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

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Message-ID: <86qzrulwve.wl-maz@kernel.org>
Date: Mon, 12 Jan 2026 14:38:13 +0000
From: Marc Zyngier <maz@...nel.org>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: Waiman Long <longman@...hat.com>,
	Thomas Gleixner <tglx@...nel.org>,
	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 Mon, 12 Jan 2026 14:08:37 +0000,
Sebastian Andrzej Siewior <bigeasy@...utronix.de> wrote:
> 
> On 2026-01-12 11:20:07 [+0000], Marc Zyngier wrote:
> > On Sun, 11 Jan 2026 16:20:45 +0000,
> > Thomas Gleixner <tglx@...nel.org> wrote:
> > > 
> > > On Sun, Jan 11 2026 at 10:38, Marc Zyngier wrote:
> > > > 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.
> > > 
> > > That's fine then. vCPUs are considered "user-space" tasks and can't be
> > > scheduled before CPUHP_AP_ACTIVE sets the CPU active for the scheduler.
> > 
> > Waiman, can you please give the following hack a go on your box? The
> > machines I have are thankfully limited to a single ITS group, so I
> > can't directly reproduce your issue.
> > 
> > Thanks,
> > 
> > 	M.
> > 
> > diff --git a/drivers/irqchip/irq-gic-v3-its.c b/drivers/irqchip/irq-gic-v3-its.c
> > index ada585bfa4517..20967000f2348 100644
> > --- a/drivers/irqchip/irq-gic-v3-its.c
> > +++ b/drivers/irqchip/irq-gic-v3-its.c
> > @@ -2896,7 +2896,7 @@ static bool allocate_vpe_l2_table(int cpu, u32 id)
> >  	return true;
> >  }
> >  
> > -static int allocate_vpe_l1_table(void)
> > +static int allocate_vpe_l1_table(unsigned int cpu)
> >  {
> >  	void __iomem *vlpi_base = gic_data_rdist_vlpi_base();
> >  	u64 val, gpsz, npg, pa;
> > @@ -3012,10 +3012,11 @@ static int allocate_vpe_l1_table(void)
> >  
> >  out:
> >  	gicr_write_vpropbaser(val, vlpi_base + GICR_VPROPBASER);
> > -	cpumask_set_cpu(smp_processor_id(), gic_data_rdist()->vpe_table_mask);
> > +	cpumask_set_cpu(cpu, gic_data_rdist()->vpe_table_mask);
> > +	dsb(sy);
> >  
> >  	pr_debug("CPU%d: VPROPBASER = %llx %*pbl\n",
> > -		 smp_processor_id(), val,
> > +		 cpu, val,
> >  		 cpumask_pr_args(gic_data_rdist()->vpe_table_mask));
> >  
> >  	return 0;
> > @@ -3264,15 +3265,9 @@ static void its_cpu_init_lpis(void)
> >  		val = its_clear_vpend_valid(vlpi_base, 0, 0);
> >  	}
> >  
> > -	if (allocate_vpe_l1_table()) {
> > -		/*
> > -		 * If the allocation has failed, we're in massive trouble.
> > -		 * Disable direct injection, and pray that no VM was
> > -		 * already running...
> > -		 */
> > -		gic_rdists->has_rvpeid = false;
> > -		gic_rdists->has_vlpis = false;
> > -	}
> > +	if (smp_processor_id() == 0)
> > +		cpuhp_setup_state(CPUHP_AP_ONLINE_DYN, "irqchip/arm/gicv3:vpe",
> > +				  allocate_vpe_l1_table, NULL);
> 
> If you move it the online state then you could also
> s/GFP_ATOMIC/GFP_KERNEL.
> 
> Also previously you checked the error code set has_rvpeid, has_vlpis on
> failure. Now you you should the same in case of a failure during
> registration.
> This also happens happens on CPU hotplug and I don't see how you avoid a
> second allocation. But I also don't understand why this registrations
> happens on CPU0. It might be just a test patch…

It's just a test hack. There is way more things that would need to
change in order to cope with moving this to CPUHP, but I want
confirmation that this indeed solves the original issue before I start
breaking more things.

Thanks,

	M.

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