linux-kernel - Re: [PATCH v9 12/26] arm64: irqflags: Use ICC_PMR

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Date:   Tue, 22 Jan 2019 15:21:49 +0000
From:   Catalin Marinas <catalin.marinas@....com>
To:     Julien Thierry <julien.thierry@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        daniel.thompson@...aro.org, joel@...lfernandes.org,
        marc.zyngier@....com, christoffer.dall@....com,
        james.morse@....com, will.deacon@....com, mark.rutland@....com,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH v9 12/26] arm64: irqflags: Use ICC_PMR_EL1 for interrupt
 masking

On Mon, Jan 21, 2019 at 03:33:31PM +0000, Julien Thierry wrote:
> diff --git a/arch/arm64/include/asm/irqflags.h b/arch/arm64/include/asm/irqflags.h
> index 24692ed..7e82a92 100644
> --- a/arch/arm64/include/asm/irqflags.h
> +++ b/arch/arm64/include/asm/irqflags.h
> @@ -18,7 +18,9 @@
>  
>  #ifdef __KERNEL__
>  
> +#include <asm/alternative.h>
>  #include <asm/ptrace.h>
> +#include <asm/sysreg.h>
>  
>  /*
>   * Aarch64 has flags for masking: Debug, Asynchronous (serror), Interrupts and
> @@ -36,33 +38,31 @@
>  /*
>   * CPU interrupt mask handling.
>   */
> -static inline unsigned long arch_local_irq_save(void)
> -{
> -	unsigned long flags;
> -	asm volatile(
> -		"mrs	%0, daif		// arch_local_irq_save\n"
> -		"msr	daifset, #2"
> -		: "=r" (flags)
> -		:
> -		: "memory");
> -	return flags;
> -}
> -
>  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",
> +		ARM64_HAS_IRQ_PRIO_MASKING)
>  		:
> +		: "r" (unmasked)
>  		: "memory");
>  }
>  
>  static inline void arch_local_irq_disable(void)
>  {
> -	asm volatile(
> -		"msr	daifset, #2		// arch_local_irq_disable"
> -		:
> +	unsigned long masked = GIC_PRIO_IRQOFF;
> +
> +	asm volatile(ALTERNATIVE(
> +		"msr	daifset, #2		// arch_local_irq_disable",
> +		"msr_s  " __stringify(SYS_ICC_PMR_EL1) ", %0",
> +		ARM64_HAS_IRQ_PRIO_MASKING)
>  		:
> +		: "r" (masked)
>  		: "memory");
>  }

Nitpicks: you could drop masked/unmasked variables here (it's up to you,
it wouldn't make any difference on the generated asm).

> @@ -71,12 +71,44 @@ static inline void arch_local_irq_disable(void)
>   */
>  static inline unsigned long arch_local_save_flags(void)
>  {
> +	unsigned long daif_bits;
>  	unsigned long flags;
> -	asm volatile(
> -		"mrs	%0, daif		// arch_local_save_flags"
> -		: "=r" (flags)
> -		:
> +
> +	daif_bits = read_sysreg(daif);
> +
> +	/*
> +	 * The asm is logically equivalent to:
> +	 *
> +	 * if (system_uses_irq_prio_masking())
> +	 *	flags = (daif_bits & PSR_I_BIT) ?
> +	 *			GIC_PRIO_IRQOFF :
> +	 *			read_sysreg_s(SYS_ICC_PMR_EL1);
> +	 * else
> +	 *	flags = daif_bits;
> +	 */
> +	asm volatile(ALTERNATIVE(
> +			"mov	%0, %1\n"
> +			"nop\n"
> +			"nop",
> +			"mrs_s	%0, " __stringify(SYS_ICC_PMR_EL1) "\n"
> +			"ands	%1, %1, " __stringify(PSR_I_BIT) "\n"
> +			"csel	%0, %0, %2, eq",
> +			ARM64_HAS_IRQ_PRIO_MASKING)
> +		: "=&r" (flags), "+r" (daif_bits)
> +		: "r" (GIC_PRIO_IRQOFF)
>  		: "memory");
> +
> +	return flags;
> +}

BTW, how's the code generated from the C version? It will have a branch
but may not be too bad. Either way is fine by me.

Reviewed-by: Catalin Marinas <catalin.marinas@....com>