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

[julien.thierry@arm.com (Julien Thierry)]

On 22/01/18 14:45, Suzuki K Poulose wrote:
> On 22/01/18 12:21, Julien Thierry wrote:
>>
>>
>> On 22/01/18 12:05, Suzuki K Poulose wrote:
>>> On 17/01/18 11:54, Julien Thierry wrote:
>>>> From: Daniel Thompson <daniel.thompson@linaro.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@linaro.org>
>>>> [julien.thierry at arm.com: check non-boot cpu missing early features, 
>>>> avoid
>>>>              duplicates between early features and normal
>>>>              features]
>>>> Signed-off-by: Julien Thierry <julien.thierry@arm.com>
>>>> Cc: Catalin Marinas <catalin.marinas@arm.com>
>>>> Cc: Will Deacon <will.deacon@arm.com>
>>>> Cc: Suzuki K Poulose <suzuki.poulose@arm.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?
>>
> 
> If we have the CONFIG enabled for using system regs, we can continue
> to panic the system. Otherwise, we should ignore the mismatch early,
> as we don't use the system register access unless all boot time active
> CPUs have it.
> 

Hmmm, we use the CPUIF (if available) in the first CPU pretty much as 
soon as we re-enable interrupts in the GICv3 driver, which is way before 
the other CPUs are brought up.

other CPUs get to die_early().

> In a nutshell, this is an early feature only if the CONFIG is enabled,
> otherwise should fall back to the normal behavior.
> 

Maybe we should just not panic and let the mismatching CPUs die.
It's a system wide feature and linux will try to make the other CPUs 
match the boot CPU's config anyway.

-- 
Julien Thierry