Subject: Re: [PATCH v3 1/2] arm64: Relax ICC_PMR_EL1 accesses when ICC_CTLR_EL1.PMHE is clear

From: Marc Zyngier <[email-masked]>
Date: Wed, 23 Oct 2019 13:13:14 +0100

Hi Wei,

On 2019-10-23 09:38, liwei (GF) wrote:
> Hi Marc,
>
> On 2019/10/2 17:06, Marc Zyngier wrote:
>> The GICv3 architecture specification is incredibly misleading when 
>> it
>> comes to PMR and the requirement for a DSB. It turns out that this 
>> DSB
>> is only required if the CPU interface sends an Upstream Control
>> message to the redistributor in order to update the RD's view of 
>> PMR.
>>
>> This message is only sent when ICC_CTLR_EL1.PMHE is set, which isn't
>> the case in Linux. It can still be set from EL3, so some special 
>> care
>> is required. But the upshot is that in the (hopefuly large) majority
>> of the cases, we can drop the DSB altogether.
>>
>> This relies on a new static key being set if the boot CPU has PMHE
>> set. The drawback is that this static key has to be exported to
>> modules.
>>
>> Cc: Catalin Marinas <[email-masked]>
>> Cc: Will Deacon <[email-masked]>
>> Cc: James Morse <[email-masked]>
>> Cc: Julien Thierry <[email-masked]>
>> Cc: Suzuki K Poulose <[email-masked]>
>> Signed-off-by: Marc Zyngier <[email-masked]>
>> ---
>>  arch/arm64/include/asm/barrier.h   | 12 ++++++++++++
>>  arch/arm64/include/asm/daifflags.h |  3 ++-
>>  arch/arm64/include/asm/irqflags.h  | 19 ++++++++++---------
>>  arch/arm64/include/asm/kvm_host.h  |  3 +--
>>  arch/arm64/kernel/entry.S          |  6 ++++--
>>  arch/arm64/kvm/hyp/switch.c        |  4 ++--
>>  drivers/irqchip/irq-gic-v3.c       | 20 ++++++++++++++++++++
>>  include/linux/irqchip/arm-gic-v3.h |  2 ++
>>  8 files changed, 53 insertions(+), 16 deletions(-)
>>
>> diff --git a/arch/arm64/include/asm/barrier.h 
>> b/arch/arm64/include/asm/barrier.h
>> index e0e2b1946f42..7d9cc5ec4971 100644
>> --- a/arch/arm64/include/asm/barrier.h
>> +++ b/arch/arm64/include/asm/barrier.h
>> @@ -29,6 +29,18 @@
>>  						 SB_BARRIER_INSN"nop\n",	\
>>  						 ARM64_HAS_SB))
>>
>> +#ifdef CONFIG_ARM64_PSEUDO_NMI
>> +#define pmr_sync()						\
>> +	do {							\
>> +		extern struct static_key_false gic_pmr_sync;	\
>> +								\
>> +		if (static_branch_unlikely(&gic_pmr_sync))	\
>> +			dsb(sy);				\
>> +	} while(0)
>> +#else
>> +#define pmr_sync()	do {} while (0)
>> +#endif
>> +
>
> Thank you for solving this problem, it helps a lot indeed.
>
> The pmr_sync() will call dsb(sy) when ARM64_PSEUDO_NMI=y and
> gic_pmr_sync=force,
> but if pseudo nmi is not enabled through boot option, it just take 
> one more
> redundant calling than before at the following two place.
>
> I think change dsb(sy) to
> +                       asm volatile(ALTERNATIVE("nop", "dsb sy",     
>  \
> +                               ARM64_HAS_IRQ_PRIO_MASKING)     \
> +                               : : : "memory");                \
> may be more appropriate.

I'm not sure I understand what you mean. The static key defaults to 
false,
so if pseudo_nmi is not enabled, this dsb(sy) is simply never executed.

Am I missing something obvious?

Thanks,

         M.

>
> Thanks,
> Wei
>
>>
>> @@ -34,14 +35,14 @@ static inline void arch_local_irq_enable(void)
>>  	}
>>
>>  	asm volatile(ALTERNATIVE(
>> -		"msr	daifclr, #2		// arch_local_irq_enable\n"
>> -		"nop",
>> -		__msr_s(SYS_ICC_PMR_EL1, "%0")
>> -		"dsb	sy",
>> +		"msr	daifclr, #2		// arch_local_irq_enable",
>> +		__msr_s(SYS_ICC_PMR_EL1, "%0"),
>>  		ARM64_HAS_IRQ_PRIO_MASKING)
>>  		:
>>  		: "r" ((unsigned long) GIC_PRIO_IRQON)
>>  		: "memory");
>> +
>> +	pmr_sync();
>>  }
>>
>>  static inline void arch_local_irq_disable(void)
>> @@ -116,14 +117,14 @@ static inline unsigned long 
>> arch_local_irq_save(void)
>>  static inline void arch_local_irq_restore(unsigned long flags)
>>  {
>>  	asm volatile(ALTERNATIVE(
>> -			"msr	daif, %0\n"
>> -			"nop",
>> -			__msr_s(SYS_ICC_PMR_EL1, "%0")
>> -			"dsb	sy",
>> -			ARM64_HAS_IRQ_PRIO_MASKING)
>> +		"msr	daif, %0",
>> +		__msr_s(SYS_ICC_PMR_EL1, "%0"),
>> +		ARM64_HAS_IRQ_PRIO_MASKING)
>>  		:
>>  		: "r" (flags)
>>  		: "memory");
>> +
>> +	pmr_sync();
>>  }
>>

-- 
Jazz is not dead. It just smells funny...

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