Re: [PATCH v4 18/26] arm64: smp: Support non-SGIs for IPIs

[Jonathan Cameron <Jonathan.Cameron@huawei.com>]

On Tue, 13 May 2025 19:48:11 +0200
Lorenzo Pieralisi <lpieralisi@kernel.org> wrote:

> From: Marc Zyngier <maz@kernel.org>
> 
> The arm64 arch has relied so far on GIC architectural software
> generated interrupt (SGIs) to handle IPIs. Those are per-cpu
> software generated interrupts.
> 
> arm64 architecture code that allocates the IPIs virtual IRQs and
> IRQ descriptors was written accordingly.
> 
> On GICv5 systems, IPIs are implemented using LPIs that are not
> per-cpu interrupts - they are just normal routable IRQs.
> 
> Add arch code to set-up IPIs on systems where they are handled
> using normal routable IRQs.
> 
> For those systems, force the IRQ affinity (and make it immutable)
> to the cpu a given IRQ was assigned to.
> 
> Signed-off-by: Marc Zyngier <maz@kernel.org>
> [timothy.hayes@arm.com: fixed ipi/irq conversion, irq flags]
> Signed-off-by: Timothy Hayes <timothy.hayes@arm.com>
> [lpieralisi: changed affinity set-up, log]
> Signed-off-by: Lorenzo Pieralisi <lpieralisi@kernel.org>
> Cc: Will Deacon <will@kernel.org>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
Hi Lorenzo,

A few trivial comments inline.

> +
> +static int ipi_to_irq(int ipi, int cpu)

Maybe this naming needs a breadcrumb to indicate this only
applies only to lpi case as it's directly computed in the old ppi code?
A comment might do the job.

> +{
> +	return ipi_irq_base + (cpu * nr_ipi) + ipi;
> +}
> +
> +static int irq_to_ipi(int irq)
> +{
> +	return (irq - ipi_irq_base) % nr_ipi;
> +}


> +static void ipi_setup_lpi(int ipi, int ncpus)
> +{
> +	for (int cpu = 0; cpu < ncpus; cpu++) {
> +		int err, irq;
> +
> +		irq = ipi_to_irq(ipi, cpu);
> +
> +		err = irq_force_affinity(irq, cpumask_of(cpu));
> +
Trivial local consistency thing but maybe no blank line here or...
> +		WARN(err, "Could not force affinity IRQ %d, err=%d\n", irq, err);
> +
> +		err = request_irq(irq, ipi_handler, IRQF_NO_AUTOEN, "IPI",
> +				  &irq_stat);
> +
here to match the style in ipi_setup_ppi()

> +		WARN(err, "Could not request IRQ %d, err=%d\n", irq, err);
> +
> +		irq_set_status_flags(irq, (IRQ_HIDDEN | IRQ_NO_BALANCING_MASK));
> +
> +		get_ipi_desc(cpu, ipi) = irq_to_desc(irq);
> +	}
> +}