linux-kernel - Re: [PATCH v8 12/26] arm64: irqflags: Use ICC_PMR

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Message-ID: <20190108153957.GA5840@e103592.cambridge.arm.com>
Date:   Tue, 8 Jan 2019 15:40:13 +0000
From:   Dave Martin <Dave.Martin@....com>
To:     Julien Thierry <julien.thierry@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, mark.rutland@....com,
        daniel.thompson@...aro.org, marc.zyngier@....com,
        catalin.marinas@....com,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        will.deacon@....com, linux-kernel@...r.kernel.org,
        christoffer.dall@....com, james.morse@....com,
        Oleg Nesterov <oleg@...hat.com>, joel@...lfernandes.org
Subject: Re: [PATCH v8 12/26] arm64: irqflags: Use ICC_PMR_EL1 for interrupt
 masking

On Tue, Jan 08, 2019 at 02:07:30PM +0000, Julien Thierry wrote:
> Instead disabling interrupts by setting the PSR.I bit, use a priority
> higher than the one used for interrupts to mask them via PMR.
> 
> When using PMR to disable interrupts, the value of PMR will be used
> instead of PSR.[DAIF] for the irqflags.
> 
> Signed-off-by: Julien Thierry <julien.thierry@....com>
> Suggested-by: Daniel Thompson <daniel.thompson@...aro.org>
> Cc: Catalin Marinas <catalin.marinas@....com>
> Cc: Will Deacon <will.deacon@....com>
> Cc: Ard Biesheuvel <ard.biesheuvel@...aro.org>
> Cc: Oleg Nesterov <oleg@...hat.com>
> ---
>  arch/arm64/include/asm/efi.h      |  11 ++++
>  arch/arm64/include/asm/irqflags.h | 123 +++++++++++++++++++++++++++++---------
>  2 files changed, 106 insertions(+), 28 deletions(-)

[...]

> diff --git a/arch/arm64/include/asm/irqflags.h b/arch/arm64/include/asm/irqflags.h
> index 24692ed..fa3b06f 100644
> --- a/arch/arm64/include/asm/irqflags.h
> +++ b/arch/arm64/include/asm/irqflags.h
> @@ -18,7 +18,9 @@

[...]

>  static inline void arch_local_irq_enable(void)
>  {
> -	asm volatile(
> -		"msr	daifclr, #2		// arch_local_irq_enable"
> -		:
> +	unsigned long unmasked = GIC_PRIO_IRQON;
> +
> +	asm volatile(ALTERNATIVE(
> +		"msr	daifclr, #2		// arch_local_irq_enable\n"
> +		"nop",
> +		"msr_s  " __stringify(SYS_ICC_PMR_EL1) ",%0\n"
> +		"dsb	sy",

I'm still not convinced these dsbs are needed.

Without the dsb, we are probably not guaranteed to take a pending
interrupt _immediately_ on unmasking, but I'm not sure that's a
problem.

What goes wrong if we omit them?

(My attempts to answer these questions using the GIC architecture spec
have met with limited success so far...)

[...]

Cheers
---Dave