linux-kernel - Re: [PATCH] irqchip/gic-v3-its: Don't acquire rt_spin_lock in allocate_vpe_l1

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Message-ID: <86wm1qlq7l.wl-maz@kernel.org>
Date: Fri, 09 Jan 2026 16:13:02 +0000
From: Marc Zyngier <maz@...nel.org>
To: Thomas Gleixner <tglx@...nel.org>
Cc: Waiman Long <longman@...hat.com>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Clark Williams
 <clrkwllms@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org,
	linux-rt-devel@...ts.linux.dev
Subject: Re: [PATCH] irqchip/gic-v3-its: Don't acquire rt_spin_lock in allocate_vpe_l1_table()

On Thu, 08 Jan 2026 22:11:33 +0000,
Thomas Gleixner <tglx@...nel.org> wrote:
> 
> On Thu, Jan 08 2026 at 08:26, Marc Zyngier wrote:
> > Err, no. That's horrible. I can see three ways to address this in a
> > more appealing way:
> >
> > - you give RT a generic allocator that works for (small) atomic
> >   allocations. I appreciate that's not easy, and even probably
> >   contrary to the RT goals. But I'm also pretty sure that the GIC code
> >   is not the only pile of crap being caught doing that.
> >
> > - you pre-compute upfront how many cpumasks you are going to require,
> >   based on the actual GIC topology. You do that on CPU0, outside of
> >   the hotplug constraints, and allocate what you need. This is
> >   difficult as you need to ensure the RD<->CPU matching without the
> >   CPUs having booted, which means wading through the DT/ACPI gunk to
> >   try and guess what you have.
> >
> > - you delay the allocation of L1 tables to a context where you can
> >   perform allocations, and before we have a chance of running a guest
> >   on this CPU. That's probably the simplest option (though dealing
> >   with late onlining while guests are already running could be
> >   interesting...).
> 
> At the point where a CPU is brought up, the topology should be known
> already, which means this can be allocated on the control CPU _before_
> the new CPU comes up, no?

No. Each CPU finds *itself* in the forest of redistributors, and from
there tries to find whether it has some shared resource with a CPU
that has booted before it. That's because firmware is absolutely awful
and can't present a consistent view of the system.

Anyway, I expect it could be solved by moving this part of the init to
an ONLINE HP callback.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.