linux-kernel - Re: [PATCH v6 15/24] arm64: Switch to PMR masking when starting CPUs

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Date:   Tue, 4 Dec 2018 17:51:42 +0000
From:   Catalin Marinas <catalin.marinas@....com>
To:     Julien Thierry <julien.thierry@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, daniel.thompson@...aro.org,
        marc.zyngier@....com, will.deacon@....com,
        linux-kernel@...r.kernel.org, christoffer.dall@....com,
        james.morse@....com, joel@...lfernandes.org
Subject: Re: [PATCH v6 15/24] arm64: Switch to PMR masking when starting CPUs

On Mon, Nov 12, 2018 at 11:57:06AM +0000, Julien Thierry wrote:
> diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
> index 8dc9dde..e495360 100644
> --- a/arch/arm64/kernel/smp.c
> +++ b/arch/arm64/kernel/smp.c
> @@ -35,6 +35,7 @@
>  #include <linux/smp.h>
>  #include <linux/seq_file.h>
>  #include <linux/irq.h>
> +#include <linux/irqchip/arm-gic-v3.h>
>  #include <linux/percpu.h>
>  #include <linux/clockchips.h>
>  #include <linux/completion.h>
> @@ -175,6 +176,25 @@ int __cpu_up(unsigned int cpu, struct task_struct *idle)
>  	return ret;
>  }
>  
> +static void init_gic_priority_masking(void)
> +{
> +	u32 gic_sre = gic_read_sre();
> +	u32 cpuflags;
> +
> +	if (WARN_ON(!(gic_sre & ICC_SRE_EL1_SRE)))
> +		return;
> +
> +	WARN_ON(!irqs_disabled());
> +
> +	gic_write_pmr(GIC_PRIO_IRQOFF);
> +
> +	cpuflags = read_sysreg(daif);
> +
> +	/* We can only unmask PSR.I if we can take aborts */
> +	if (!(cpuflags & PSR_A_BIT))
> +		write_sysreg(cpuflags & ~PSR_I_BIT, daif);

I don't understand this. If you don't switch off PSR_I_BIT here, where
does it happen? In which scenario do we actually have the A bit still
set? At a quick look, smp_prepare_boot_cpu() would have the A bit
cleared previously by setup_arch(). We have secondary_start_kernel()
where you call init_gic_priority_masking() before local_daif_restore().

So what happens if you always turn off PSR_I_BIT here?

-- 
Catalin