On 18/01/2019 17:33, Catalin Marinas wrote:

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

quoted

On Fri, Jan 18, 2019 at 04:57:32PM +0000, Julien Thierry wrote:

quoted

On 18/01/2019 16:09, Catalin Marinas wrote:

quoted

On Tue, Jan 08, 2019 at 02:07:30PM +0000, Julien Thierry wrote:

quoted

+	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.

quoted

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.

I see, thanks. I'll avoid that superfluous variable.

Thanks,

-- 
Julien Thierry

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