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Re: [PATCH v2] target/ppc: cpu_init: Clean up stop state on cpu reset

From: Fabiano Rosas
Subject: Re: [PATCH v2] target/ppc: cpu_init: Clean up stop state on cpu reset
Date: Fri, 17 Jun 2022 11:57:31 -0300

Frederic Barrat <fbarrat@linux.ibm.com> writes:

> The 'resume_as_sreset' attribute of a cpu is set when a thread is
> entering a stop state on ppc books. It causes the thread to be
> re-routed to vector 0x100 when woken up by an exception. So it must be
> cleared on reset or a thread might be re-routed unexpectedly after a
> reset, when it was not in a stop state and/or when the appropriate
> exception handler isn't set up yet.
> Using skiboot, it can be tested by resetting the system when it is
> quiet and most threads are idle and in stop state.
> After the reset occurs, skiboot elects a primary thread and all the
> others wait in secondary_wait. The primary thread does all the system
> initialization from main_cpu_entry() and at some point, the
> decrementer interrupt starts ticking. The exception vector for the
> decrementer interrupt is in place, so that shouldn't be a
> problem. However, if that primary thread was in stop state prior to
> the reset, and because the resume_as_sreset parameters is still set,
> it is re-routed to exception vector 0x100. Which, at that time, is
> still defined as the entry point for BML. So that primary thread
> restarts as new and ends up being treated like any other secondary
> thread. All threads are now waiting in secondary_wait.
> It results in a full system hang with no message on the console, as
> the uart hasn't been init'ed yet. It's actually not obvious to realise
> what's happening if not tracing reset (-d cpu_reset). The fix is
> simply to clear the 'resume_as_sreset' attribute on reset.
> Signed-off-by: Frederic Barrat <fbarrat@linux.ibm.com>

Reviewed-by: Fabiano Rosas <farosas@linux.ibm.com>

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