Subject: Re: [PATCH v10 12/25] arm64: irqflags: Use ICC_PMR_EL1 for interrupt masking

From: Nathan Chancellor <[email-masked]>
Date: Thu, 7 Feb 2019 21:35:43 -0700

On Thu, Jan 31, 2019 at 02:58:50PM +0000, Julien Thierry wrote:
> Instead disabling interrupts by setting the PSR.I bit, use a priority
> higher than the one used for interrupts to mask them via PMR.
> 
> When using PMR to disable interrupts, the value of PMR will be used
> instead of PSR.[DAIF] for the irqflags.
> 
> Signed-off-by: Julien Thierry <[email-masked]>
> Suggested-by: Daniel Thompson <[email-masked]>
> Acked-by: Ard Biesheuvel <[email-masked]>
> Reviewed-by: Catalin Marinas <[email-masked]>
> Cc: Catalin Marinas <[email-masked]>
> Cc: Will Deacon <[email-masked]>
> Cc: Ard Biesheuvel <[email-masked]>
> Cc: Oleg Nesterov <[email-masked]>
> ---
>  arch/arm64/include/asm/efi.h      |  11 +++++
>  arch/arm64/include/asm/irqflags.h | 100 +++++++++++++++++++++++++++-----------
>  2 files changed, 83 insertions(+), 28 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/efi.h b/arch/arm64/include/asm/efi.h
> index 7ed3208..c9e9a69 100644
> --- a/arch/arm64/include/asm/efi.h
> +++ b/arch/arm64/include/asm/efi.h
> @@ -44,6 +44,17 @@
>  
>  #define ARCH_EFI_IRQ_FLAGS_MASK (PSR_D_BIT | PSR_A_BIT | PSR_I_BIT | PSR_F_BIT)
>  
> +/*
> + * Even when Linux uses IRQ priorities for IRQ disabling, EFI does not.
> + * And EFI shouldn't really play around with priority masking as it is not aware
> + * which priorities the OS has assigned to its interrupts.
> + */
> +#define arch_efi_save_flags(state_flags)		\
> +	((void)((state_flags) = read_sysreg(daif)))
> +
> +#define arch_efi_restore_flags(state_flags)	write_sysreg(state_flags, daif)
> +
> +
>  /* arch specific definitions used by the stub code */
>  
>  /*
> diff --git a/arch/arm64/include/asm/irqflags.h b/arch/arm64/include/asm/irqflags.h
> index 24692ed..d4597b2 100644
> --- a/arch/arm64/include/asm/irqflags.h
> +++ b/arch/arm64/include/asm/irqflags.h
> @@ -18,7 +18,9 @@
>  
>  #ifdef __KERNEL__
>  
> +#include <asm/alternative.h>
>  #include <asm/ptrace.h>
> +#include <asm/sysreg.h>
>  
>  /*
>   * Aarch64 has flags for masking: Debug, Asynchronous (serror), Interrupts and
> @@ -36,33 +38,27 @@
>  /*
>   * CPU interrupt mask handling.
>   */
> -static inline unsigned long arch_local_irq_save(void)
> -{
> -	unsigned long flags;
> -	asm volatile(
> -		"mrs	%0, daif		// arch_local_irq_save\n"
> -		"msr	daifset, #2"
> -		: "=r" (flags)
> -		:
> -		: "memory");
> -	return flags;
> -}
> -
>  static inline void arch_local_irq_enable(void)
>  {
> -	asm volatile(
> -		"msr	daifclr, #2		// arch_local_irq_enable"
> -		:
> +	asm volatile(ALTERNATIVE(
