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

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Date:   Fri, 18 Jan 2019 16:09:20 +0000
From:   Catalin Marinas <catalin.marinas@....com>
To:     Julien Thierry <julien.thierry@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, mark.rutland@....com,
        daniel.thompson@...aro.org, marc.zyngier@....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

Hi Julien,

On Tue, Jan 08, 2019 at 02:07:30PM +0000, Julien Thierry wrote:
> + * Having two ways to control interrupt status is a bit complicated. Some
> + * locations like exception entries will have PSR.I bit set by the architecture
> + * while PMR is unmasked.
> + * We need the irqflags to represent that interrupts are disabled in such cases.
> + *
> + * For this, we lower the value read from PMR when the I bit is set so it is
> + * considered as an irq masking priority. (With PMR, lower value means masking
> + * more interrupts).
> + */
> +#define _get_irqflags(daif_bits, pmr)					\
> +({									\
> +	unsigned long flags;						\
> +									\
> +	BUILD_BUG_ON(GIC_PRIO_IRQOFF < (GIC_PRIO_IRQON & ~PSR_I_BIT));	\
> +	asm volatile(ALTERNATIVE(					\
> +		"mov	%0, %1\n"					\
> +		"nop\n"							\
> +		"nop",							\
> +		"and	%0, %1, #" __stringify(PSR_I_BIT) "\n"		\
> +		"mvn	%0, %0\n"					\
> +		"and	%0, %0, %2",					\
> +		ARM64_HAS_IRQ_PRIO_MASKING)				\

Can you write the last two instructions as a single:

		bic	%0, %2, %0

> +		: "=&r" (flags)						\
> +		: "r" (daif_bits), "r" (pmr)				\
> +		: "memory");						\
> +									\
> +	flags;								\
> +})
> +
> +/*
>   * Save the current interrupt enable state.
>   */
>  static inline unsigned long arch_local_save_flags(void)
>  {
> -	unsigned long flags;
> -	asm volatile(
> -		"mrs	%0, daif		// arch_local_save_flags"
> -		: "=r" (flags)
> +	unsigned long daif_bits;
> +	unsigned long pmr; // Only used if alternative is on
> +
> +	daif_bits = read_sysreg(daif);
> +
> +	// Get PMR

Nitpick: don't use C++ (or arm asm) comment style in C code.

> +	asm volatile(ALTERNATIVE(
> +			"nop",
> +			"mrs_s	%0, " __stringify(SYS_ICC_PMR_EL1),
> +			ARM64_HAS_IRQ_PRIO_MASKING)
> +		: "=&r" (pmr)
>  		:
>  		: "memory");
> +
> +	return _get_irqflags(daif_bits, pmr);
> +}

I find this confusing spread over two inline asm statements. IIUC, you
want something like below (it could be written as inline asm but I need
to understand it first):

	daif_bits = read_sysreg(daif);

	if (system_uses_irq_prio_masking()) {
		pmr = read_gicreg(ICC_PMR_EL1);
		flags = pmr & ~(daif_bits & PSR_I_BIT);
	} else {
		flags = daif_bits;
	}

	return flags;

In the case where the interrupts are disabled at the PSR level, is the
PMR value still relevant? Could we just return the GIC_PRIO_IRQOFF?
Something like:

	flags = read_sysreg(daif);

	if (system_uses_irq_prio_masking())
		flags = flags & PSR_I_BIT ?
			GIC_PRIO_IRQOFF : read_gicreg(ICC_PMR_EL1);

-- 
Catalin