Re: [RFC PATCH v2 39/45] arm64: smp: Fall back to IRQ when IPI NMI request fails

[Jinjie Ruan <ruanjinjie@huawei.com>]

在 2026/7/28 0:34, Vladimir Murzin 写道:
> Existing IPI setup logic relies on ipi_should_be_nmi() to decide
> whether to request an NMI (if supported) or an ordinary IRQ. If NMI is
> not supported, using an ordinary IRQ is acceptable.
> 
> FEAT_NMI requires complementary support in the interrupt
> controller. If FEAT_NMI is present but interrupt controller support is
> absent, request_percpu_nmi() can fail.
> 
> One way to prevent this would be to extend ipi_should_be_nmi() to also
> check for interrupt controller support. However, since falling back to
> an ordinary IRQ is acceptable, treat ipi_should_be_nmi() as a hint
> instead. Whether an IPI is actually registered as an NMI is determined
> by the success or failure of request_percpu_nmi(). If the request
> fails, fall back to request_irq().
> 
> Track which IPIs were actually registered as NMIs and use that state
> in the per-CPU enable and disable paths. This avoids calling the
> percpu NMI helpers for an IPI that fell back to regular IRQ handling.
> 
> Signed-off-by: Vladimir Murzin <vladimir.murzin@arm.com>


Should we replace ipi_should_be_nmi() with test_bit(ipi,
get_ipi_nmi_bitmap(cpu) in smp_send_stop()?

irq_desc_get_irq(get_ipi_desc(cpu, ipi)) has been used more than 3
times, can we define a new macro for use?

git grep "irq_desc_get_irq(get_ipi_desc(cpu, ipi))"
arch/arm64/kernel/smp.c:
enable_nmi(irq_desc_get_irq(get_ipi_desc(cpu, ipi)));
arch/arm64/kernel/smp.c:
enable_irq(irq_desc_get_irq(get_ipi_desc(cpu, ipi)));
arch/arm64/kernel/smp.c:
disable_nmi_nosync(irq_desc_get_irq(get_ipi_desc(cpu, ipi)));
arch/arm64/kernel/smp.c:
disable_irq(irq_desc_get_irq(get_ipi_desc(cpu, ipi)));


> ---
>  arch/arm64/kernel/smp.c | 31 +++++++++++++++++++++++--------
>  1 file changed, 23 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
> index 3dd4bc02caed..9bacdbb2aa44 100644
> --- a/arch/arm64/kernel/smp.c
> +++ b/arch/arm64/kernel/smp.c
> @@ -69,11 +69,13 @@ static int nr_ipi __ro_after_init = NR_IPI;
>  
>  struct ipi_descs {
>  	struct irq_desc *descs[MAX_IPI];
> +	DECLARE_BITMAP(nmi_bitmap, MAX_IPI);
>  };
>  
>  static DEFINE_PER_CPU_READ_MOSTLY(struct ipi_descs, pcpu_ipi_desc);
>  
>  #define get_ipi_desc(__cpu, __ipi) (per_cpu_ptr(&pcpu_ipi_desc, __cpu)->descs[__ipi])
> +#define get_ipi_nmi_bitmap(__cpu) (per_cpu_ptr(&pcpu_ipi_desc, __cpu)->nmi_bitmap)
>  
>  struct ipi_irq_ops {
>  	void (*setup)(int ipi, int ncpus);
> @@ -1051,31 +1053,44 @@ static bool ipi_should_be_nmi(enum ipi_msg_type ipi)
>  
>  static void ipi_sgi_setup(int ipi, int ncpus)
>  {
> +	bool is_nmi = ipi_should_be_nmi(ipi);
>  	int err, irq, cpu;
>  
>  	if (WARN_ON_ONCE(ncpus))
>  		return;
>  
> -
>  	irq = ipi_irq_base + ipi;
>  
> -	if (ipi_should_be_nmi(ipi)) {
> -		err = request_percpu_nmi(irq, ipi_handler, "IPI", NULL, &irq_stat);
> -		WARN(err, "Could not request IRQ %d as NMI, err=%d\n", irq, err);
> -	} else {
> +	/*
> +	 * Try to request NMI if it should be
> +	 */
> +	if (is_nmi && request_percpu_nmi(irq, ipi_handler, "IPI", NULL, &irq_stat)) {
> +		/*
> +		 * Fallback to regular IRQ
> +		 */
> +		is_nmi = false;
> +	}
> +
> +       /*
> +        * Otherwise regular IRQ
> +        */
> +	if (!is_nmi) {
>  		err = request_percpu_irq(irq, ipi_handler, "IPI", &irq_stat);
>  		WARN(err, "Could not request IRQ %d as IRQ, err=%d\n", irq, err);
>  	}
>  
> -	for_each_possible_cpu(cpu)
> +	for_each_possible_cpu(cpu) {
>  		get_ipi_desc(cpu, ipi) = irq_to_desc(irq);
> +		if (is_nmi)
> +			set_bit(ipi, get_ipi_nmi_bitmap(cpu));
> +	}
>  
>  	irq_set_status_flags(irq, IRQ_HIDDEN);
>  }
>  
>  static void ipi_sgi_enable(int cpu, int ipi)
>  {
> -	if (ipi_should_be_nmi(ipi)) {
> +	if (test_bit(ipi, get_ipi_nmi_bitmap(cpu))) {
>  		prepare_percpu_nmi(ipi_irq_base + ipi);
>  		enable_percpu_nmi(ipi_irq_base + ipi, 0);
>  	} else {
> @@ -1085,7 +1100,7 @@ static void ipi_sgi_enable(int cpu, int ipi)
>  
>  static void ipi_sgi_disable(int cpu, int ipi)
>  {
> -	if (ipi_should_be_nmi(ipi)) {
> +	if (test_bit(ipi, get_ipi_nmi_bitmap(cpu))) {
>  		disable_percpu_nmi(ipi_irq_base + ipi);
>  		teardown_percpu_nmi(ipi_irq_base + ipi);
>  	} else {