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

[Marc Zyngier]

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@xxxxxxxxxx>
>>>>
>>>> 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@xxxxxxxxxx>
>>>> [julien.thierry@xxxxxxx: check non-boot cpu missing early features, avoid
>>>>              duplicates between early features and normal
>>>>              features]
>>>> Signed-off-by: Julien Thierry <julien.thierry@xxxxxxx>
>>>> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
>>>> Cc: Will Deacon <will.deacon@xxxxxxx>
>>>> Cc: Suzuki K Poulose <suzuki.poulose@xxxxxxx>
>>>> ---
>>>>   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]).

Thanks,

	M.

[1]
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/irqchip/irq-gic-v3.c#n536
[2]
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/arm64/kernel/cpufeature.c#n798
-- 
Jazz is not dead. It just smells funny...