Hello Nick, On Fri, 2020-05-15 at 17:30 +1000, Nicholas Piggin wrote: > Excerpts from Leonardo Bras's message of May 15, 2020 9:51 am: > > Implement rtas_call_reentrant() for reentrant rtas-calls: > > "ibm,int-on", "ibm,int-off",ibm,get-xive" and "ibm,set-xive". > > > > On LoPAPR Version 1.1 (March 24, 2016), from 7.3.10.1 to 7.3.10.4, > > items 2 and 3 say: > > > > 2 - For the PowerPC External Interrupt option: The * call must be > > reentrant to the number of processors on the platform. > > 3 - For the PowerPC External Interrupt option: The * argument call > > buffer for each simultaneous call must be physically unique. > > > > So, these rtas-calls can be called in a lockless way, if using > > a different buffer for each cpu doing such rtas call. > > What about rtas_call_unlocked? Do the callers need to take the rtas > lock? > > Machine checks must call ibm,nmi-interlock too, which we really don't > want to take a lock for either. Hopefully that's in a class of its own > and we can essentially ignore with respect to other rtas calls. > > The spec is pretty vague too :( > > "The ibm,get-xive call must be reentrant to the number of processors on > the platform." > > This suggests ibm,get-xive can be called concurrently by multiple > processors. It doesn't say anything about being re-entrant against any > of the other re-entrant calls. Maybe that could be reasonably assumed, > but I don't know if it's reasonable to assume it can be called > concurrently with a *non-reentrant* call, is it?
This was discussed on a previous version of the patchset: https://lore.kernel.org/linuxppc-dev/875zcy2v8o....@linux.ibm.com/ He checked with partition firmware development and these calls can be used concurrently with arbitrary other RTAS calls. > > > For this, it was suggested to add the buffer (struct rtas_args) > > in the PACA struct, so each cpu can have it's own buffer. > > You can't do this, paca is not limited to RTAS_INSTANTIATE_MAX. > Which is good, because I didn't want you to add another 88 bytes to the > paca :) Can you make it a pointer and allocate it separately? Check > the slb_shadow allocation, you could use a similar pattern. Sure, I will send the next version with this change. > > The other option would be to have just one more rtas args, and have the > crashing CPU always that. That would skirt the re-entrancy issue -- the > concurrency is only ever a last resort. Would be a bit tricker though. It seems a good idea, but I would like to try the previous alternative first. > Thanks, > Nick Thank you Nick!
