part!
> > I agree: reentrant calls that way don't look as useful on Linux than I
> > previously thought.
> > 
> > OTOH, I think that instead of reverting the change, we could make use of the
> > correct information and fix the current implementation. (This could help when we
> > do the same rtas call in multiple cpus)
> 
> Hmm I'm happy to be mistaken here, but I doubt we ever really need to do
> that. I'm not seeing the need.
> 
> > I have an idea of a patch to fix this. 
> > Do you think it would be ok if I sent that, to prospect being an alternative to
> > this reversion?
> 
> It is my preference, and I believe it is more common, to revert to the
> well-understood prior state, imperfect as it may be. The revert can be
> backported to -stable and distros while development and review of
> another approach proceeds.

Ok then, as long as you are aware of the kdump bug, I'm good.

FWIW:
Reviewed-by: Leonardo Bras <leobras.c@gmail.com>