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

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Message-ID: <86tswrkrh4.wl-maz@kernel.org>
Date: Mon, 12 Jan 2026 11:20:07 +0000
From: Marc Zyngier <maz@...nel.org>
To: Waiman Long <longman@...hat.com>, Thomas Gleixner <tglx@...nel.org>
Cc: 	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 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);
 
 	/* Make sure the GIC has seen the above */
 	dsb(sy);


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