On Mon, 2022-09-12 at 14:58 -0500, Nathan Lynch wrote: > Leonardo Brás <leobra...@gmail.com> writes: > > On Fri, 2022-09-09 at 09:04 -0500, Nathan Lynch wrote: > > > Leonardo Brás <leobra...@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....@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 <leobra...@gmail.com>