linux-kernel - Re: [PATCH v3 1/3] arm64: Add BBM Level 2 cpu feature

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Message-ID: <d3a15aaf-16a6-4a88-a6c5-9b9afaa5f370@arm.com>
Date: Fri, 14 Mar 2025 09:26:56 +0000
From: Ryan Roberts <ryan.roberts@....com>
To: Mikołaj Lenczewski <miko.lenczewski@....com>
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Subject: Re: [PATCH v3 1/3] arm64: Add BBM Level 2 cpu feature

On 13/03/2025 18:08, Mikołaj Lenczewski wrote:
> On Thu, Mar 13, 2025 at 04:13:22PM +0000, Ryan Roberts wrote:
>> On 13/03/2025 10:41, Mikołaj Lenczewski wrote: 
>>> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
>>> index d561cf3b8ac7..b936e0805161 100644
>>> --- a/arch/arm64/kernel/cpufeature.c
>>> +++ b/arch/arm64/kernel/cpufeature.c
>>> @@ -2176,6 +2176,76 @@ static bool hvhe_possible(const struct arm64_cpu_capabilities *entry,
>>>  	return arm64_test_sw_feature_override(ARM64_SW_FEATURE_OVERRIDE_HVHE);
>>>  }
>>>  
>>> +static inline bool bbml2_possible(void)
>>> +{
>>> +	return !arm64_test_sw_feature_override(ARM64_SW_FEATURE_OVERRIDE_NOBBML2);
>>
>> If you're going to keep this helper, I think it really needs to be:
>>
>> return IS_ENABLED(CONFIG_ARM64_BBML2_NOABORT) &&
>>        !arm64_test_sw_feature_override(ARM64_SW_FEATURE_OVERRIDE_NOBBML2);
>>
>> Then you would simplify the caller to remove it's own
>> IS_ENABLED(CONFIG_ARM64_BBML2_NOABORT) check.
>>
>> But personally I would remove the helper and just fold the test into
>> has_bbml2_noabort().
>>
>> Thanks,
>> Ryan
> 
> I was debating folding it into has_bbml2_noabort(), but went ahead and
> implemented it separately to match hvhe_possible(), which was another sw
> feature helper.

hvhe_possible() is a .matches function, so there is nothing to fold it into.

> 
> But I agree, folding it will be simpler and read just as easily (if not
> easier). Will do so.
> 
>>> +}
>>> +
>>> +static bool cpu_has_bbml2_noabort(unsigned int cpu_midr)
>>> +{
>>> +	/* We want to allow usage of bbml2 in as wide a range of kernel contexts
>>> +	 * as possible. This list is therefore an allow-list of known-good
>>> +	 * implementations that both support bbml2 and additionally, fulfill the
>>> +	 * extra constraint of never generating TLB conflict aborts when using
>>> +	 * the relaxed bbml2 semantics (such aborts make use of bbml2 in certain
>>> +	 * kernel contexts difficult to prove safe against recursive aborts).
>>> +	 *
>>> +	 * Note that implementations can only be considered "known-good" if their
>>> +	 * implementors attest to the fact that the implementation never raises
>>> +	 * TLBI conflict aborts for bbml2 mapping granularity changes.
>>> +	 */
>>> +	static const struct midr_range supports_bbml2_noabort_list[] = {
>>> +		MIDR_REV_RANGE(MIDR_CORTEX_X4, 0, 3, 0xf),
>>> +		MIDR_REV_RANGE(MIDR_NEOVERSE_V3, 0, 2, 0xf),
>>> +		{}
>>> +	};
>>> +
>>> +	return is_midr_in_range_list(cpu_midr, supports_bbml2_noabort_list);
>>> +}
>>> +
>>> +static inline unsigned int __cpu_read_midr(int cpu)
>>
>> nit: why the double underscrore prefix?
> 
> Again copying other helpers I saw that seemed to do similar things.
> Didn't know if this was the expected style, so did as other helpers did.
> Will remove.

Often those double underscores are used when you have a public function wrapping
into a private function, like this:

static void __do_a_thing(bool modify_behaviour_in_some_way);

void do_a_thing(void)
{
	__do_a_thing(false);
}

I'm sure the coding style offers a better explanation.

Thanks,
Ryan

> 
> Thank you for the review.
>