Re: [PATCH 1/1] powerpc/rtas: Implement reentrant rtas call

Hi Leonardo,

Leonardo Bras [off-list ref] writes:

Hello Nathan, thanks for the feedback!

On Fri, 2020-04-10 at 14:28 -0500, Nathan Lynch wrote:

quoted

Leonardo Bras [off-list ref] writes:

quoted

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 call.

quoted

From the language in the spec it's clear that these calls are intended
to be reentrant with respect to themselves, but it's less clear to me
that they are safe to call simultaneously with respect to each other or
arbitrary other RTAS methods.

In my viewpoint, being reentrant to themselves, without being reentrant
to others would be very difficult to do, considering the way the
rtas_call is crafted to work.

I mean, I have no experience in rtas code, it's my viewpoint. In my
thoughts there is something like this:

common_path -> selects function by token -> reentrant function
					|-> non-reentrant function

If there is one function that is reentrant, it means the common_path
and function selection by token would need to be reentrant too.

I checked with partition firmware development and these calls can be
used concurrently with arbitrary other RTAS calls, which confirms your
interpretation. Thanks for bearing with me.