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? > 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. 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. Thanks, Nick