Re: [PATCH] irqchip/gic-v3-its: Don't acquire rt_spin_lock in allocate_vpe_l1_table()

[Waiman Long]

On 1/21/26 3:38 AM, Marc Zyngier wrote:
> On Mon, 12 Jan 2026 11:20:07 +0000,
> Marc Zyngier <maz@xxxxxxxxxx> wrote:
> > On Sun, 11 Jan 2026 16:20:45 +0000,
> > Thomas Gleixner <tglx@xxxxxxxxxx> wrote:
> > > On Sun, Jan 11 2026 at 10:38, Marc Zyngier wrote:
> > > > On Sun, 11 Jan 2026 09:39:07 +0000,
> > > > Thomas Gleixner <tglx@xxxxxxxxxx> wrote:
> > > > > On Fri, Jan 09 2026 at 16:13, Marc Zyngier wrote:
> > > > > > On Thu, 08 Jan 2026 22:11:33 +0000,
> > > > > > Thomas Gleixner <tglx@xxxxxxxxxx> 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.
> Have you managed to try this hack? I may be able to spend some time
> addressing the issue in the next cycle if I have an indication that
> I'm on the right track.

I am sorry that I was busy working on other stuff. Will try out the hack 
today and report back ASAP.

Cheers,
Longman