> +		"msr	daifclr, #2		// arch_local_irq_enable\n"
> +		"nop",
> +		"msr_s  " __stringify(SYS_ICC_PMR_EL1) ",%0\n"
> +		"dsb	sy",
> +		ARM64_HAS_IRQ_PRIO_MASKING)
>  		:
> +		: "r" (GIC_PRIO_IRQON)
>  		: "memory");
>  }
>  
>  static inline void arch_local_irq_disable(void)
>  {
> -	asm volatile(
> -		"msr	daifset, #2		// arch_local_irq_disable"
> -		:
> +	asm volatile(ALTERNATIVE(
> +		"msr	daifset, #2		// arch_local_irq_disable",
> +		"msr_s  " __stringify(SYS_ICC_PMR_EL1) ", %0",
> +		ARM64_HAS_IRQ_PRIO_MASKING)
>  		:
> +		: "r" (GIC_PRIO_IRQOFF)
>  		: "memory");
>  }
>  
> @@ -71,12 +67,44 @@ static inline void arch_local_irq_disable(void)
>   */
>  static inline unsigned long arch_local_save_flags(void)
>  {
> +	unsigned long daif_bits;
>  	unsigned long flags;
> -	asm volatile(
> -		"mrs	%0, daif		// arch_local_save_flags"
> -		: "=r" (flags)
> -		:
> +
> +	daif_bits = read_sysreg(daif);
> +
> +	/*
> +	 * The asm is logically equivalent to:
> +	 *
> +	 * if (system_uses_irq_prio_masking())
> +	 *	flags = (daif_bits & PSR_I_BIT) ?
> +	 *			GIC_PRIO_IRQOFF :
> +	 *			read_sysreg_s(SYS_ICC_PMR_EL1);
> +	 * else
> +	 *	flags = daif_bits;
> +	 */
> +	asm volatile(ALTERNATIVE(
> +			"mov	%0, %1\n"
> +			"nop\n"
> +			"nop",
> +			"mrs_s	%0, " __stringify(SYS_ICC_PMR_EL1) "\n"
> +			"ands	%1, %1, " __stringify(PSR_I_BIT) "\n"
> +			"csel	%0, %0, %2, eq",
> +			ARM64_HAS_IRQ_PRIO_MASKING)
> +		: "=&r" (flags), "+r" (daif_bits)
> +		: "r" (GIC_PRIO_IRQOFF)
>  		: "memory");
> +
> +	return flags;
> +}
> +
> +static inline unsigned long arch_local_irq_save(void)
> +{
> +	unsigned long flags;
> +
> +	flags = arch_local_save_flags();
> +
> +	arch_local_irq_disable();
> +
>  	return flags;
>  }
>  
> @@ -85,16 +113,32 @@ static inline unsigned long arch_local_save_flags(void)
>   */
>  static inline void arch_local_irq_restore(unsigned long flags)
>  {
> -	asm volatile(
> -		"msr	daif, %0		// arch_local_irq_restore"
> -	:
> -	: "r" (flags)
> -	: "memory");
> +	asm volatile(ALTERNATIVE(
> +			"msr	daif, %0\n"
> +			"nop",
> +			"msr_s	" __stringify(SYS_ICC_PMR_EL1) ", %0\n"
> +			"dsb	sy",
> +			ARM64_HAS_IRQ_PRIO_MASKING)
> +		: "+r" (flags)
> +		:
> +		: "memory");
>  }
>  
>  static inline int arch_irqs_disabled_flags(unsigned long flags)
>  {
> -	return flags & PSR_I_BIT;
> +	int res;
> +
> +	asm volatile(ALTERNATIVE(
> +			"and	%w0, %w1, #" __stringify(PSR_I_BIT) "\n"
> +			"nop",
> +			"cmp	%w1, #" __stringify(GIC_PRIO_IRQOFF) "\n"
> +			"cset	%w0, ls",
> +			ARM64_HAS_IRQ_PRIO_MASKING)
> +		: "=&r" (res)
> +		: "r" ((int) flags)
> +		: "memory");
> +
> +	return res;
>  }
>  #endif
>  #endif
> -- 
> 1.9.1
> 

Hi Julien,

This patch introduced a slew of Clang warnings:

