lkml.org 
[lkml]   [2020]   [May]   [16]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [PATCH v4 2/2] powerpc/rtas: Implement reentrant rtas call
From
Date
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.fsf@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!

\
 
 \ /
  Last update: 2020-05-16 06:08    [W:0.042 / U:0.508 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site