linux-kernel - Re: [PATCH v3 1/4] arm64: smp: Introduce a new IPI as IPI_CALL_NMI

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Message-ID: <05a195374cc81008e95e258221fe7d2b@kernel.org>
Date:   Thu, 03 Sep 2020 17:36:49 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Sumit Garg <sumit.garg@...aro.org>
Cc:     catalin.marinas@....com, will@...nel.org,
        linux-arm-kernel@...ts.infradead.org, tglx@...utronix.de,
        jason@...edaemon.net, julien.thierry.kdev@...il.com,
        dianders@...omium.org, daniel.thompson@...aro.org,
        jason.wessel@...driver.com, kgdb-bugreport@...ts.sourceforge.net,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/4] arm64: smp: Introduce a new IPI as
 IPI_CALL_NMI_FUNC

On 2020-09-03 13:05, Sumit Garg wrote:
> Introduce a new inter processor interrupt as IPI_CALL_NMI_FUNC that
> can be invoked to run special handlers in NMI context. One such handler
> example is kgdb_nmicallback() which is invoked in order to round up 
> CPUs
> to enter kgdb context.
> 
> As currently pseudo NMIs are supported on specific arm64 platforms 
> which
> incorporates GICv3 or later version of interrupt controller. In case a
> particular platform doesn't support pseudo NMIs, IPI_CALL_NMI_FUNC will
> act as a normal IPI which can still be used to invoke special handlers.
> 
> Signed-off-by: Sumit Garg <sumit.garg@...aro.org>
> ---
>  arch/arm64/include/asm/smp.h |  1 +
>  arch/arm64/kernel/smp.c      | 11 +++++++++++
>  2 files changed, 12 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/smp.h 
> b/arch/arm64/include/asm/smp.h
> index 2e7f529..e85f5d5 100644
> --- a/arch/arm64/include/asm/smp.h
> +++ b/arch/arm64/include/asm/smp.h
> @@ -89,6 +89,7 @@ extern void secondary_entry(void);
> 
>  extern void arch_send_call_function_single_ipi(int cpu);
>  extern void arch_send_call_function_ipi_mask(const struct cpumask 
> *mask);
> +extern void arch_send_call_nmi_func_ipi_mask(const struct cpumask 
> *mask);
> 
>  #ifdef CONFIG_ARM64_ACPI_PARKING_PROTOCOL
>  extern void arch_send_wakeup_ipi_mask(const struct cpumask *mask);
> diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
> index b6bde26..1b4c07c 100644
> --- a/arch/arm64/kernel/smp.c
> +++ b/arch/arm64/kernel/smp.c
> @@ -74,6 +74,7 @@ enum ipi_msg_type {
>  	IPI_TIMER,
>  	IPI_IRQ_WORK,
>  	IPI_WAKEUP,
> +	IPI_CALL_NMI_FUNC,
>  	NR_IPI
>  };
> 
> @@ -793,6 +794,7 @@ static const char *ipi_types[NR_IPI] 
> __tracepoint_string = {
>  	S(IPI_TIMER, "Timer broadcast interrupts"),
>  	S(IPI_IRQ_WORK, "IRQ work interrupts"),
>  	S(IPI_WAKEUP, "CPU wake-up interrupts"),
> +	S(IPI_CALL_NMI_FUNC, "NMI function call interrupts"),
>  };
> 
>  static void smp_cross_call(const struct cpumask *target, unsigned int 
> ipinr);
> @@ -840,6 +842,11 @@ void arch_irq_work_raise(void)
>  }
>  #endif
> 
> +void arch_send_call_nmi_func_ipi_mask(const struct cpumask *mask)
> +{
> +	smp_cross_call(mask, IPI_CALL_NMI_FUNC);
> +}
> +
>  static void local_cpu_stop(void)
>  {
>  	set_cpu_online(smp_processor_id(), false);
> @@ -932,6 +939,10 @@ static void do_handle_IPI(int ipinr)
>  		break;
>  #endif
> 
> +	case IPI_CALL_NMI_FUNC:
> +		/* nop, IPI handlers for special features can be added here. */
> +		break;
> +
>  	default:
>  		pr_crit("CPU%u: Unknown IPI message 0x%x\n", cpu, ipinr);
>  		break;

I'm really not keen on adding more IPIs to the SMP code. One of the
main reasons for using these SGIs as normal IRQs was to make them
"requestable" from non-arch code as if they were standard percpu
interrupts.

What prevents you from moving that all the way to the kgdb code?

         M.
-- 
Jazz is not dead. It just smells funny...