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

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Message-ID: <86v7h8l9ht.wl-maz@kernel.org>
Date: Sun, 11 Jan 2026 10:38:38 +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 Sun, 11 Jan 2026 09:39:07 +0000,
Thomas Gleixner <tglx@...nel.org> wrote:
> 
> On Fri, Jan 09 2026 at 16:13, Marc Zyngier wrote:
> > On Thu, 08 Jan 2026 22:11:33 +0000,
> > Thomas Gleixner <tglx@...nel.org> wrote:
> >> 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.
> 
> Groan....
>
> > Anyway, I expect it could be solved by moving this part of the init to
> > an ONLINE HP callback.
> 
> Which needs to be before CPUHP_AP_IRQ_AFFINITY_ONLINE, but even that
> might be to late because there are callbacks in the STARTING section,
> i.e. timer, perf, which might rely on interrupts being accessible.

Nah. This stuff is only for direct injection of vLPIs into guests, so
as long as this is done before we can schedule a vcpu on this physical
CPU, we're good. No physical interrupt is concerned with this code.

> Also that patch seems to be incomplete because there is another
> allocation further down in allocate_vpe_l1_table()....

Yeah, I wondered why page allocation wasn't affected by this issue,
but didn't try to find out.

	M.

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