linux-kernel - Re: [PATCH v6 08/24] arm64: Unmask PMR before going idle

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Message-ID: <20181129174436.avqjydyzvv6ubnjd@lakrids.cambridge.arm.com>
Date:   Thu, 29 Nov 2018 17:44:37 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Julien Thierry <julien.thierry@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        daniel.thompson@...aro.org, joel@...lfernandes.org,
        marc.zyngier@....com, christoffer.dall@....com,
        james.morse@....com, catalin.marinas@....com, will.deacon@....com
Subject: Re: [PATCH v6 08/24] arm64: Unmask PMR before going idle

On Mon, Nov 12, 2018 at 11:56:59AM +0000, Julien Thierry wrote:
> CPU does not received signals for interrupts with a priority masked by
> ICC_PMR_EL1. This means the CPU might not come back from a WFI
> instruction.
> 
> Make sure ICC_PMR_EL1 does not mask interrupts when doing a WFI.
> 
> Signed-off-by: Julien Thierry <julien.thierry@....com>
> Suggested-by: Daniel Thompson <daniel.thompson@...aro.org>
> Cc: Catalin Marinas <catalin.marinas@....com>
> Cc: Will Deacon <will.deacon@....com>
> ---
>  arch/arm64/mm/proc.S | 18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)
> 
> diff --git a/arch/arm64/mm/proc.S b/arch/arm64/mm/proc.S
> index 2c75b0b..3c7064c 100644
> --- a/arch/arm64/mm/proc.S
> +++ b/arch/arm64/mm/proc.S
> @@ -20,6 +20,7 @@
>  
>  #include <linux/init.h>
>  #include <linux/linkage.h>
> +#include <linux/irqchip/arm-gic-v3.h>
>  #include <asm/assembler.h>
>  #include <asm/asm-offsets.h>
>  #include <asm/hwcap.h>
> @@ -53,10 +54,27 @@
>   *	cpu_do_idle()
>   *
>   *	Idle the processor (wait for interrupt).
> + *
> + *	If the CPU supports priority masking we must do additional work to
> + *	ensure that interrupts are not masked at the PMR (because the core will
> + *	not wake up if we block the wake up signal in the interrupt controller).
>   */
>  ENTRY(cpu_do_idle)
> +alternative_if_not ARM64_HAS_IRQ_PRIO_MASKING
> +	dsb	sy				// WFI may enter a low-power mode
> +	wfi
> +	ret
> +alternative_else
> +	mrs	x0, daif			// save I bit
> +	msr	daifset, #2			// set I bit
> +	mrs_s	x1, SYS_ICC_PMR_EL1		// save PMR
> +alternative_endif
> +	mov	x2, #GIC_PRIO_IRQON
> +	msr_s	SYS_ICC_PMR_EL1, x2		// unmask PMR
>  	dsb	sy				// WFI may enter a low-power mode

Is the DSB SY sufficient and necessary to synchronise the update of
SYS_ICC_PMR_EL1? We don't need an ISB too?

>  	wfi
> +	msr_s	SYS_ICC_PMR_EL1, x1		// restore PMR

Likewise, we don't need any barriers here before we poke DAIF?

> +	msr	daif, x0			// restore I bit
>  	ret
>  ENDPROC(cpu_do_idle)

If we build without CONFIG_ARM64_PSEUDO_NMI surely we don't want to emit
the alternative?

How about we move this to C, and have something like the below?

For the !CONFIG_ARM64_PSEUDO_NMI case it generates identical assembly to the
existing cpu_do_idle(). Note that I've assumed we don't need barriers, which
(as above) I'm not certain of.

Thanks,
Mark.

---->8----
diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
index 7f1628effe6d..ccd2ad8c5e2f 100644
--- a/arch/arm64/kernel/process.c
+++ b/arch/arm64/kernel/process.c
@@ -73,6 +73,40 @@ EXPORT_SYMBOL_GPL(pm_power_off);
 
 void (*arm_pm_restart)(enum reboot_mode reboot_mode, const char *cmd);
 
+static inline void __cpu_do_idle(void)
+{
+       /* WFI may enter a low-power mode */
+       dsb(sy);
+       wfi();
+}
+
+/*
+ * When using priority masking we need to take extra care, etc.
+ */
+static inline void __cpu_do_idle_irqprio(void)
+{
+       unsigned long flags = arch_local_irq_save();
+       unsigned long pmr = gic_read_pmr();
+
+       gic_write_pmr(GIC_PRIO_IRQON);
+
+       __cpu_do_idle();
+
+       gic_write_pmr(pmr);
+       arch_local_irq_enable();
+}
+
+/*
+ * Idle the processor (wait for interrupt).
+ */
+void cpu_do_idle(void)
+{
+       if (system_uses_irq_prio_masking())
+               __cpu_do_idle_irqprio();
+       else
+               __cpu_do_idle();
+}
+
 /*
  * This is our default idle handler.
  */
diff --git a/arch/arm64/mm/proc.S b/arch/arm64/mm/proc.S
index 03646e6a2ef4..38c0171e52e2 100644
--- a/arch/arm64/mm/proc.S
+++ b/arch/arm64/mm/proc.S
@@ -49,17 +49,6 @@
 
 #define MAIR(attr, mt) ((attr) << ((mt) * 8))
 
-/*
- *     cpu_do_idle()
- *
- *     Idle the processor (wait for interrupt).
- */
-ENTRY(cpu_do_idle)
-       dsb     sy                              // WFI may enter a low-power mode
-       wfi
-       ret
-ENDPROC(cpu_do_idle)
-
 #ifdef CONFIG_CPU_PM
 /**
  * cpu_do_suspend - save CPU registers context