Re: [PATCH v5 03/27] arm64: alternative: Apply alternatives early in boot process

[Julien Thierry <julien.thierry@arm.com>]

Hi James,

On 12/09/18 11:29, James Morse wrote:
> Hi Julien,
> 
> On 28/08/18 16:51, 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.
>>
>> Introduce apply_boot_alternatives() to allow some alternatives to be
>> applied immediately after we detect the CPU features of the boot CPU.
> 
> 
>> diff --git a/arch/arm64/kernel/alternative.c b/arch/arm64/kernel/alternative.c
>> index b5d6039..70c2604 100644
>> --- a/arch/arm64/kernel/alternative.c
>> +++ b/arch/arm64/kernel/alternative.c
>> @@ -145,7 +145,8 @@ static void clean_dcache_range_nopatch(u64 start, u64 end)
>>   	} while (cur += d_size, cur < end);
>>   }
>>   
>> -static void __apply_alternatives(void *alt_region, bool is_module)
>> +static void __apply_alternatives(void *alt_region,  bool is_module,
>> +				 unsigned long feature_mask)
> 
> Shouldn't feature_mask be a DECLARE_BITMAP() maybe-array like cpu_hwcaps?
> This means it keeps working when NR_CAPS grows over 64, which might happen
> sooner than we think for backported errata...
> 
> 
>> @@ -155,6 +156,9 @@ static void __apply_alternatives(void *alt_region, bool is_module)
>>   	for (alt = region->begin; alt < region->end; alt++) {
>>   		int nr_inst;
>>   
>> +		if ((BIT(alt->cpufeature) & feature_mask) == 0)
>> +			continue;
>> +
>>   		/* Use ARM64_CB_PATCH as an unconditional patch */
>>   		if (alt->cpufeature < ARM64_CB_PATCH &&
>>   		    !cpus_have_cap(alt->cpufeature))
>> @@ -213,7 +217,7 @@ static int __apply_alternatives_multi_stop(void *unused)
>>   		isb();
>>   	} else {
>>   		BUG_ON(alternatives_applied);
>> -		__apply_alternatives(&region, false);
>> +		__apply_alternatives(&region, false, ~boot_capabilities);
> 
> Ah, this is tricky. There is a bitmap_complement() for the DECLARE_BITMAP()
> stuff, but we'd need a second array...
> 
> We could pass the scope around, but then __apply_alternatives() would need to
> lookup the struct arm64_cpu_capabilities up every time. This is only a problem
> as we have one cap-number-space for errata/features, but separate sparse lists.
> 

Since for each alternative we know the cpufeature associated with it, 
the "lookup" is really just accessing an array with the given index, so 
that could be an option.

> (I think applying the alternatives one cap at a time is a bad idea as we would
> need to walk the alternative region NR_CAPS times)
> 
> 
>> @@ -227,6 +231,24 @@ void __init apply_alternatives_all(void)
>>   	stop_machine(__apply_alternatives_multi_stop, NULL, cpu_online_mask);
>>   }
>>   
>> +/*
>> + * 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 __init apply_boot_alternatives(void)
>> +{
>> +	struct alt_region region = {
>> +		.begin	= (struct alt_instr *)__alt_instructions,
>> +		.end	= (struct alt_instr *)__alt_instructions_end,
>> +	};
>> +
>> +	/* If called on non-boot cpu things could go wrong */
>> +	WARN_ON(smp_processor_id() != 0);
> 
> Isn't the problem if there are multiple CPUs online?
> 

Yes, that makes more sense. I'll change this.

> 
>> +	__apply_alternatives(&region, false, boot_capabilities);
>> +}
>> +
>>   #ifdef CONFIG_MODULES
>>   void apply_alternatives_module(void *start, size_t length)
>>   {
> 
>> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
>> index 3bc1c8b..0d1e41e 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);
>>   
>> +unsigned long boot_capabilities;
>> +
>>   /*
>>    * Flag to indicate if we have computed the system wide
>>    * capabilities based on the boot time active CPUs. This
>> @@ -1375,6 +1377,9 @@ static void __update_cpu_capabilities(const struct arm64_cpu_capabilities *caps,
>>   		if (!cpus_have_cap(caps->capability) && caps->desc)
>>   			pr_info("%s %s\n", info, caps->desc);
>>   		cpus_set_cap(caps->capability);
> 
> Hmm, the bitmap behind cpus_set_cap() is what cpus_have_cap() in
> __apply_alternatives() looks at. If you had a call to __apply_alternatives after
> update_cpu_capabilities(SCOPE_BOOT_CPU), but before any others, it would only
> apply those alternatives...
> 
> (I don't think there is a problem re-applying the same alternative, but I
> haven't checked).
> 

Interesting idea. If someone can confirm that patching alternatives 
twice is safe, I think it would make things simpler.

Thanks,

-- 
Julien Thierry