Re: [PATCH v2 2/6] arm64: alternative: Apply alternatives early in boot process

[Suzuki K Poulose <suzuki.poulose@arm.com>]

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.


> 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.

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
>>
>