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

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Message-ID: <83c04cc2-a170-4d4d-82e3-23aec9749352@arm.com>
Date: Thu, 13 Mar 2025 18:17:02 +0000
From: Ryan Roberts <ryan.roberts@....com>
To: Mikołaj Lenczewski <miko.lenczewski@....com>,
 Yang Shi <yang@...amperecomputing.com>
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Subject: Re: [PATCH v3 1/3] arm64: Add BBM Level 2 cpu feature

On 13/03/2025 18:13, Mikołaj Lenczewski wrote:
> On Thu, Mar 13, 2025 at 10:21:51AM -0700, Yang Shi wrote:
>> On 3/13/25 3:41 AM, Mikołaj Lenczewski wrote:
>>> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
>>> index fb8752b42ec8..3e4cc917a07e 100644
>>> --- a/Documentation/admin-guide/kernel-parameters.txt
>>> +++ b/Documentation/admin-guide/kernel-parameters.txt
>>> @@ -453,6 +453,9 @@
>>>   	arm64.no32bit_el0 [ARM64] Unconditionally disable the execution of
>>>   			32 bit applications.
>>>   
>>> +	arm64.nobbml2	[ARM64] Unconditionally disable Break-Before-Make Level
>>> +			2 support
>>
>> Hi Miko,
>>
>> A question about the kernel boot parameter. Can this parameter be used 
>> in early boot stage? A quick look at the code shows it should be ok, for 
>> example, cpu_has_bti() is called in map_kernel(). But I'd like to double 
>> check because my patchset needs to check this parameter in map_mem() to 
>> determine whether large block mapping can be used or not.
>>
>> And a nit below.
> 
> I will need to double check exactly when the arm64 software overrides
> are finalised, but as long as those values are finalised in / before (?)
> the early boot stage then it should be fine? Will reply again once I
> check and have an answer.

This will work fine. The override is setup in the PI code before start_kernel().

>    
>>> +static inline bool bbml2_possible(void)
>>> +{
>>> +	return !arm64_test_sw_feature_override(ARM64_SW_FEATURE_OVERRIDE_NOBBML2);
>>> +}
>>
>> Can this be moved to cpufeature.h? My patch will use this, anyway I can 
>> do it in my patchset.

I'd prefer to do the moving as part of the series that needs it moved.

Thanks,
Ryan

>>
>> Thanks,
>> Yang
> 
> I can do so. In fact, on second thought, I will probably extend this to
> also include the `IS_ENABLED(CONFIG_ARM64_BBML2_NOABORT)` check as well,
> and then move it to cpufeature.h, instead of folding said check into
> has_bbml2_noabort().
>