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

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Message-ID: <20260112140837.UNQYT563@linutronix.de>
Date: Mon, 12 Jan 2026 15:08:37 +0100
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Marc Zyngier <maz@...nel.org>
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 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…

>  
>  	/* Make sure the GIC has seen the above */
>  	dsb(sy);

Sebastian