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From:   Julien Thierry <julien.thierry@arm.com>
Subject: Re: [PATCH v2 2/6] arm64: alternative: Apply alternatives early in
 boot process
To:     Suzuki K Poulose <suzuki.poulose@arm.com>,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org
Cc:     mark.rutland@arm.com, marc.zyngier@arm.com, james.morse@arm.com,
        daniel.thompson@linaro.org,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>
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Date:   Fri, 11 May 2018 09:12:59 +0100
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On 09/05/18 22:52, Suzuki K Poulose wrote:
> On 05/04/2018 11:06 AM, Julien Thierry wrote:
>> Hi,
>>
>> In order to prepare the v3 of this patchset, I'd like people's opinion 
>> on what this patch does. More below.
>>
>> On 17/01/18 11:54, Julien Thierry wrote:
>>> From: Daniel Thompson <daniel.thompson@linaro.org>
>>>
>>> Currently alternatives are applied very late in the boot process (and
>>> a long time after we enable scheduling). Some alternative sequences,
>>> such as those that alter the way CPU context is stored, must be applied
>>> much earlier in the boot sequence.
> 
>>> +/*
>>> + * early-apply features are detected using only the boot CPU and 
>>> checked on
>>> + * secondary CPUs startup, even then,
>>> + * These early-apply features should only include features where we 
>>> must
>>> + * patch the kernel very early in the boot process.
>>> + *
>>> + * Note that the cpufeature logic *must* be made aware of early-apply
>>> + * features to ensure they are reported as enabled without waiting
>>> + * for other CPUs to boot.
>>> + */
>>> +#define EARLY_APPLY_FEATURE_MASK BIT(ARM64_HAS_SYSREG_GIC_CPUIF)
>>> +
>>
>> Following the change in the cpufeature infrastructure, 
>> ARM64_HAS_SYSREG_GIC_CPUIF will have the scope 
>> ARM64_CPUCAP_SCOPE_BOOT_CPU in order to be checked early in the boot 
>> process.
> 
> Thats correct.
> 
>>
>> Now, regarding the early application of alternative, I am wondering 
>> whether we can apply all the alternatives associated with SCOPE_BOOT 
>> features that *do not* have a cpu_enable callback.
>>
> 
> I don't understand why would you skip the ones that have a "cpu_enable" 
> callback. Could you explain this a bit ? Ideally you should be able to
> apply the alternatives for features with the SCOPE_BOOT, provided the
> cpu_enable() callback is written properly.
> 

In my mind the "cpu_enable" callback is the setup a cpu should perform 
before using the feature (i.e. the code getting patched in by the 
alternative). So I was worried about the code getting patched by the 
boot cpu and then have the secondary cpus ending up executing patched 
code before the cpu_enable for the corresponding feature gets called.
Or is there a requirement for secondary cpu startup code to be free of 
alternative code?

> 
>> Otherwise we can keep the macro to list individually each feature that 
>> is patchable at boot time as the current patch does (or put this info 
>> in a flag within the arm64_cpu_capabilities structure)
> 
> You may be able to build up the mask of *available* capabilities with 
> SCOPE_BOOT at boot time by playing some trick in the 
> setup_boot_cpu_capabilities(), rather than embedding it in the 
> capabilities (and then parsing the entire table(s)) or manually keeping
> track of the capabilities by having a separate mask.
> 

Yes, I like that idea.

Thanks,

> Suzuki
> 
>>
>> Any thoughts or preferences on this?
>>
>> Thanks,
>>
>>>   #define __ALT_PTR(a,f)        ((void *)&(a)->f + (a)->f)
>>>   #define ALT_ORIG_PTR(a)        __ALT_PTR(a, orig_offset)
>>>   #define ALT_REPL_PTR(a)        __ALT_PTR(a, alt_offset)
>>> @@ -105,7 +117,8 @@ static u32 get_alt_insn(struct alt_instr *alt, 
>>> __le32 *insnptr, __le32 *altinsnp
>>>       return insn;
>>>   }
>>>
>>> -static void __apply_alternatives(void *alt_region, bool 
>>> use_linear_alias)
>>> +static void __apply_alternatives(void *alt_region,  bool 
>>> use_linear_alias,
>>> +                 unsigned long feature_mask)
>>>   {
>>>       struct alt_instr *alt;
>>>       struct alt_region *region = alt_region;
>>> @@ -115,6 +128,9 @@ static void __apply_alternatives(void 
>>> *alt_region, bool use_linear_alias)
>>>           u32 insn;
>>>           int i, nr_inst;
>>>
>>> +        if ((BIT(alt->cpufeature) & feature_mask) == 0)
>>> +            continue;
>>> +
>>>           if (!cpus_have_cap(alt->cpufeature))
>>>               continue;
>>>
>>> @@ -138,6 +154,21 @@ static void __apply_alternatives(void 
>>> *alt_region, bool use_linear_alias)
>>>   }
>>>
>>>   /*
>>> + * This is called very early in the boot process (directly after we run
>>> + * a feature detect on the boot CPU). No need to worry about other CPUs
>>> + * here.
>>> + */
>>> +void apply_alternatives_early(void)
>>> +{
>>> +    struct alt_region region = {
>>> +        .begin    = (struct alt_instr *)__alt_instructions,
>>> +        .end    = (struct alt_instr *)__alt_instructions_end,
>>> +    };
>>> +
>>> +    __apply_alternatives(&region, true, EARLY_APPLY_FEATURE_MASK);
>>> +}
>>> +
>>> +/*
>>>    * We might be patching the stop_machine state machine, so implement a
>>>    * really simple polling protocol here.
>>>    */
>>> @@ -156,7 +187,9 @@ static int __apply_alternatives_multi_stop(void 
>>> *unused)
>>>           isb();
>>>       } else {
>>>           BUG_ON(patched);
>>> -        __apply_alternatives(&region, true);
>>> +
>>> +        __apply_alternatives(&region, true, ~EARLY_APPLY_FEATURE_MASK);
>>> +
>>>           /* Barriers provided by the cache flushing */
>>>           WRITE_ONCE(patched, 1);
>>>       }
>>> @@ -177,5 +210,5 @@ void apply_alternatives(void *start, size_t length)
>>>           .end    = start + length,
>>>       };
>>>
>>> -    __apply_alternatives(&region, false);
>>> +    __apply_alternatives(&region, false, -1);
>>>   }
>>> diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
>>> index 551eb07..37361b5 100644
>>> --- a/arch/arm64/kernel/smp.c
>>> +++ b/arch/arm64/kernel/smp.c
>>> @@ -453,6 +453,12 @@ void __init smp_prepare_boot_cpu(void)
>>>        * cpuinfo_store_boot_cpu() above.
>>>        */
>>>       update_cpu_errata_workarounds();
>>> +    /*
>>> +     * We now know enough about the boot CPU to apply the
>>> +     * alternatives that cannot wait until interrupt handling
>>> +     * and/or scheduling is enabled.
>>> +     */
>>> +    apply_alternatives_early();
>>>   }
>>>
>>>   static u64 __init of_get_cpu_mpidr(struct device_node *dn)
>>> -- 
>>> 1.9.1
>>>
>>
> 

-- 
Julien Thierry