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Subject: Re: [PATCH v2 1/6] arm64: cpufeature: Allow early detect of specific
 features
To:     Marc Zyngier <marc.zyngier@arm.com>,
        Daniel Thompson <daniel.thompson@linaro.org>,
        Suzuki K Poulose <Suzuki.Poulose@arm.com>
Cc:     linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        mark.rutland@arm.com, james.morse@arm.com,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>
References: <1516190084-18978-1-git-send-email-julien.thierry@arm.com>
 <1516190084-18978-2-git-send-email-julien.thierry@arm.com>
 <ce6469ab-b864-fec7-7d97-d44340f5b2e0@arm.com>
 <2f2a406b-fddf-6c30-4052-650f57ac317a@arm.com>
 <20180122133848.546zixs3crkwvuid@oak.lan>
 <65f82425-977d-49f5-b9c5-b678d90984cf@arm.com>
From:   Julien Thierry <julien.thierry@arm.com>
Message-ID: <973f14ae-df25-f8e0-56e6-0b65e62b64e2@arm.com>
Date:   Mon, 22 Jan 2018 14:14:46 +0000
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On 22/01/18 13:57, Marc Zyngier wrote:
> On 22/01/18 13:38, Daniel Thompson wrote:
>> On Mon, Jan 22, 2018 at 12:21:55PM +0000, 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@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.
>>
>> Shouldn't each CPU be sharing the same GIC anyway? It so its not some
>> have GICv3+ and some have GICv2. The theoretical system described above
>> *has* a GICv3+ but some participants in the cluster are not able to
>> talk to it as like a co-processor.
> 
> There is some level of confusion between the GIC CPU interface (which is
> really in the CPU) and the GIC itself. You can easily end-up in a
> situation where you do have the HW, but it is configured in a way that
> prevents you from using it. Case in point: GICv3 with GICv2
> compatibility used in virtualization.
> 
>> The ARM ARM is a little vague about whether, if a GIC implements a
>> system register interface, then a core must provide access to it. Even
>> so, first question is whether such a system is architecture compliant?
> 
> Again, it is not the GIC that implements the system registers. And no,
> these system registers are not required to be accessible (see
> ICC_SRE_EL2.Enable == 0 for example).
> 
> So I believe there is value in checking those as early as possible, and
> set the expectations accordingly (such as in [1] and [2]).
> 

So in the end, if we boot on a CPU that can access ICC_CPUIF, it looks 
like we'll prevent bringing up the CPUs that cannot access the 
ICC_CPUIF, and if we boot on a CPU that cannot access ICC_CPUIF, 
everything that gets brought up afterwards will be run on GICv2 
compatibility mode?
We never run different GIC driver on different CPUs, right?


In the patch, check_early_cpu_features panics when features don't match, 
but nothing really prevents us to use cpu_die_early instead.

Would that solve the issue Suzuki?

Cheers,

-- 
Julien Thierry