In file included from arch/arm64/kernel/signal.c:21:
In file included from include/linux/compat.h:10:
In file included from include/linux/time.h:6:
In file included from include/linux/seqlock.h:36:
In file included from include/linux/spinlock.h:54:
In file included from include/linux/irqflags.h:16:
arch/arm64/include/asm/irqflags.h:50:10: warning: value size does not match register size specified by the constraint and modifier [-Wasm-operand-widths]
                : "r" (GIC_PRIO_IRQON)
                       ^
arch/arm64/include/asm/ptrace.h:39:25: note: expanded from macro 'GIC_PRIO_IRQON'
#define GIC_PRIO_IRQON          0xf0
                                ^
arch/arm64/include/asm/irqflags.h:46:44: note: use constraint modifier "w"
                "msr_s  " __stringify(SYS_ICC_PMR_EL1) ",%0\n"
                                                         ^~
                                                         %w0
arch/arm64/include/asm/alternative.h:286:29: note: expanded from macro 'ALTERNATIVE'
        _ALTERNATIVE_CFG(oldinstr, newinstr, __VA_ARGS__, 1)
                                   ^
arch/arm64/include/asm/alternative.h:88:30: note: expanded from macro '_ALTERNATIVE_CFG'
        __ALTERNATIVE_CFG(oldinstr, newinstr, feature, IS_ENABLED(cfg), 0)
                                    ^
arch/arm64/include/asm/alternative.h:76:2: note: expanded from macro '__ALTERNATIVE_CFG'
        newinstr "\n"                                                   \
        ^
In file included from arch/arm64/kernel/signal.c:21:
In file included from include/linux/compat.h:10:
In file included from include/linux/time.h:6:
In file included from include/linux/seqlock.h:36:
In file included from include/linux/spinlock.h:54:
In file included from include/linux/irqflags.h:16:
arch/arm64/include/asm/irqflags.h:61:10: warning: value size does not match register size specified by the constraint and modifier [-Wasm-operand-widths]
                : "r" (GIC_PRIO_IRQOFF)
                       ^
arch/arm64/include/asm/ptrace.h:40:26: note: expanded from macro 'GIC_PRIO_IRQOFF'
#define GIC_PRIO_IRQOFF         (GIC_PRIO_IRQON & ~0x80)
                                ^
arch/arm64/include/asm/irqflags.h:58:45: note: use constraint modifier "w"
                "msr_s  " __stringify(SYS_ICC_PMR_EL1) ", %0",
                                                          ^
arch/arm64/include/asm/irqflags.h:94:10: warning: value size does not match register size specified by the constraint and modifier [-Wasm-operand-widths]
                : "r" (GIC_PRIO_IRQOFF)
                       ^
arch/arm64/include/asm/ptrace.h:40:26: note: expanded from macro 'GIC_PRIO_IRQOFF'
#define GIC_PRIO_IRQOFF         (GIC_PRIO_IRQON & ~0x80)
                                ^
arch/arm64/include/asm/irqflags.h:91:18: note: use constraint modifier "w"
                        "csel   %0, %0, %2, eq",
                                        ^~
                                        %w2
arch/arm64/include/asm/alternative.h:286:29: note: expanded from macro 'ALTERNATIVE'
        _ALTERNATIVE_CFG(oldinstr, newinstr, __VA_ARGS__, 1)
                                   ^
arch/arm64/include/asm/alternative.h:88:30: note: expanded from macro '_ALTERNATIVE_CFG'
        __ALTERNATIVE_CFG(oldinstr, newinstr, feature, IS_ENABLED(cfg), 0)
                                    ^
arch/arm64/include/asm/alternative.h:76:2: note: expanded from macro '__ALTERNATIVE_CFG'
        newinstr "\n"                                                   \
        ^
3 warnings generated.

I am not sure if they should be fixed with Clang's suggestion of a
constraint modifier or a cast like commit 1b57ec8c7527 ("arm64: io:
Ensure value passed to __iormb() is held in a 64-bit register"), hence
this message.

Thanks,
Nathan

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