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

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Date:   Thu, 14 May 2020 14:04:55 -0500
From:   Nathan Lynch <nathanl@...ux.ibm.com>
To:     Leonardo Bras <leonardo@...ux.ibm.com>
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Allison Randal <allison@...utok.net>,
        "Gautham R. Shenoy" <ego@...ux.vnet.ibm.com>,
        Nadav Amit <namit@...are.com>
Subject: Re: [PATCH 1/1] powerpc/rtas: Implement reentrant rtas call

Hi Leonardo,

Leonardo Bras <leonardo@...ux.ibm.com> writes:
> Hello Nathan, thanks for the feedback!
>
> On Fri, 2020-04-10 at 14:28 -0500, Nathan Lynch wrote:
>> Leonardo Bras <leonardo@...ux.ibm.com> writes:
>> > 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.
>> > 
>
>> 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.