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

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Date:   Fri, 18 Jan 2019 17:30:02 +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

On Fri, Jan 18, 2019 at 04:57:32PM +0000, Julien Thierry wrote:
> On 18/01/2019 16:09, Catalin Marinas wrote:
> > On Tue, Jan 08, 2019 at 02:07:30PM +0000, Julien Thierry wrote:
> >> +	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);
> > 
> 
> You're right, returning GIC_PRIO_IRQOFF should be good enough (it is
> actually what happens in this version because GIC_PRIO_IRQOFF ==
> GIC_PRIO_IRQON & ~PSR_I_BIT happens to be true).

This wasn't entirely clear to me, I got confused by:

+       BUILD_BUG_ON(GIC_PRIO_IRQOFF < (GIC_PRIO_IRQON & ~PSR_I_BIT));  \

and I thought there isn't necessarily an equality between the two.

> Your suggestion would
> make things easier to reason about. Maybe something like:
> 
> 
> static inline unsigned long arch_local_save_flags(void)
> {
> 	unsigned long daif_bits;
> 	unsigned long prio_off = GIC_PRIO_IRQOFF;
> 
> 	daif_bits = read_sysreg(daif);
> 
> 	asm volatile(ALTERNATIVE(
> 		"mov	%0, %1\n"
> 		"nop\n"
> 		"nop",
> 		"mrs	%0, SYS_ICC_PMR_EL1\n"
> 		"ands	%1, %1, PSR_I_BIT\n"
> 		"csel	%0, %0, %2, eq")
> 	: "=&r" (flags)
> 	: "r" (daif_bits), "r" (prio_off)
> 	: "memory");
> 
> 	return flags;
> }

It looks fine. If you turn the BUILD_BUG_ON into a !=, you could
probably simplify the asm a bit (though the number of instructions
generated would probably be the same). Untested:

static inline unsigned long arch_local_save_flags(void)
{
	unsigned long flags;

	flags = read_sysreg(daif);

	asm volatile(ALTERNATIVE(
		"nop",
		"bic	%0, %1, %2")
	: "=&r" (flags)
	: "r" (flags & PSR_I_BIT), "r" (GIC_PRIO_IRQOFF)
	: "memory");

	return flags;
}

-- 
Catalin