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

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Date:   Fri, 18 Jan 2019 17:33:09 +0000
From:   Catalin Marinas <catalin.marinas@....com>
To:     Julien Thierry <julien.thierry@....com>
Cc:     mark.rutland@....com, daniel.thompson@...aro.org,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        marc.zyngier@....com, will.deacon@....com,
        linux-kernel@...r.kernel.org, christoffer.dall@....com,
        james.morse@....com, Oleg Nesterov <oleg@...hat.com>,
        joel@...lfernandes.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v8 12/26] arm64: irqflags: Use ICC_PMR_EL1 for interrupt
 masking

On Fri, Jan 18, 2019 at 05:30:02PM +0000, Catalin Marinas wrote:
> 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");

Ah, I missed a read from SYS_ICC_PMR_EL1 here. Anyway, the idea was that
you don't need to set prio_off to a variable, just pass "r" (constant)
here and the compiler does the trick.

-- 
Catalin