Re: [PATCH v1 3/6] irqchip/gic-v3: support SGI broadcast

[Marc Zyngier <maz@kernel.org>]

On Fri, 25 Oct 2024 18:31:01 +0100,
Yu Zhao <yuzhao@google.com> wrote:
> 
> On Fri, Oct 25, 2024 at 10:15 AM Marc Zyngier <maz@kernel.org> wrote:
> >
> > On Fri, 25 Oct 2024 06:07:45 +0100,
> > Yu Zhao <yuzhao@google.com> wrote:
> > >
> > > Hi Marc,
> > >
> > > On Tue, Oct 22, 2024 at 9:03 AM Marc Zyngier <maz@kernel.org> wrote:
> > > >
> > > > On Mon, 21 Oct 2024 05:22:15 +0100,
> > > > Yu Zhao <yuzhao@google.com> wrote:
> > > > >
> > > > > @@ -1407,6 +1418,13 @@ static void gic_ipi_send_mask(struct irq_data *d, const struct cpumask *mask)
> > > > >        */
> > > > >       dsb(ishst);
> > > > >
> > > > > +     cpumask_copy(&broadcast, cpu_present_mask);
> > > >
> > > > Why cpu_present_mask? I'd expect that cpu_online_mask should be the
> > > > correct mask to use -- we don't IPI offline CPUs, in general.
> > >
> > > This is exactly because "we don't IPI offline CPUs, in general",
> > > assuming "we" means the kernel, not GIC.
> > >
> > > My interpretation of what the GIC spec says ("0b1: Interrupts routed
> > > to all PEs in the system, excluding self") is that it broadcasts IPIs to
> > > "cpu_present_mask" (minus the local one). So if the kernel uses
> > > "cpu_online_mask" here, GIC would send IPIs to offline CPUs
> > > (cpu_present_mask ^ cpu_online_mask), which I don't know whether it's
> > > a defined behavior.
> 
> Thanks for clarifying.
> 
> > Offline CPUs are not known to the kernel.
> 
> I assume it wouldn't matter to firmware either, correct? IOW, we

Firmware is on the secure side of the stack.

> wouldn't cause firmware any trouble by letting GIC send IPIs to
> (cpu_present_mask ^ cpu_online_mask), assuming those two masks can be
> different on arm64 when hotplug is enabled?

You can't send SGIs from non-secure to secure using ICC_SGI1R_EL1. You
would need to use ICC_ASGI1R_EL1, and have secure to allow such
brokenness via a configuration of GICR_NSACR. Linux doesn't use the
former, and no sane software touches the latter.

	M.

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