linux-kernel - Re: [PATCH v2 1/6] arm64: cpufeature: Allow early detect of specific features

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Message-ID: <2f2a406b-fddf-6c30-4052-650f57ac317a@arm.com>
Date:   Mon, 22 Jan 2018 12:21:55 +0000
From:   Julien Thierry <julien.thierry@....com>
To:     Suzuki K Poulose <Suzuki.Poulose@....com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Cc:     mark.rutland@....com, marc.zyngier@....com, james.morse@....com,
        daniel.thompson@...aro.org,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>
Subject: Re: [PATCH v2 1/6] arm64: cpufeature: Allow early detect of specific
 features



On 22/01/18 12:05, Suzuki K Poulose wrote:
> On 17/01/18 11:54, Julien Thierry wrote:
>> From: Daniel Thompson <daniel.thompson@...aro.org>
>>
>> Currently it is not possible to detect features of the boot CPU
>> until the other CPUs have been brought up.
>>
>> This prevents us from reacting to features of the boot CPU until
>> fairly late in the boot process. To solve this we allow a subset
>> of features (that are likely to be common to all clusters) to be
>> detected based on the boot CPU alone.
>>
>> Signed-off-by: Daniel Thompson <daniel.thompson@...aro.org>
>> [julien.thierry@....com: check non-boot cpu missing early features, avoid
>>              duplicates between early features and normal
>>              features]
>> Signed-off-by: Julien Thierry <julien.thierry@....com>
>> Cc: Catalin Marinas <catalin.marinas@....com>
>> Cc: Will Deacon <will.deacon@....com>
>> Cc: Suzuki K Poulose <suzuki.poulose@....com>
>> ---
>>   arch/arm64/kernel/cpufeature.c | 69 
>> ++++++++++++++++++++++++++++--------------
>>   1 file changed, 47 insertions(+), 22 deletions(-)
>>
>> diff --git a/arch/arm64/kernel/cpufeature.c 
>> b/arch/arm64/kernel/cpufeature.c
>> index a73a592..6698404 100644
>> --- a/arch/arm64/kernel/cpufeature.c
>> +++ b/arch/arm64/kernel/cpufeature.c
>> @@ -52,6 +52,8 @@
>>   DECLARE_BITMAP(cpu_hwcaps, ARM64_NCAPS);
>>   EXPORT_SYMBOL(cpu_hwcaps);
>>
>> +static void __init setup_early_feature_capabilities(void);
>> +
>>   /*
>>    * Flag to indicate if we have computed the system wide
>>    * capabilities based on the boot time active CPUs. This
>> @@ -542,6 +544,8 @@ void __init init_cpu_features(struct cpuinfo_arm64 
>> *info)
>>           init_cpu_ftr_reg(SYS_ZCR_EL1, info->reg_zcr);
>>           sve_init_vq_map();
>>       }
>> +
>> +    setup_early_feature_capabilities();
>>   }
>>
>>   static void update_cpu_ftr_reg(struct arm64_ftr_reg *reg, u64 new)
>> @@ -846,7 +850,7 @@ static bool has_no_fpsimd(const struct 
>> arm64_cpu_capabilities *entry, int __unus
>>                       ID_AA64PFR0_FP_SHIFT) < 0;
>>   }
>>
>> -static const struct arm64_cpu_capabilities arm64_features[] = {
>> +static const struct arm64_cpu_capabilities arm64_early_features[] = {
>>       {
>>           .desc = "GIC system register CPU interface",
>>           .capability = ARM64_HAS_SYSREG_GIC_CPUIF,
>> @@ -857,6 +861,10 @@ static bool has_no_fpsimd(const struct 
>> arm64_cpu_capabilities *entry, int __unus
>>           .sign = FTR_UNSIGNED,
>>           .min_field_value = 1,
>>       },
>> +    {}
>> +};
>> +
> 
> 
> Julien,
> 
> One potential problem with this is that we don't have a way
> to make this work on a "theoretical" system with and without
> GIC system reg interface. i.e, if we don't have the CONFIG
> enabled for using ICC system regs for IRQ flags, the kernel
> could still panic. I understand this is not a "normal" configuration
> but, may be we could make the panic option based on whether
> we actually use the system regs early enough ?
> 

I see, however I'm not sure what happens in the GIC drivers if we have a 
CPU running with a GICv3 and other CPUs with something else... But of 
course this is not technically limited by the arm64 capabilities handling.

What behaviour would you be looking for? A way to prevent the CPU to be 
brought up instead of panicking?

> Btw, I am rewriting the capabilities infrastructure to allow per-cap
> control on how it should be treated. I might add an EARLY scope for
> caps which could cover this and may be VHE.

Thanks,

-- 
Julien Thierry