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SubjectRe: [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

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