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Date:   Sat, 16 May 2020 01:08:24 -0300
From:   Leonardo Bras <>
To:     Nicholas Piggin <>,
        Allison Randal <>,
        Benjamin Herrenschmidt <>,
        "Gautham R. Shenoy" <>,
        Greg Kroah-Hartman <>,
        Michael Ellerman <>,
        Nadav Amit <>,
        Nathan Lynch <>,
        Paul Mackerras <>,
        Thomas Gleixner <>
Subject: Re: [PATCH v4 2/2] powerpc/rtas: Implement reentrant rtas call

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 to,
> > 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:

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

> Thanks,
> Nick

Thank you Nick! 

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