From:   Valentin Schneider <valentin.schneider@arm.com>
To:     Michael Ellerman <mpe@ellerman.id.au>,
        linux-kernel@vger.kernel.org, kasan-dev@googlegroups.com,
        linuxppc-dev@lists.ozlabs.org, linux-kbuild@vger.kernel.org
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>,
        Frederic Weisbecker <frederic@kernel.org>,
        Mike Galbraith <efault@gmx.de>, Marco Elver <elver@google.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Michal Marek <michal.lkml@markovi.net>,
        Nick Desaulniers <ndesaulniers@google.com>
Subject: Re: [PATCH v2 3/5] powerpc: Use preemption model accessors
In-Reply-To: <87o86rmgu8.fsf@mpe.ellerman.id.au>
References: <20211110202448.4054153-1-valentin.schneider@arm.com> <20211110202448.4054153-4-valentin.schneider@arm.com> <87o86rmgu8.fsf@mpe.ellerman.id.au>
Date:   Mon, 15 Nov 2021 15:29:53 +0000
Message-ID: <87lf1pqvwu.mognet@arm.com>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk


Doh, thought I had sent this one out already...

On 11/11/21 15:55, Michael Ellerman wrote:
> Valentin Schneider <valentin.schneider@arm.com> writes:
>> Per PREEMPT_DYNAMIC, checking CONFIG_PREEMPT doesn't tell you the actual
>> preemption model of the live kernel. Use the newly-introduced accessors
>> instead.
>>
>> sched_init() -> preempt_dynamic_init() happens way before IRQs are set up,
>> so this should be fine.
>
> Despite the name interrupt_exit_kernel_prepare() is called before IRQs
> are setup, traps and page faults are "interrupts" here.
>
> So I'm not sure about adding that call there, because it will trigger a
> WARN if called early in boot, which will trigger a trap and depending on
> the context we may not survive.
>
> I'd be happier if we can make it a build-time check.
>

This can't be done at build-time for PREEMPT_DYNAMIC, but that can be
punted off to whoever will implement ppc support for that :-) AFAICT if
this can't use preempt_dynamic_mode (due to how "late" it is setup), the
preempt_schedule_irq() needs to go and ppc needs to use irqentry_exit() /
irqentry_exit_cond_resched().

I dropped that for v2.

> cheers
>